blk-mq: Drop explicit timeout sync in hotplug

[karo-tx-linux.git] / block / blk-mq.c

/*
 * Block multiqueue core code
 *
 * Copyright (C) 2013-2014 Jens Axboe
 * Copyright (C) 2013-2014 Christoph Hellwig
 */
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/backing-dev.h>
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/kmemleak.h>
#include <linux/mm.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/workqueue.h>
#include <linux/smp.h>
#include <linux/llist.h>
#include <linux/list_sort.h>
#include <linux/cpu.h>
#include <linux/cache.h>
#include <linux/sched/sysctl.h>
#include <linux/delay.h>
#include <linux/crash_dump.h>
#include <linux/prefetch.h>

#include <trace/events/block.h>

#include <linux/blk-mq.h>
#include "blk.h"
#include "blk-mq.h"
#include "blk-mq-tag.h"
#include "blk-stat.h"
#include "blk-wbt.h"

static DEFINE_MUTEX(all_q_mutex);
static LIST_HEAD(all_q_list);

/*
 * Check if any of the ctx's have pending work in this hardware queue
 */
static bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx)
{
        return sbitmap_any_bit_set(&hctx->ctx_map);
}

/*
 * Mark this ctx as having pending work in this hardware queue
 */
static void blk_mq_hctx_mark_pending(struct blk_mq_hw_ctx *hctx,
                                     struct blk_mq_ctx *ctx)
{
        if (!sbitmap_test_bit(&hctx->ctx_map, ctx->index_hw))
                sbitmap_set_bit(&hctx->ctx_map, ctx->index_hw);
}

static void blk_mq_hctx_clear_pending(struct blk_mq_hw_ctx *hctx,
                                      struct blk_mq_ctx *ctx)
{
        sbitmap_clear_bit(&hctx->ctx_map, ctx->index_hw);
}

void blk_mq_freeze_queue_start(struct request_queue *q)
{
        int freeze_depth;

        freeze_depth = atomic_inc_return(&q->mq_freeze_depth);
        if (freeze_depth == 1) {
                percpu_ref_kill(&q->q_usage_counter);
                blk_mq_run_hw_queues(q, false);
        }
}
EXPORT_SYMBOL_GPL(blk_mq_freeze_queue_start);

static void blk_mq_freeze_queue_wait(struct request_queue *q)
{
        wait_event(q->mq_freeze_wq, percpu_ref_is_zero(&q->q_usage_counter));
}

/*
 * Guarantee no request is in use, so we can change any data structure of
 * the queue afterward.
 */
void blk_freeze_queue(struct request_queue *q)
{
        /*
         * In the !blk_mq case we are only calling this to kill the
         * q_usage_counter, otherwise this increases the freeze depth
         * and waits for it to return to zero.  For this reason there is
         * no blk_unfreeze_queue(), and blk_freeze_queue() is not
         * exported to drivers as the only user for unfreeze is blk_mq.
         */
        blk_mq_freeze_queue_start(q);
        blk_mq_freeze_queue_wait(q);
}

void blk_mq_freeze_queue(struct request_queue *q)
{
        /*
         * ...just an alias to keep freeze and unfreeze actions balanced
         * in the blk_mq_* namespace
         */
        blk_freeze_queue(q);
}
EXPORT_SYMBOL_GPL(blk_mq_freeze_queue);

void blk_mq_unfreeze_queue(struct request_queue *q)
{
        int freeze_depth;

        freeze_depth = atomic_dec_return(&q->mq_freeze_depth);
        WARN_ON_ONCE(freeze_depth < 0);
        if (!freeze_depth) {
                percpu_ref_reinit(&q->q_usage_counter);
                wake_up_all(&q->mq_freeze_wq);
        }
}
EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);

/**
 * blk_mq_quiesce_queue() - wait until all ongoing queue_rq calls have finished
 * @q: request queue.
 *
 * Note: this function does not prevent that the struct request end_io()
 * callback function is invoked. Additionally, it is not prevented that
 * new queue_rq() calls occur unless the queue has been stopped first.
 */
void blk_mq_quiesce_queue(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;
        bool rcu = false;

        blk_mq_stop_hw_queues(q);

        queue_for_each_hw_ctx(q, hctx, i) {
                if (hctx->flags & BLK_MQ_F_BLOCKING)
                        synchronize_srcu(&hctx->queue_rq_srcu);
                else
                        rcu = true;
        }
        if (rcu)
                synchronize_rcu();
}
EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);

void blk_mq_wake_waiters(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;

        queue_for_each_hw_ctx(q, hctx, i)
                if (blk_mq_hw_queue_mapped(hctx))
                        blk_mq_tag_wakeup_all(hctx->tags, true);

        /*
         * If we are called because the queue has now been marked as
         * dying, we need to ensure that processes currently waiting on
         * the queue are notified as well.
         */
        wake_up_all(&q->mq_freeze_wq);
}

bool blk_mq_can_queue(struct blk_mq_hw_ctx *hctx)
{
        return blk_mq_has_free_tags(hctx->tags);
}
EXPORT_SYMBOL(blk_mq_can_queue);

static void blk_mq_rq_ctx_init(struct request_queue *q, struct blk_mq_ctx *ctx,
                               struct request *rq, unsigned int op)
{
        INIT_LIST_HEAD(&rq->queuelist);
        /* csd/requeue_work/fifo_time is initialized before use */
        rq->q = q;
        rq->mq_ctx = ctx;
        rq->cmd_flags = op;
        if (blk_queue_io_stat(q))
                rq->rq_flags |= RQF_IO_STAT;
        /* do not touch atomic flags, it needs atomic ops against the timer */
        rq->cpu = -1;
        INIT_HLIST_NODE(&rq->hash);
        RB_CLEAR_NODE(&rq->rb_node);
        rq->rq_disk = NULL;
        rq->part = NULL;
        rq->start_time = jiffies;
#ifdef CONFIG_BLK_CGROUP
        rq->rl = NULL;
        set_start_time_ns(rq);
        rq->io_start_time_ns = 0;
#endif
        rq->nr_phys_segments = 0;
#if defined(CONFIG_BLK_DEV_INTEGRITY)
        rq->nr_integrity_segments = 0;
#endif
        rq->special = NULL;
        /* tag was already set */
        rq->errors = 0;

        rq->cmd = rq->__cmd;

        rq->extra_len = 0;
        rq->sense_len = 0;
        rq->resid_len = 0;
        rq->sense = NULL;

        INIT_LIST_HEAD(&rq->timeout_list);
        rq->timeout = 0;

        rq->end_io = NULL;
        rq->end_io_data = NULL;
        rq->next_rq = NULL;

        ctx->rq_dispatched[op_is_sync(op)]++;
}

static struct request *
__blk_mq_alloc_request(struct blk_mq_alloc_data *data, unsigned int op)
{
        struct request *rq;
        unsigned int tag;

        tag = blk_mq_get_tag(data);
        if (tag != BLK_MQ_TAG_FAIL) {
                rq = data->hctx->tags->rqs[tag];

                if (blk_mq_tag_busy(data->hctx)) {
                        rq->rq_flags = RQF_MQ_INFLIGHT;
                        atomic_inc(&data->hctx->nr_active);
                }

                rq->tag = tag;
                blk_mq_rq_ctx_init(data->q, data->ctx, rq, op);
                return rq;
        }

        return NULL;
}

struct request *blk_mq_alloc_request(struct request_queue *q, int rw,
                unsigned int flags)
{
        struct blk_mq_ctx *ctx;
        struct blk_mq_hw_ctx *hctx;
        struct request *rq;
        struct blk_mq_alloc_data alloc_data;
        int ret;

        ret = blk_queue_enter(q, flags & BLK_MQ_REQ_NOWAIT);
        if (ret)
                return ERR_PTR(ret);

        ctx = blk_mq_get_ctx(q);
        hctx = blk_mq_map_queue(q, ctx->cpu);
        blk_mq_set_alloc_data(&alloc_data, q, flags, ctx, hctx);
        rq = __blk_mq_alloc_request(&alloc_data, rw);
        blk_mq_put_ctx(ctx);

        if (!rq) {
                blk_queue_exit(q);
                return ERR_PTR(-EWOULDBLOCK);
        }

        rq->__data_len = 0;
        rq->__sector = (sector_t) -1;
        rq->bio = rq->biotail = NULL;
        return rq;
}
EXPORT_SYMBOL(blk_mq_alloc_request);

struct request *blk_mq_alloc_request_hctx(struct request_queue *q, int rw,
                unsigned int flags, unsigned int hctx_idx)
{
        struct blk_mq_hw_ctx *hctx;
        struct blk_mq_ctx *ctx;
        struct request *rq;
        struct blk_mq_alloc_data alloc_data;
        int ret;

        /*
         * If the tag allocator sleeps we could get an allocation for a
         * different hardware context.  No need to complicate the low level
         * allocator for this for the rare use case of a command tied to
         * a specific queue.
         */
        if (WARN_ON_ONCE(!(flags & BLK_MQ_REQ_NOWAIT)))
                return ERR_PTR(-EINVAL);

        if (hctx_idx >= q->nr_hw_queues)
                return ERR_PTR(-EIO);

        ret = blk_queue_enter(q, true);
        if (ret)
                return ERR_PTR(ret);

        /*
         * Check if the hardware context is actually mapped to anything.
         * If not tell the caller that it should skip this queue.
         */
        hctx = q->queue_hw_ctx[hctx_idx];
        if (!blk_mq_hw_queue_mapped(hctx)) {
                ret = -EXDEV;
                goto out_queue_exit;
        }
        ctx = __blk_mq_get_ctx(q, cpumask_first(hctx->cpumask));

        blk_mq_set_alloc_data(&alloc_data, q, flags, ctx, hctx);
        rq = __blk_mq_alloc_request(&alloc_data, rw);
        if (!rq) {
                ret = -EWOULDBLOCK;
                goto out_queue_exit;
        }

        return rq;

out_queue_exit:
        blk_queue_exit(q);
        return ERR_PTR(ret);
}
EXPORT_SYMBOL_GPL(blk_mq_alloc_request_hctx);

static void __blk_mq_free_request(struct blk_mq_hw_ctx *hctx,
                                  struct blk_mq_ctx *ctx, struct request *rq)
{
        const int tag = rq->tag;
        struct request_queue *q = rq->q;

        if (rq->rq_flags & RQF_MQ_INFLIGHT)
                atomic_dec(&hctx->nr_active);

        wbt_done(q->rq_wb, &rq->issue_stat);
        rq->rq_flags = 0;

        clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
        clear_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);
        blk_mq_put_tag(hctx, ctx, tag);
        blk_queue_exit(q);
}

void blk_mq_free_hctx_request(struct blk_mq_hw_ctx *hctx, struct request *rq)
{
        struct blk_mq_ctx *ctx = rq->mq_ctx;

        ctx->rq_completed[rq_is_sync(rq)]++;
        __blk_mq_free_request(hctx, ctx, rq);

}
EXPORT_SYMBOL_GPL(blk_mq_free_hctx_request);

void blk_mq_free_request(struct request *rq)
{
        blk_mq_free_hctx_request(blk_mq_map_queue(rq->q, rq->mq_ctx->cpu), rq);
}
EXPORT_SYMBOL_GPL(blk_mq_free_request);

inline void __blk_mq_end_request(struct request *rq, int error)
{
        blk_account_io_done(rq);

        if (rq->end_io) {
                wbt_done(rq->q->rq_wb, &rq->issue_stat);
                rq->end_io(rq, error);
        } else {
                if (unlikely(blk_bidi_rq(rq)))
                        blk_mq_free_request(rq->next_rq);
                blk_mq_free_request(rq);
        }
}
EXPORT_SYMBOL(__blk_mq_end_request);

void blk_mq_end_request(struct request *rq, int error)
{
        if (blk_update_request(rq, error, blk_rq_bytes(rq)))
                BUG();
        __blk_mq_end_request(rq, error);
}
EXPORT_SYMBOL(blk_mq_end_request);

static void __blk_mq_complete_request_remote(void *data)
{
        struct request *rq = data;

        rq->q->softirq_done_fn(rq);
}

static void blk_mq_ipi_complete_request(struct request *rq)
{
        struct blk_mq_ctx *ctx = rq->mq_ctx;
        bool shared = false;
        int cpu;

        if (!test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags)) {
                rq->q->softirq_done_fn(rq);
                return;
        }

        cpu = get_cpu();
        if (!test_bit(QUEUE_FLAG_SAME_FORCE, &rq->q->queue_flags))
                shared = cpus_share_cache(cpu, ctx->cpu);

        if (cpu != ctx->cpu && !shared && cpu_online(ctx->cpu)) {
                rq->csd.func = __blk_mq_complete_request_remote;
                rq->csd.info = rq;
                rq->csd.flags = 0;
                smp_call_function_single_async(ctx->cpu, &rq->csd);
        } else {
                rq->q->softirq_done_fn(rq);
        }
        put_cpu();
}

static void blk_mq_stat_add(struct request *rq)
{
        if (rq->rq_flags & RQF_STATS) {
                /*
                 * We could rq->mq_ctx here, but there's less of a risk
                 * of races if we have the completion event add the stats
                 * to the local software queue.
                 */
                struct blk_mq_ctx *ctx;

                ctx = __blk_mq_get_ctx(rq->q, raw_smp_processor_id());
                blk_stat_add(&ctx->stat[rq_data_dir(rq)], rq);
        }
}

static void __blk_mq_complete_request(struct request *rq)
{
        struct request_queue *q = rq->q;

        blk_mq_stat_add(rq);

        if (!q->softirq_done_fn)
                blk_mq_end_request(rq, rq->errors);
        else
                blk_mq_ipi_complete_request(rq);
}

/**
 * blk_mq_complete_request - end I/O on a request
 * @rq:         the request being processed
 *
 * Description:
 *      Ends all I/O on a request. It does not handle partial completions.
 *      The actual completion happens out-of-order, through a IPI handler.
 **/
void blk_mq_complete_request(struct request *rq, int error)
{
        struct request_queue *q = rq->q;

        if (unlikely(blk_should_fake_timeout(q)))
                return;
        if (!blk_mark_rq_complete(rq)) {
                rq->errors = error;
                __blk_mq_complete_request(rq);
        }
}
EXPORT_SYMBOL(blk_mq_complete_request);

int blk_mq_request_started(struct request *rq)
{
        return test_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
}
EXPORT_SYMBOL_GPL(blk_mq_request_started);

void blk_mq_start_request(struct request *rq)
{
        struct request_queue *q = rq->q;

        trace_block_rq_issue(q, rq);

        rq->resid_len = blk_rq_bytes(rq);
        if (unlikely(blk_bidi_rq(rq)))
                rq->next_rq->resid_len = blk_rq_bytes(rq->next_rq);

        if (test_bit(QUEUE_FLAG_STATS, &q->queue_flags)) {
                blk_stat_set_issue_time(&rq->issue_stat);
                rq->rq_flags |= RQF_STATS;
                wbt_issue(q->rq_wb, &rq->issue_stat);
        }

        blk_add_timer(rq);

        /*
         * Ensure that ->deadline is visible before set the started
         * flag and clear the completed flag.
         */
        smp_mb__before_atomic();

        /*
         * Mark us as started and clear complete. Complete might have been
         * set if requeue raced with timeout, which then marked it as
         * complete. So be sure to clear complete again when we start
         * the request, otherwise we'll ignore the completion event.
         */
        if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags))
                set_bit(REQ_ATOM_STARTED, &rq->atomic_flags);
        if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
                clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);

        if (q->dma_drain_size && blk_rq_bytes(rq)) {
                /*
                 * Make sure space for the drain appears.  We know we can do
                 * this because max_hw_segments has been adjusted to be one
                 * fewer than the device can handle.
                 */
                rq->nr_phys_segments++;
        }
}
EXPORT_SYMBOL(blk_mq_start_request);

static void __blk_mq_requeue_request(struct request *rq)
{
        struct request_queue *q = rq->q;

        trace_block_rq_requeue(q, rq);
        wbt_requeue(q->rq_wb, &rq->issue_stat);

        if (test_and_clear_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
                if (q->dma_drain_size && blk_rq_bytes(rq))
                        rq->nr_phys_segments--;
        }
}

void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list)
{
        __blk_mq_requeue_request(rq);

        BUG_ON(blk_queued_rq(rq));
        blk_mq_add_to_requeue_list(rq, true, kick_requeue_list);
}
EXPORT_SYMBOL(blk_mq_requeue_request);

static void blk_mq_requeue_work(struct work_struct *work)
{
        struct request_queue *q =
                container_of(work, struct request_queue, requeue_work.work);
        LIST_HEAD(rq_list);
        struct request *rq, *next;
        unsigned long flags;

        spin_lock_irqsave(&q->requeue_lock, flags);
        list_splice_init(&q->requeue_list, &rq_list);
        spin_unlock_irqrestore(&q->requeue_lock, flags);

        list_for_each_entry_safe(rq, next, &rq_list, queuelist) {
                if (!(rq->rq_flags & RQF_SOFTBARRIER))
                        continue;

                rq->rq_flags &= ~RQF_SOFTBARRIER;
                list_del_init(&rq->queuelist);
                blk_mq_insert_request(rq, true, false, false);
        }

        while (!list_empty(&rq_list)) {
                rq = list_entry(rq_list.next, struct request, queuelist);
                list_del_init(&rq->queuelist);
                blk_mq_insert_request(rq, false, false, false);
        }

        blk_mq_run_hw_queues(q, false);
}

void blk_mq_add_to_requeue_list(struct request *rq, bool at_head,
                                bool kick_requeue_list)
{
        struct request_queue *q = rq->q;
        unsigned long flags;

        /*
         * We abuse this flag that is otherwise used by the I/O scheduler to
         * request head insertation from the workqueue.
         */
        BUG_ON(rq->rq_flags & RQF_SOFTBARRIER);

        spin_lock_irqsave(&q->requeue_lock, flags);
        if (at_head) {
                rq->rq_flags |= RQF_SOFTBARRIER;
                list_add(&rq->queuelist, &q->requeue_list);
        } else {
                list_add_tail(&rq->queuelist, &q->requeue_list);
        }
        spin_unlock_irqrestore(&q->requeue_lock, flags);

        if (kick_requeue_list)
                blk_mq_kick_requeue_list(q);
}
EXPORT_SYMBOL(blk_mq_add_to_requeue_list);

void blk_mq_kick_requeue_list(struct request_queue *q)
{
        kblockd_schedule_delayed_work(&q->requeue_work, 0);
}
EXPORT_SYMBOL(blk_mq_kick_requeue_list);

void blk_mq_delay_kick_requeue_list(struct request_queue *q,
                                    unsigned long msecs)
{
        kblockd_schedule_delayed_work(&q->requeue_work,
                                      msecs_to_jiffies(msecs));
}
EXPORT_SYMBOL(blk_mq_delay_kick_requeue_list);

void blk_mq_abort_requeue_list(struct request_queue *q)
{
        unsigned long flags;
        LIST_HEAD(rq_list);

        spin_lock_irqsave(&q->requeue_lock, flags);
        list_splice_init(&q->requeue_list, &rq_list);
        spin_unlock_irqrestore(&q->requeue_lock, flags);

        while (!list_empty(&rq_list)) {
                struct request *rq;

                rq = list_first_entry(&rq_list, struct request, queuelist);
                list_del_init(&rq->queuelist);
                rq->errors = -EIO;
                blk_mq_end_request(rq, rq->errors);
        }
}
EXPORT_SYMBOL(blk_mq_abort_requeue_list);

struct request *blk_mq_tag_to_rq(struct blk_mq_tags *tags, unsigned int tag)
{
        if (tag < tags->nr_tags) {
                prefetch(tags->rqs[tag]);
                return tags->rqs[tag];
        }

        return NULL;
}
EXPORT_SYMBOL(blk_mq_tag_to_rq);

struct blk_mq_timeout_data {
        unsigned long next;
        unsigned int next_set;
};

void blk_mq_rq_timed_out(struct request *req, bool reserved)
{
        struct blk_mq_ops *ops = req->q->mq_ops;
        enum blk_eh_timer_return ret = BLK_EH_RESET_TIMER;

        /*
         * We know that complete is set at this point. If STARTED isn't set
         * anymore, then the request isn't active and the "timeout" should
         * just be ignored. This can happen due to the bitflag ordering.
         * Timeout first checks if STARTED is set, and if it is, assumes
         * the request is active. But if we race with completion, then
         * we both flags will get cleared. So check here again, and ignore
         * a timeout event with a request that isn't active.
         */
        if (!test_bit(REQ_ATOM_STARTED, &req->atomic_flags))
                return;

        if (ops->timeout)
                ret = ops->timeout(req, reserved);

        switch (ret) {
        case BLK_EH_HANDLED:
                __blk_mq_complete_request(req);
                break;
        case BLK_EH_RESET_TIMER:
                blk_add_timer(req);
                blk_clear_rq_complete(req);
                break;
        case BLK_EH_NOT_HANDLED:
                break;
        default:
                printk(KERN_ERR "block: bad eh return: %d\n", ret);
                break;
        }
}

static void blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
                struct request *rq, void *priv, bool reserved)
{
        struct blk_mq_timeout_data *data = priv;

        if (!test_bit(REQ_ATOM_STARTED, &rq->atomic_flags)) {
                /*
                 * If a request wasn't started before the queue was
                 * marked dying, kill it here or it'll go unnoticed.
                 */
                if (unlikely(blk_queue_dying(rq->q))) {
                        rq->errors = -EIO;
                        blk_mq_end_request(rq, rq->errors);
                }
                return;
        }

        if (time_after_eq(jiffies, rq->deadline)) {
                if (!blk_mark_rq_complete(rq))
                        blk_mq_rq_timed_out(rq, reserved);
        } else if (!data->next_set || time_after(data->next, rq->deadline)) {
                data->next = rq->deadline;
                data->next_set = 1;
        }
}

static void blk_mq_timeout_work(struct work_struct *work)
{
        struct request_queue *q =
                container_of(work, struct request_queue, timeout_work);
        struct blk_mq_timeout_data data = {
                .next           = 0,
                .next_set       = 0,
        };
        int i;

        /* A deadlock might occur if a request is stuck requiring a
         * timeout at the same time a queue freeze is waiting
         * completion, since the timeout code would not be able to
         * acquire the queue reference here.
         *
         * That's why we don't use blk_queue_enter here; instead, we use
         * percpu_ref_tryget directly, because we need to be able to
         * obtain a reference even in the short window between the queue
         * starting to freeze, by dropping the first reference in
         * blk_mq_freeze_queue_start, and the moment the last request is
         * consumed, marked by the instant q_usage_counter reaches
         * zero.
         */
        if (!percpu_ref_tryget(&q->q_usage_counter))
                return;

        blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data);

        if (data.next_set) {
                data.next = blk_rq_timeout(round_jiffies_up(data.next));
                mod_timer(&q->timeout, data.next);
        } else {
                struct blk_mq_hw_ctx *hctx;

                queue_for_each_hw_ctx(q, hctx, i) {
                        /* the hctx may be unmapped, so check it here */
                        if (blk_mq_hw_queue_mapped(hctx))
                                blk_mq_tag_idle(hctx);
                }
        }
        blk_queue_exit(q);
}

/*
 * Reverse check our software queue for entries that we could potentially
 * merge with. Currently includes a hand-wavy stop count of 8, to not spend
 * too much time checking for merges.
 */
static bool blk_mq_attempt_merge(struct request_queue *q,
                                 struct blk_mq_ctx *ctx, struct bio *bio)
{
        struct request *rq;
        int checked = 8;

        list_for_each_entry_reverse(rq, &ctx->rq_list, queuelist) {
                int el_ret;

                if (!checked--)
                        break;

                if (!blk_rq_merge_ok(rq, bio))
                        continue;

                el_ret = blk_try_merge(rq, bio);
                if (el_ret == ELEVATOR_BACK_MERGE) {
                        if (bio_attempt_back_merge(q, rq, bio)) {
                                ctx->rq_merged++;
                                return true;
                        }
                        break;
                } else if (el_ret == ELEVATOR_FRONT_MERGE) {
                        if (bio_attempt_front_merge(q, rq, bio)) {
                                ctx->rq_merged++;
                                return true;
                        }
                        break;
                }
        }

        return false;
}

struct flush_busy_ctx_data {
        struct blk_mq_hw_ctx *hctx;
        struct list_head *list;
};

static bool flush_busy_ctx(struct sbitmap *sb, unsigned int bitnr, void *data)
{
        struct flush_busy_ctx_data *flush_data = data;
        struct blk_mq_hw_ctx *hctx = flush_data->hctx;
        struct blk_mq_ctx *ctx = hctx->ctxs[bitnr];

        sbitmap_clear_bit(sb, bitnr);
        spin_lock(&ctx->lock);
        list_splice_tail_init(&ctx->rq_list, flush_data->list);
        spin_unlock(&ctx->lock);
        return true;
}

/*
 * Process software queues that have been marked busy, splicing them
 * to the for-dispatch
 */
static void flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list)
{
        struct flush_busy_ctx_data data = {
                .hctx = hctx,
                .list = list,
        };

        sbitmap_for_each_set(&hctx->ctx_map, flush_busy_ctx, &data);
}

static inline unsigned int queued_to_index(unsigned int queued)
{
        if (!queued)
                return 0;

        return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
}

/*
 * Run this hardware queue, pulling any software queues mapped to it in.
 * Note that this function currently has various problems around ordering
 * of IO. In particular, we'd like FIFO behaviour on handling existing
 * items on the hctx->dispatch list. Ignore that for now.
 */
static void blk_mq_process_rq_list(struct blk_mq_hw_ctx *hctx)
{
        struct request_queue *q = hctx->queue;
        struct request *rq;
        LIST_HEAD(rq_list);
        LIST_HEAD(driver_list);
        struct list_head *dptr;
        int queued;

        if (unlikely(blk_mq_hctx_stopped(hctx)))
                return;

        hctx->run++;

        /*
         * Touch any software queue that has pending entries.
         */
        flush_busy_ctxs(hctx, &rq_list);

        /*
         * If we have previous entries on our dispatch list, grab them
         * and stuff them at the front for more fair dispatch.
         */
        if (!list_empty_careful(&hctx->dispatch)) {
                spin_lock(&hctx->lock);
                if (!list_empty(&hctx->dispatch))
                        list_splice_init(&hctx->dispatch, &rq_list);
                spin_unlock(&hctx->lock);
        }

        /*
         * Start off with dptr being NULL, so we start the first request
         * immediately, even if we have more pending.
         */
        dptr = NULL;

        /*
         * Now process all the entries, sending them to the driver.
         */
        queued = 0;
        while (!list_empty(&rq_list)) {
                struct blk_mq_queue_data bd;
                int ret;

                rq = list_first_entry(&rq_list, struct request, queuelist);
                list_del_init(&rq->queuelist);

                bd.rq = rq;
                bd.list = dptr;
                bd.last = list_empty(&rq_list);

                ret = q->mq_ops->queue_rq(hctx, &bd);
                switch (ret) {
                case BLK_MQ_RQ_QUEUE_OK:
                        queued++;
                        break;
                case BLK_MQ_RQ_QUEUE_BUSY:
                        list_add(&rq->queuelist, &rq_list);
                        __blk_mq_requeue_request(rq);
                        break;
                default:
                        pr_err("blk-mq: bad return on queue: %d\n", ret);
                case BLK_MQ_RQ_QUEUE_ERROR:
                        rq->errors = -EIO;
                        blk_mq_end_request(rq, rq->errors);
                        break;
                }

                if (ret == BLK_MQ_RQ_QUEUE_BUSY)
                        break;

                /*
                 * We've done the first request. If we have more than 1
                 * left in the list, set dptr to defer issue.
                 */
                if (!dptr && rq_list.next != rq_list.prev)
                        dptr = &driver_list;
        }

        hctx->dispatched[queued_to_index(queued)]++;

        /*
         * Any items that need requeuing? Stuff them into hctx->dispatch,
         * that is where we will continue on next queue run.
         */
        if (!list_empty(&rq_list)) {
                spin_lock(&hctx->lock);
                list_splice(&rq_list, &hctx->dispatch);
                spin_unlock(&hctx->lock);
                /*
                 * the queue is expected stopped with BLK_MQ_RQ_QUEUE_BUSY, but
                 * it's possible the queue is stopped and restarted again
                 * before this. Queue restart will dispatch requests. And since
                 * requests in rq_list aren't added into hctx->dispatch yet,
                 * the requests in rq_list might get lost.
                 *
                 * blk_mq_run_hw_queue() already checks the STOPPED bit
                 **/
                blk_mq_run_hw_queue(hctx, true);
        }
}

static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
{
        int srcu_idx;

        WARN_ON(!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
                cpu_online(hctx->next_cpu));

        if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
                rcu_read_lock();
                blk_mq_process_rq_list(hctx);
                rcu_read_unlock();
        } else {
                srcu_idx = srcu_read_lock(&hctx->queue_rq_srcu);
                blk_mq_process_rq_list(hctx);
                srcu_read_unlock(&hctx->queue_rq_srcu, srcu_idx);
        }
}

/*
 * It'd be great if the workqueue API had a way to pass
 * in a mask and had some smarts for more clever placement.
 * For now we just round-robin here, switching for every
 * BLK_MQ_CPU_WORK_BATCH queued items.
 */
static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
{
        if (hctx->queue->nr_hw_queues == 1)
                return WORK_CPU_UNBOUND;

        if (--hctx->next_cpu_batch <= 0) {
                int next_cpu;

                next_cpu = cpumask_next(hctx->next_cpu, hctx->cpumask);
                if (next_cpu >= nr_cpu_ids)
                        next_cpu = cpumask_first(hctx->cpumask);

                hctx->next_cpu = next_cpu;
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }

        return hctx->next_cpu;
}

void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
{
        if (unlikely(blk_mq_hctx_stopped(hctx) ||
                     !blk_mq_hw_queue_mapped(hctx)))
                return;

        if (!async && !(hctx->flags & BLK_MQ_F_BLOCKING)) {
                int cpu = get_cpu();
                if (cpumask_test_cpu(cpu, hctx->cpumask)) {
                        __blk_mq_run_hw_queue(hctx);
                        put_cpu();
                        return;
                }

                put_cpu();
        }

        kblockd_schedule_work_on(blk_mq_hctx_next_cpu(hctx), &hctx->run_work);
}

void blk_mq_run_hw_queues(struct request_queue *q, bool async)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if ((!blk_mq_hctx_has_pending(hctx) &&
                    list_empty_careful(&hctx->dispatch)) ||
                    blk_mq_hctx_stopped(hctx))
                        continue;

                blk_mq_run_hw_queue(hctx, async);
        }
}
EXPORT_SYMBOL(blk_mq_run_hw_queues);

/**
 * blk_mq_queue_stopped() - check whether one or more hctxs have been stopped
 * @q: request queue.
 *
 * The caller is responsible for serializing this function against
 * blk_mq_{start,stop}_hw_queue().
 */
bool blk_mq_queue_stopped(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i)
                if (blk_mq_hctx_stopped(hctx))
                        return true;

        return false;
}
EXPORT_SYMBOL(blk_mq_queue_stopped);

void blk_mq_stop_hw_queue(struct blk_mq_hw_ctx *hctx)
{
        cancel_work(&hctx->run_work);
        cancel_delayed_work(&hctx->delay_work);
        set_bit(BLK_MQ_S_STOPPED, &hctx->state);
}
EXPORT_SYMBOL(blk_mq_stop_hw_queue);

void blk_mq_stop_hw_queues(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i)
                blk_mq_stop_hw_queue(hctx);
}
EXPORT_SYMBOL(blk_mq_stop_hw_queues);

void blk_mq_start_hw_queue(struct blk_mq_hw_ctx *hctx)
{
        clear_bit(BLK_MQ_S_STOPPED, &hctx->state);

        blk_mq_run_hw_queue(hctx, false);
}
EXPORT_SYMBOL(blk_mq_start_hw_queue);

void blk_mq_start_hw_queues(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i)
                blk_mq_start_hw_queue(hctx);
}
EXPORT_SYMBOL(blk_mq_start_hw_queues);

void blk_mq_start_stopped_hw_queues(struct request_queue *q, bool async)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (!blk_mq_hctx_stopped(hctx))
                        continue;

                clear_bit(BLK_MQ_S_STOPPED, &hctx->state);
                blk_mq_run_hw_queue(hctx, async);
        }
}
EXPORT_SYMBOL(blk_mq_start_stopped_hw_queues);

static void blk_mq_run_work_fn(struct work_struct *work)
{
        struct blk_mq_hw_ctx *hctx;

        hctx = container_of(work, struct blk_mq_hw_ctx, run_work);

        __blk_mq_run_hw_queue(hctx);
}

static void blk_mq_delay_work_fn(struct work_struct *work)
{
        struct blk_mq_hw_ctx *hctx;

        hctx = container_of(work, struct blk_mq_hw_ctx, delay_work.work);

        if (test_and_clear_bit(BLK_MQ_S_STOPPED, &hctx->state))
                __blk_mq_run_hw_queue(hctx);
}

void blk_mq_delay_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
{
        if (unlikely(!blk_mq_hw_queue_mapped(hctx)))
                return;

        kblockd_schedule_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
                        &hctx->delay_work, msecs_to_jiffies(msecs));
}
EXPORT_SYMBOL(blk_mq_delay_queue);

static inline void __blk_mq_insert_req_list(struct blk_mq_hw_ctx *hctx,
                                            struct request *rq,
                                            bool at_head)
{
        struct blk_mq_ctx *ctx = rq->mq_ctx;

        trace_block_rq_insert(hctx->queue, rq);

        if (at_head)
                list_add(&rq->queuelist, &ctx->rq_list);
        else
                list_add_tail(&rq->queuelist, &ctx->rq_list);
}

static void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx,
                                    struct request *rq, bool at_head)
{
        struct blk_mq_ctx *ctx = rq->mq_ctx;

        __blk_mq_insert_req_list(hctx, rq, at_head);
        blk_mq_hctx_mark_pending(hctx, ctx);
}

void blk_mq_insert_request(struct request *rq, bool at_head, bool run_queue,
                           bool async)
{
        struct blk_mq_ctx *ctx = rq->mq_ctx;
        struct request_queue *q = rq->q;
        struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);

        spin_lock(&ctx->lock);
        __blk_mq_insert_request(hctx, rq, at_head);
        spin_unlock(&ctx->lock);

        if (run_queue)
                blk_mq_run_hw_queue(hctx, async);
}

static void blk_mq_insert_requests(struct request_queue *q,
                                     struct blk_mq_ctx *ctx,
                                     struct list_head *list,
                                     int depth,
                                     bool from_schedule)

{
        struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, ctx->cpu);

        trace_block_unplug(q, depth, !from_schedule);

        /*
         * preemption doesn't flush plug list, so it's possible ctx->cpu is
         * offline now
         */
        spin_lock(&ctx->lock);
        while (!list_empty(list)) {
                struct request *rq;

                rq = list_first_entry(list, struct request, queuelist);
                BUG_ON(rq->mq_ctx != ctx);
                list_del_init(&rq->queuelist);
                __blk_mq_insert_req_list(hctx, rq, false);
        }
        blk_mq_hctx_mark_pending(hctx, ctx);
        spin_unlock(&ctx->lock);

        blk_mq_run_hw_queue(hctx, from_schedule);
}

static int plug_ctx_cmp(void *priv, struct list_head *a, struct list_head *b)
{
        struct request *rqa = container_of(a, struct request, queuelist);
        struct request *rqb = container_of(b, struct request, queuelist);

        return !(rqa->mq_ctx < rqb->mq_ctx ||
                 (rqa->mq_ctx == rqb->mq_ctx &&
                  blk_rq_pos(rqa) < blk_rq_pos(rqb)));
}

void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
{
        struct blk_mq_ctx *this_ctx;
        struct request_queue *this_q;
        struct request *rq;
        LIST_HEAD(list);
        LIST_HEAD(ctx_list);
        unsigned int depth;

        list_splice_init(&plug->mq_list, &list);

        list_sort(NULL, &list, plug_ctx_cmp);

        this_q = NULL;
        this_ctx = NULL;
        depth = 0;

        while (!list_empty(&list)) {
                rq = list_entry_rq(list.next);
                list_del_init(&rq->queuelist);
                BUG_ON(!rq->q);
                if (rq->mq_ctx != this_ctx) {
                        if (this_ctx) {
                                blk_mq_insert_requests(this_q, this_ctx,
                                                        &ctx_list, depth,
                                                        from_schedule);
                        }

                        this_ctx = rq->mq_ctx;
                        this_q = rq->q;
                        depth = 0;
                }

                depth++;
                list_add_tail(&rq->queuelist, &ctx_list);
        }

        /*
         * If 'this_ctx' is set, we know we have entries to complete
         * on 'ctx_list'. Do those.
         */
        if (this_ctx) {
                blk_mq_insert_requests(this_q, this_ctx, &ctx_list, depth,
                                       from_schedule);
        }
}

static void blk_mq_bio_to_request(struct request *rq, struct bio *bio)
{
        init_request_from_bio(rq, bio);

        blk_account_io_start(rq, 1);
}

static inline bool hctx_allow_merges(struct blk_mq_hw_ctx *hctx)
{
        return (hctx->flags & BLK_MQ_F_SHOULD_MERGE) &&
                !blk_queue_nomerges(hctx->queue);
}

static inline bool blk_mq_merge_queue_io(struct blk_mq_hw_ctx *hctx,
                                         struct blk_mq_ctx *ctx,
                                         struct request *rq, struct bio *bio)
{
        if (!hctx_allow_merges(hctx) || !bio_mergeable(bio)) {
                blk_mq_bio_to_request(rq, bio);
                spin_lock(&ctx->lock);
insert_rq:
                __blk_mq_insert_request(hctx, rq, false);
                spin_unlock(&ctx->lock);
                return false;
        } else {
                struct request_queue *q = hctx->queue;

                spin_lock(&ctx->lock);
                if (!blk_mq_attempt_merge(q, ctx, bio)) {
                        blk_mq_bio_to_request(rq, bio);
                        goto insert_rq;
                }

                spin_unlock(&ctx->lock);
                __blk_mq_free_request(hctx, ctx, rq);
                return true;
        }
}

static struct request *blk_mq_map_request(struct request_queue *q,
                                          struct bio *bio,
                                          struct blk_mq_alloc_data *data)
{
        struct blk_mq_hw_ctx *hctx;
        struct blk_mq_ctx *ctx;
        struct request *rq;

        blk_queue_enter_live(q);
        ctx = blk_mq_get_ctx(q);
        hctx = blk_mq_map_queue(q, ctx->cpu);

        trace_block_getrq(q, bio, bio->bi_opf);
        blk_mq_set_alloc_data(data, q, 0, ctx, hctx);
        rq = __blk_mq_alloc_request(data, bio->bi_opf);

        data->hctx->queued++;
        return rq;
}

static void blk_mq_try_issue_directly(struct request *rq, blk_qc_t *cookie)
{
        int ret;
        struct request_queue *q = rq->q;
        struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(q, rq->mq_ctx->cpu);
        struct blk_mq_queue_data bd = {
                .rq = rq,
                .list = NULL,
                .last = 1
        };
        blk_qc_t new_cookie = blk_tag_to_qc_t(rq->tag, hctx->queue_num);

        if (blk_mq_hctx_stopped(hctx))
                goto insert;

        /*
         * For OK queue, we are done. For error, kill it. Any other
         * error (busy), just add it to our list as we previously
         * would have done
         */
        ret = q->mq_ops->queue_rq(hctx, &bd);
        if (ret == BLK_MQ_RQ_QUEUE_OK) {
                *cookie = new_cookie;
                return;
        }

        __blk_mq_requeue_request(rq);

        if (ret == BLK_MQ_RQ_QUEUE_ERROR) {
                *cookie = BLK_QC_T_NONE;
                rq->errors = -EIO;
                blk_mq_end_request(rq, rq->errors);
                return;
        }

insert:
        blk_mq_insert_request(rq, false, true, true);
}

/*
 * Multiple hardware queue variant. This will not use per-process plugs,
 * but will attempt to bypass the hctx queueing if we can go straight to
 * hardware for SYNC IO.
 */
static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
{
        const int is_sync = op_is_sync(bio->bi_opf);
        const int is_flush_fua = bio->bi_opf & (REQ_PREFLUSH | REQ_FUA);
        struct blk_mq_alloc_data data;
        struct request *rq;
        unsigned int request_count = 0, srcu_idx;
        struct blk_plug *plug;
        struct request *same_queue_rq = NULL;
        blk_qc_t cookie;
        unsigned int wb_acct;

        blk_queue_bounce(q, &bio);

        if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
                bio_io_error(bio);
                return BLK_QC_T_NONE;
        }

        blk_queue_split(q, &bio, q->bio_split);

        if (!is_flush_fua && !blk_queue_nomerges(q) &&
            blk_attempt_plug_merge(q, bio, &request_count, &same_queue_rq))
                return BLK_QC_T_NONE;

        wb_acct = wbt_wait(q->rq_wb, bio, NULL);

        rq = blk_mq_map_request(q, bio, &data);
        if (unlikely(!rq)) {
                __wbt_done(q->rq_wb, wb_acct);
                return BLK_QC_T_NONE;
        }

        wbt_track(&rq->issue_stat, wb_acct);

        cookie = blk_tag_to_qc_t(rq->tag, data.hctx->queue_num);

        if (unlikely(is_flush_fua)) {
                blk_mq_bio_to_request(rq, bio);
                blk_insert_flush(rq);
                goto run_queue;
        }

        plug = current->plug;
        /*
         * If the driver supports defer issued based on 'last', then
         * queue it up like normal since we can potentially save some
         * CPU this way.
         */
        if (((plug && !blk_queue_nomerges(q)) || is_sync) &&
            !(data.hctx->flags & BLK_MQ_F_DEFER_ISSUE)) {
                struct request *old_rq = NULL;

                blk_mq_bio_to_request(rq, bio);

                /*
                 * We do limited plugging. If the bio can be merged, do that.
                 * Otherwise the existing request in the plug list will be
                 * issued. So the plug list will have one request at most
                 */
                if (plug) {
                        /*
                         * The plug list might get flushed before this. If that
                         * happens, same_queue_rq is invalid and plug list is
                         * empty
                         */
                        if (same_queue_rq && !list_empty(&plug->mq_list)) {
                                old_rq = same_queue_rq;
                                list_del_init(&old_rq->queuelist);
                        }
                        list_add_tail(&rq->queuelist, &plug->mq_list);
                } else /* is_sync */
                        old_rq = rq;
                blk_mq_put_ctx(data.ctx);
                if (!old_rq)
                        goto done;

                if (!(data.hctx->flags & BLK_MQ_F_BLOCKING)) {
                        rcu_read_lock();
                        blk_mq_try_issue_directly(old_rq, &cookie);
                        rcu_read_unlock();
                } else {
                        srcu_idx = srcu_read_lock(&data.hctx->queue_rq_srcu);
                        blk_mq_try_issue_directly(old_rq, &cookie);
                        srcu_read_unlock(&data.hctx->queue_rq_srcu, srcu_idx);
                }
                goto done;
        }

        if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio)) {
                /*
                 * For a SYNC request, send it to the hardware immediately. For
                 * an ASYNC request, just ensure that we run it later on. The
                 * latter allows for merging opportunities and more efficient
                 * dispatching.
                 */
run_queue:
                blk_mq_run_hw_queue(data.hctx, !is_sync || is_flush_fua);
        }
        blk_mq_put_ctx(data.ctx);
done:
        return cookie;
}

/*
 * Single hardware queue variant. This will attempt to use any per-process
 * plug for merging and IO deferral.
 */
static blk_qc_t blk_sq_make_request(struct request_queue *q, struct bio *bio)
{
        const int is_sync = op_is_sync(bio->bi_opf);
        const int is_flush_fua = bio->bi_opf & (REQ_PREFLUSH | REQ_FUA);
        struct blk_plug *plug;
        unsigned int request_count = 0;
        struct blk_mq_alloc_data data;
        struct request *rq;
        blk_qc_t cookie;
        unsigned int wb_acct;

        blk_queue_bounce(q, &bio);

        if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
                bio_io_error(bio);
                return BLK_QC_T_NONE;
        }

        blk_queue_split(q, &bio, q->bio_split);

        if (!is_flush_fua && !blk_queue_nomerges(q)) {
                if (blk_attempt_plug_merge(q, bio, &request_count, NULL))
                        return BLK_QC_T_NONE;
        } else
                request_count = blk_plug_queued_count(q);

        wb_acct = wbt_wait(q->rq_wb, bio, NULL);

        rq = blk_mq_map_request(q, bio, &data);
        if (unlikely(!rq)) {
                __wbt_done(q->rq_wb, wb_acct);
                return BLK_QC_T_NONE;
        }

        wbt_track(&rq->issue_stat, wb_acct);

        cookie = blk_tag_to_qc_t(rq->tag, data.hctx->queue_num);

        if (unlikely(is_flush_fua)) {
                blk_mq_bio_to_request(rq, bio);
                blk_insert_flush(rq);
                goto run_queue;
        }

        /*
         * A task plug currently exists. Since this is completely lockless,
         * utilize that to temporarily store requests until the task is
         * either done or scheduled away.
         */
        plug = current->plug;
        if (plug) {
                struct request *last = NULL;

                blk_mq_bio_to_request(rq, bio);

                /*
                 * @request_count may become stale because of schedule
                 * out, so check the list again.
                 */
                if (list_empty(&plug->mq_list))
                        request_count = 0;
                if (!request_count)
                        trace_block_plug(q);
                else
                        last = list_entry_rq(plug->mq_list.prev);

                blk_mq_put_ctx(data.ctx);

                if (request_count >= BLK_MAX_REQUEST_COUNT || (last &&
                    blk_rq_bytes(last) >= BLK_PLUG_FLUSH_SIZE)) {
                        blk_flush_plug_list(plug, false);
                        trace_block_plug(q);
                }

                list_add_tail(&rq->queuelist, &plug->mq_list);
                return cookie;
        }

        if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio)) {
                /*
                 * For a SYNC request, send it to the hardware immediately. For
                 * an ASYNC request, just ensure that we run it later on. The
                 * latter allows for merging opportunities and more efficient
                 * dispatching.
                 */
run_queue:
                blk_mq_run_hw_queue(data.hctx, !is_sync || is_flush_fua);
        }

        blk_mq_put_ctx(data.ctx);
        return cookie;
}

static void blk_mq_free_rq_map(struct blk_mq_tag_set *set,
                struct blk_mq_tags *tags, unsigned int hctx_idx)
{
        struct page *page;

        if (tags->rqs && set->ops->exit_request) {
                int i;

                for (i = 0; i < tags->nr_tags; i++) {
                        if (!tags->rqs[i])
                                continue;
                        set->ops->exit_request(set->driver_data, tags->rqs[i],
                                                hctx_idx, i);
                        tags->rqs[i] = NULL;
                }
        }

        while (!list_empty(&tags->page_list)) {
                page = list_first_entry(&tags->page_list, struct page, lru);
                list_del_init(&page->lru);
                /*
                 * Remove kmemleak object previously allocated in
                 * blk_mq_init_rq_map().
                 */
                kmemleak_free(page_address(page));
                __free_pages(page, page->private);
        }

        kfree(tags->rqs);

        blk_mq_free_tags(tags);
}

static size_t order_to_size(unsigned int order)
{
        return (size_t)PAGE_SIZE << order;
}

static struct blk_mq_tags *blk_mq_init_rq_map(struct blk_mq_tag_set *set,
                unsigned int hctx_idx)
{
        struct blk_mq_tags *tags;
        unsigned int i, j, entries_per_page, max_order = 4;
        size_t rq_size, left;

        tags = blk_mq_init_tags(set->queue_depth, set->reserved_tags,
                                set->numa_node,
                                BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags));
        if (!tags)
                return NULL;

        INIT_LIST_HEAD(&tags->page_list);

        tags->rqs = kzalloc_node(set->queue_depth * sizeof(struct request *),
                                 GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY,
                                 set->numa_node);
        if (!tags->rqs) {
                blk_mq_free_tags(tags);
                return NULL;
        }

        /*
         * rq_size is the size of the request plus driver payload, rounded
         * to the cacheline size
         */
        rq_size = round_up(sizeof(struct request) + set->cmd_size,
                                cache_line_size());
        left = rq_size * set->queue_depth;

        for (i = 0; i < set->queue_depth; ) {
                int this_order = max_order;
                struct page *page;
                int to_do;
                void *p;

                while (this_order && left < order_to_size(this_order - 1))
                        this_order--;

                do {
                        page = alloc_pages_node(set->numa_node,
                                GFP_KERNEL | __GFP_NOWARN | __GFP_NORETRY | __GFP_ZERO,
                                this_order);
                        if (page)
                                break;
                        if (!this_order--)
                                break;
                        if (order_to_size(this_order) < rq_size)
                                break;
                } while (1);

                if (!page)
                        goto fail;

                page->private = this_order;
                list_add_tail(&page->lru, &tags->page_list);

                p = page_address(page);
                /*
                 * Allow kmemleak to scan these pages as they contain pointers
                 * to additional allocations like via ops->init_request().
                 */
                kmemleak_alloc(p, order_to_size(this_order), 1, GFP_KERNEL);
                entries_per_page = order_to_size(this_order) / rq_size;
                to_do = min(entries_per_page, set->queue_depth - i);
                left -= to_do * rq_size;
                for (j = 0; j < to_do; j++) {
                        tags->rqs[i] = p;
                        if (set->ops->init_request) {
                                if (set->ops->init_request(set->driver_data,
                                                tags->rqs[i], hctx_idx, i,
                                                set->numa_node)) {
                                        tags->rqs[i] = NULL;
                                        goto fail;
                                }
                        }

                        p += rq_size;
                        i++;
                }
        }
        return tags;

fail:
        blk_mq_free_rq_map(set, tags, hctx_idx);
        return NULL;
}

/*
 * 'cpu' is going away. splice any existing rq_list entries from this
 * software queue to the hw queue dispatch list, and ensure that it
 * gets run.
 */
static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
{
        struct blk_mq_hw_ctx *hctx;
        struct blk_mq_ctx *ctx;
        LIST_HEAD(tmp);

        hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
        ctx = __blk_mq_get_ctx(hctx->queue, cpu);

        spin_lock(&ctx->lock);
        if (!list_empty(&ctx->rq_list)) {
                list_splice_init(&ctx->rq_list, &tmp);
                blk_mq_hctx_clear_pending(hctx, ctx);
        }
        spin_unlock(&ctx->lock);

        if (list_empty(&tmp))
                return 0;

        spin_lock(&hctx->lock);
        list_splice_tail_init(&tmp, &hctx->dispatch);
        spin_unlock(&hctx->lock);

        blk_mq_run_hw_queue(hctx, true);
        return 0;
}

static void blk_mq_remove_cpuhp(struct blk_mq_hw_ctx *hctx)
{
        cpuhp_state_remove_instance_nocalls(CPUHP_BLK_MQ_DEAD,
                                            &hctx->cpuhp_dead);
}

/* hctx->ctxs will be freed in queue's release handler */
static void blk_mq_exit_hctx(struct request_queue *q,
                struct blk_mq_tag_set *set,
                struct blk_mq_hw_ctx *hctx, unsigned int hctx_idx)
{
        unsigned flush_start_tag = set->queue_depth;

        blk_mq_tag_idle(hctx);

        if (set->ops->exit_request)
                set->ops->exit_request(set->driver_data,
                                       hctx->fq->flush_rq, hctx_idx,
                                       flush_start_tag + hctx_idx);

        if (set->ops->exit_hctx)
                set->ops->exit_hctx(hctx, hctx_idx);

        if (hctx->flags & BLK_MQ_F_BLOCKING)
                cleanup_srcu_struct(&hctx->queue_rq_srcu);

        blk_mq_remove_cpuhp(hctx);
        blk_free_flush_queue(hctx->fq);
        sbitmap_free(&hctx->ctx_map);
}

static void blk_mq_exit_hw_queues(struct request_queue *q,
                struct blk_mq_tag_set *set, int nr_queue)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (i == nr_queue)
                        break;
                blk_mq_exit_hctx(q, set, hctx, i);
        }
}

static void blk_mq_free_hw_queues(struct request_queue *q,
                struct blk_mq_tag_set *set)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;

        queue_for_each_hw_ctx(q, hctx, i)
                free_cpumask_var(hctx->cpumask);
}

static int blk_mq_init_hctx(struct request_queue *q,
                struct blk_mq_tag_set *set,
                struct blk_mq_hw_ctx *hctx, unsigned hctx_idx)
{
        int node;
        unsigned flush_start_tag = set->queue_depth;

        node = hctx->numa_node;
        if (node == NUMA_NO_NODE)
                node = hctx->numa_node = set->numa_node;

        INIT_WORK(&hctx->run_work, blk_mq_run_work_fn);
        INIT_DELAYED_WORK(&hctx->delay_work, blk_mq_delay_work_fn);
        spin_lock_init(&hctx->lock);
        INIT_LIST_HEAD(&hctx->dispatch);
        hctx->queue = q;
        hctx->queue_num = hctx_idx;
        hctx->flags = set->flags & ~BLK_MQ_F_TAG_SHARED;

        cpuhp_state_add_instance_nocalls(CPUHP_BLK_MQ_DEAD, &hctx->cpuhp_dead);

        hctx->tags = set->tags[hctx_idx];

        /*
         * Allocate space for all possible cpus to avoid allocation at
         * runtime
         */
        hctx->ctxs = kmalloc_node(nr_cpu_ids * sizeof(void *),
                                        GFP_KERNEL, node);
        if (!hctx->ctxs)
                goto unregister_cpu_notifier;

        if (sbitmap_init_node(&hctx->ctx_map, nr_cpu_ids, ilog2(8), GFP_KERNEL,
                              node))
                goto free_ctxs;

        hctx->nr_ctx = 0;

        if (set->ops->init_hctx &&
            set->ops->init_hctx(hctx, set->driver_data, hctx_idx))
                goto free_bitmap;

        hctx->fq = blk_alloc_flush_queue(q, hctx->numa_node, set->cmd_size);
        if (!hctx->fq)
                goto exit_hctx;

        if (set->ops->init_request &&
            set->ops->init_request(set->driver_data,
                                   hctx->fq->flush_rq, hctx_idx,
                                   flush_start_tag + hctx_idx, node))
                goto free_fq;

        if (hctx->flags & BLK_MQ_F_BLOCKING)
                init_srcu_struct(&hctx->queue_rq_srcu);

        return 0;

 free_fq:
        kfree(hctx->fq);
 exit_hctx:
        if (set->ops->exit_hctx)
                set->ops->exit_hctx(hctx, hctx_idx);
 free_bitmap:
        sbitmap_free(&hctx->ctx_map);
 free_ctxs:
        kfree(hctx->ctxs);
 unregister_cpu_notifier:
        blk_mq_remove_cpuhp(hctx);
        return -1;
}

static void blk_mq_init_cpu_queues(struct request_queue *q,
                                   unsigned int nr_hw_queues)
{
        unsigned int i;

        for_each_possible_cpu(i) {
                struct blk_mq_ctx *__ctx = per_cpu_ptr(q->queue_ctx, i);
                struct blk_mq_hw_ctx *hctx;

                memset(__ctx, 0, sizeof(*__ctx));
                __ctx->cpu = i;
                spin_lock_init(&__ctx->lock);
                INIT_LIST_HEAD(&__ctx->rq_list);
                __ctx->queue = q;
                blk_stat_init(&__ctx->stat[BLK_STAT_READ]);
                blk_stat_init(&__ctx->stat[BLK_STAT_WRITE]);

                /* If the cpu isn't online, the cpu is mapped to first hctx */
                if (!cpu_online(i))
                        continue;

                hctx = blk_mq_map_queue(q, i);

                /*
                 * Set local node, IFF we have more than one hw queue. If
                 * not, we remain on the home node of the device
                 */
                if (nr_hw_queues > 1 && hctx->numa_node == NUMA_NO_NODE)
                        hctx->numa_node = local_memory_node(cpu_to_node(i));
        }
}

static void blk_mq_map_swqueue(struct request_queue *q,
                               const struct cpumask *online_mask)
{
        unsigned int i;
        struct blk_mq_hw_ctx *hctx;
        struct blk_mq_ctx *ctx;
        struct blk_mq_tag_set *set = q->tag_set;

        /*
         * Avoid others reading imcomplete hctx->cpumask through sysfs
         */
        mutex_lock(&q->sysfs_lock);

        queue_for_each_hw_ctx(q, hctx, i) {
                cpumask_clear(hctx->cpumask);
                hctx->nr_ctx = 0;
        }

        /*
         * Map software to hardware queues
         */
        for_each_possible_cpu(i) {
                /* If the cpu isn't online, the cpu is mapped to first hctx */
                if (!cpumask_test_cpu(i, online_mask))
                        continue;

                ctx = per_cpu_ptr(q->queue_ctx, i);
                hctx = blk_mq_map_queue(q, i);

                cpumask_set_cpu(i, hctx->cpumask);
                ctx->index_hw = hctx->nr_ctx;
                hctx->ctxs[hctx->nr_ctx++] = ctx;
        }

        mutex_unlock(&q->sysfs_lock);

        queue_for_each_hw_ctx(q, hctx, i) {
                /*
                 * If no software queues are mapped to this hardware queue,
                 * disable it and free the request entries.
                 */
                if (!hctx->nr_ctx) {
                        if (set->tags[i]) {
                                blk_mq_free_rq_map(set, set->tags[i], i);
                                set->tags[i] = NULL;
                        }
                        hctx->tags = NULL;
                        continue;
                }

                /* unmapped hw queue can be remapped after CPU topo changed */
                if (!set->tags[i])
                        set->tags[i] = blk_mq_init_rq_map(set, i);
                hctx->tags = set->tags[i];
                WARN_ON(!hctx->tags);

                /*
                 * Set the map size to the number of mapped software queues.
                 * This is more accurate and more efficient than looping
                 * over all possibly mapped software queues.
                 */
                sbitmap_resize(&hctx->ctx_map, hctx->nr_ctx);

                /*
                 * Initialize batch roundrobin counts
                 */
                hctx->next_cpu = cpumask_first(hctx->cpumask);
                hctx->next_cpu_batch = BLK_MQ_CPU_WORK_BATCH;
        }
}

static void queue_set_hctx_shared(struct request_queue *q, bool shared)
{
        struct blk_mq_hw_ctx *hctx;
        int i;

        queue_for_each_hw_ctx(q, hctx, i) {
                if (shared)
                        hctx->flags |= BLK_MQ_F_TAG_SHARED;
                else
                        hctx->flags &= ~BLK_MQ_F_TAG_SHARED;
        }
}

static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set, bool shared)
{
        struct request_queue *q;

        list_for_each_entry(q, &set->tag_list, tag_set_list) {
                blk_mq_freeze_queue(q);
                queue_set_hctx_shared(q, shared);
                blk_mq_unfreeze_queue(q);
        }
}

static void blk_mq_del_queue_tag_set(struct request_queue *q)
{
        struct blk_mq_tag_set *set = q->tag_set;

        mutex_lock(&set->tag_list_lock);
        list_del_init(&q->tag_set_list);
        if (list_is_singular(&set->tag_list)) {
                /* just transitioned to unshared */
                set->flags &= ~BLK_MQ_F_TAG_SHARED;
                /* update existing queue */
                blk_mq_update_tag_set_depth(set, false);
        }
        mutex_unlock(&set->tag_list_lock);
}

static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
                                     struct request_queue *q)
{
        q->tag_set = set;

        mutex_lock(&set->tag_list_lock);

        /* Check to see if we're transitioning to shared (from 1 to 2 queues). */
        if (!list_empty(&set->tag_list) && !(set->flags & BLK_MQ_F_TAG_SHARED)) {
                set->flags |= BLK_MQ_F_TAG_SHARED;
                /* update existing queue */
                blk_mq_update_tag_set_depth(set, true);
        }
        if (set->flags & BLK_MQ_F_TAG_SHARED)
                queue_set_hctx_shared(q, true);
        list_add_tail(&q->tag_set_list, &set->tag_list);

        mutex_unlock(&set->tag_list_lock);
}

/*
 * It is the actual release handler for mq, but we do it from
 * request queue's release handler for avoiding use-after-free
 * and headache because q->mq_kobj shouldn't have been introduced,
 * but we can't group ctx/kctx kobj without it.
 */
void blk_mq_release(struct request_queue *q)
{
        struct blk_mq_hw_ctx *hctx;
        unsigned int i;

        /* hctx kobj stays in hctx */
        queue_for_each_hw_ctx(q, hctx, i) {
                if (!hctx)
                        continue;
                kfree(hctx->ctxs);
                kfree(hctx);
        }

        q->mq_map = NULL;

        kfree(q->queue_hw_ctx);

        /* ctx kobj stays in queue_ctx */
        free_percpu(q->queue_ctx);
}

struct request_queue *blk_mq_init_queue(struct blk_mq_tag_set *set)
{
        struct request_queue *uninit_q, *q;

        uninit_q = blk_alloc_queue_node(GFP_KERNEL, set->numa_node);
        if (!uninit_q)
                return ERR_PTR(-ENOMEM);

        q = blk_mq_init_allocated_queue(set, uninit_q);
        if (IS_ERR(q))
                blk_cleanup_queue(uninit_q);

        return q;
}
EXPORT_SYMBOL(blk_mq_init_queue);

static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
                                                struct request_queue *q)
{
        int i, j;
        struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx;

        blk_mq_sysfs_unregister(q);
        for (i = 0; i < set->nr_hw_queues; i++) {
                int node;

                if (hctxs[i])
                        continue;

                node = blk_mq_hw_queue_to_node(q->mq_map, i);
                hctxs[i] = kzalloc_node(sizeof(struct blk_mq_hw_ctx),
                                        GFP_KERNEL, node);
                if (!hctxs[i])
                        break;

                if (!zalloc_cpumask_var_node(&hctxs[i]->cpumask, GFP_KERNEL,
                                                node)) {
                        kfree(hctxs[i]);
                        hctxs[i] = NULL;
                        break;
                }

                atomic_set(&hctxs[i]->nr_active, 0);
                hctxs[i]->numa_node = node;
                hctxs[i]->queue_num = i;

                if (blk_mq_init_hctx(q, set, hctxs[i], i)) {
                        free_cpumask_var(hctxs[i]->cpumask);
                        kfree(hctxs[i]);
                        hctxs[i] = NULL;
                        break;
                }
                blk_mq_hctx_kobj_init(hctxs[i]);
        }
        for (j = i; j < q->nr_hw_queues; j++) {
                struct blk_mq_hw_ctx *hctx = hctxs[j];

                if (hctx) {
                        if (hctx->tags) {
                                blk_mq_free_rq_map(set, hctx->tags, j);
                                set->tags[j] = NULL;
                        }
                        blk_mq_exit_hctx(q, set, hctx, j);
                        free_cpumask_var(hctx->cpumask);
                        kobject_put(&hctx->kobj);
                        kfree(hctx->ctxs);
                        kfree(hctx);
                        hctxs[j] = NULL;

                }
        }
        q->nr_hw_queues = i;
        blk_mq_sysfs_register(q);
}

struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
                                                  struct request_queue *q)
{
        /* mark the queue as mq asap */
        q->mq_ops = set->ops;

        q->queue_ctx = alloc_percpu(struct blk_mq_ctx);
        if (!q->queue_ctx)
                goto err_exit;

        q->queue_hw_ctx = kzalloc_node(nr_cpu_ids * sizeof(*(q->queue_hw_ctx)),
                                                GFP_KERNEL, set->numa_node);
        if (!q->queue_hw_ctx)
                goto err_percpu;

        q->mq_map = set->mq_map;

        blk_mq_realloc_hw_ctxs(set, q);
        if (!q->nr_hw_queues)
                goto err_hctxs;

        INIT_WORK(&q->timeout_work, blk_mq_timeout_work);
        blk_queue_rq_timeout(q, set->timeout ? set->timeout : 30 * HZ);

        q->nr_queues = nr_cpu_ids;

        q->queue_flags |= QUEUE_FLAG_MQ_DEFAULT;

        if (!(set->flags & BLK_MQ_F_SG_MERGE))
                q->queue_flags |= 1 << QUEUE_FLAG_NO_SG_MERGE;

        q->sg_reserved_size = INT_MAX;

        INIT_DELAYED_WORK(&q->requeue_work, blk_mq_requeue_work);
        INIT_LIST_HEAD(&q->requeue_list);
        spin_lock_init(&q->requeue_lock);

        if (q->nr_hw_queues > 1)
                blk_queue_make_request(q, blk_mq_make_request);
        else
                blk_queue_make_request(q, blk_sq_make_request);

        /*
         * Do this after blk_queue_make_request() overrides it...
         */
        q->nr_requests = set->queue_depth;

        /*
         * Default to classic polling
         */
        q->poll_nsec = -1;

        if (set->ops->complete)
                blk_queue_softirq_done(q, set->ops->complete);

        blk_mq_init_cpu_queues(q, set->nr_hw_queues);

        get_online_cpus();
        mutex_lock(&all_q_mutex);

        list_add_tail(&q->all_q_node, &all_q_list);
        blk_mq_add_queue_tag_set(set, q);
        blk_mq_map_swqueue(q, cpu_online_mask);

        mutex_unlock(&all_q_mutex);
        put_online_cpus();

        return q;

err_hctxs:
        kfree(q->queue_hw_ctx);
err_percpu:
        free_percpu(q->queue_ctx);
err_exit:
        q->mq_ops = NULL;
        return ERR_PTR(-ENOMEM);
}
EXPORT_SYMBOL(blk_mq_init_allocated_queue);

void blk_mq_free_queue(struct request_queue *q)
{
        struct blk_mq_tag_set   *set = q->tag_set;

        mutex_lock(&all_q_mutex);
        list_del_init(&q->all_q_node);
        mutex_unlock(&all_q_mutex);

        wbt_exit(q);

        blk_mq_del_queue_tag_set(q);

        blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
        blk_mq_free_hw_queues(q, set);
}

/* Basically redo blk_mq_init_queue with queue frozen */
static void blk_mq_queue_reinit(struct request_queue *q,
                                const struct cpumask *online_mask)
{
        WARN_ON_ONCE(!atomic_read(&q->mq_freeze_depth));

        blk_mq_sysfs_unregister(q);

        /*
         * redo blk_mq_init_cpu_queues and blk_mq_init_hw_queues. FIXME: maybe
         * we should change hctx numa_node according to new topology (this
         * involves free and re-allocate memory, worthy doing?)
         */

        blk_mq_map_swqueue(q, online_mask);

        blk_mq_sysfs_register(q);
}

/*
 * New online cpumask which is going to be set in this hotplug event.
 * Declare this cpumasks as global as cpu-hotplug operation is invoked
 * one-by-one and dynamically allocating this could result in a failure.
 */
static struct cpumask cpuhp_online_new;

static void blk_mq_queue_reinit_work(void)
{
        struct request_queue *q;

        mutex_lock(&all_q_mutex);
        /*
         * We need to freeze and reinit all existing queues.  Freezing
         * involves synchronous wait for an RCU grace period and doing it
         * one by one may take a long time.  Start freezing all queues in
         * one swoop and then wait for the completions so that freezing can
         * take place in parallel.
         */
        list_for_each_entry(q, &all_q_list, all_q_node)
                blk_mq_freeze_queue_start(q);
        list_for_each_entry(q, &all_q_list, all_q_node)
                blk_mq_freeze_queue_wait(q);

        list_for_each_entry(q, &all_q_list, all_q_node)
                blk_mq_queue_reinit(q, &cpuhp_online_new);

        list_for_each_entry(q, &all_q_list, all_q_node)
                blk_mq_unfreeze_queue(q);

        mutex_unlock(&all_q_mutex);
}

static int blk_mq_queue_reinit_dead(unsigned int cpu)
{
        cpumask_copy(&cpuhp_online_new, cpu_online_mask);
        blk_mq_queue_reinit_work();
        return 0;
}

/*
 * Before hotadded cpu starts handling requests, new mappings must be
 * established.  Otherwise, these requests in hw queue might never be
 * dispatched.
 *
 * For example, there is a single hw queue (hctx) and two CPU queues (ctx0
 * for CPU0, and ctx1 for CPU1).
 *
 * Now CPU1 is just onlined and a request is inserted into ctx1->rq_list
 * and set bit0 in pending bitmap as ctx1->index_hw is still zero.
 *
 * And then while running hw queue, flush_busy_ctxs() finds bit0 is set in
 * pending bitmap and tries to retrieve requests in hctx->ctxs[0]->rq_list.
 * But htx->ctxs[0] is a pointer to ctx0, so the request in ctx1->rq_list
 * is ignored.
 */
static int blk_mq_queue_reinit_prepare(unsigned int cpu)
{
        cpumask_copy(&cpuhp_online_new, cpu_online_mask);
        cpumask_set_cpu(cpu, &cpuhp_online_new);
        blk_mq_queue_reinit_work();
        return 0;
}

static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
{
        int i;

        for (i = 0; i < set->nr_hw_queues; i++) {
                set->tags[i] = blk_mq_init_rq_map(set, i);
                if (!set->tags[i])
                        goto out_unwind;
        }

        return 0;

out_unwind:
        while (--i >= 0)
                blk_mq_free_rq_map(set, set->tags[i], i);

        return -ENOMEM;
}

/*
 * Allocate the request maps associated with this tag_set. Note that this
 * may reduce the depth asked for, if memory is tight. set->queue_depth
 * will be updated to reflect the allocated depth.
 */
static int blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)
{
        unsigned int depth;
        int err;

        depth = set->queue_depth;
        do {
                err = __blk_mq_alloc_rq_maps(set);
                if (!err)
                        break;

                set->queue_depth >>= 1;
                if (set->queue_depth < set->reserved_tags + BLK_MQ_TAG_MIN) {
                        err = -ENOMEM;
                        break;
                }
        } while (set->queue_depth);

        if (!set->queue_depth || err) {
                pr_err("blk-mq: failed to allocate request map\n");
                return -ENOMEM;
        }

        if (depth != set->queue_depth)
                pr_info("blk-mq: reduced tag depth (%u -> %u)\n",
                                                depth, set->queue_depth);

        return 0;
}

/*
 * Alloc a tag set to be associated with one or more request queues.
 * May fail with EINVAL for various error conditions. May adjust the
 * requested depth down, if if it too large. In that case, the set
 * value will be stored in set->queue_depth.
 */
int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
{
        int ret;

        BUILD_BUG_ON(BLK_MQ_MAX_DEPTH > 1 << BLK_MQ_UNIQUE_TAG_BITS);

        if (!set->nr_hw_queues)
                return -EINVAL;
        if (!set->queue_depth)
                return -EINVAL;
        if (set->queue_depth < set->reserved_tags + BLK_MQ_TAG_MIN)
                return -EINVAL;

        if (!set->ops->queue_rq)
                return -EINVAL;

        if (set->queue_depth > BLK_MQ_MAX_DEPTH) {
                pr_info("blk-mq: reduced tag depth to %u\n",
                        BLK_MQ_MAX_DEPTH);
                set->queue_depth = BLK_MQ_MAX_DEPTH;
        }

        /*
         * If a crashdump is active, then we are potentially in a very
         * memory constrained environment. Limit us to 1 queue and
         * 64 tags to prevent using too much memory.
         */
        if (is_kdump_kernel()) {
                set->nr_hw_queues = 1;
                set->queue_depth = min(64U, set->queue_depth);
        }
        /*
         * There is no use for more h/w queues than cpus.
         */
        if (set->nr_hw_queues > nr_cpu_ids)
                set->nr_hw_queues = nr_cpu_ids;

        set->tags = kzalloc_node(nr_cpu_ids * sizeof(struct blk_mq_tags *),
                                 GFP_KERNEL, set->numa_node);
        if (!set->tags)
                return -ENOMEM;

        ret = -ENOMEM;
        set->mq_map = kzalloc_node(sizeof(*set->mq_map) * nr_cpu_ids,
                        GFP_KERNEL, set->numa_node);
        if (!set->mq_map)
                goto out_free_tags;

        if (set->ops->map_queues)
                ret = set->ops->map_queues(set);
        else
                ret = blk_mq_map_queues(set);
        if (ret)
                goto out_free_mq_map;

        ret = blk_mq_alloc_rq_maps(set);
        if (ret)
                goto out_free_mq_map;

        mutex_init(&set->tag_list_lock);
        INIT_LIST_HEAD(&set->tag_list);

        return 0;

out_free_mq_map:
        kfree(set->mq_map);
        set->mq_map = NULL;
out_free_tags:
        kfree(set->tags);
        set->tags = NULL;
        return ret;
}
EXPORT_SYMBOL(blk_mq_alloc_tag_set);

void blk_mq_free_tag_set(struct blk_mq_tag_set *set)
{
        int i;

        for (i = 0; i < nr_cpu_ids; i++) {
                if (set->tags[i])
                        blk_mq_free_rq_map(set, set->tags[i], i);
        }

        kfree(set->mq_map);
        set->mq_map = NULL;

        kfree(set->tags);
        set->tags = NULL;
}
EXPORT_SYMBOL(blk_mq_free_tag_set);

int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
{
        struct blk_mq_tag_set *set = q->tag_set;
        struct blk_mq_hw_ctx *hctx;
        int i, ret;

        if (!set || nr > set->queue_depth)
                return -EINVAL;

        ret = 0;
        queue_for_each_hw_ctx(q, hctx, i) {
                if (!hctx->tags)
                        continue;
                ret = blk_mq_tag_update_depth(hctx->tags, nr);
                if (ret)
                        break;
        }

        if (!ret)
                q->nr_requests = nr;

        return ret;
}

void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
{
        struct request_queue *q;

        if (nr_hw_queues > nr_cpu_ids)
                nr_hw_queues = nr_cpu_ids;
        if (nr_hw_queues < 1 || nr_hw_queues == set->nr_hw_queues)
                return;

        list_for_each_entry(q, &set->tag_list, tag_set_list)
                blk_mq_freeze_queue(q);

        set->nr_hw_queues = nr_hw_queues;
        list_for_each_entry(q, &set->tag_list, tag_set_list) {
                blk_mq_realloc_hw_ctxs(set, q);

                if (q->nr_hw_queues > 1)
                        blk_queue_make_request(q, blk_mq_make_request);
                else
                        blk_queue_make_request(q, blk_sq_make_request);

                blk_mq_queue_reinit(q, cpu_online_mask);
        }

        list_for_each_entry(q, &set->tag_list, tag_set_list)
                blk_mq_unfreeze_queue(q);
}
EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);

static unsigned long blk_mq_poll_nsecs(struct request_queue *q,
                                       struct blk_mq_hw_ctx *hctx,
                                       struct request *rq)
{
        struct blk_rq_stat stat[2];
        unsigned long ret = 0;

        /*
         * If stats collection isn't on, don't sleep but turn it on for
         * future users
         */
        if (!blk_stat_enable(q))
                return 0;

        /*
         * We don't have to do this once per IO, should optimize this
         * to just use the current window of stats until it changes
         */
        memset(&stat, 0, sizeof(stat));
        blk_hctx_stat_get(hctx, stat);

        /*
         * As an optimistic guess, use half of the mean service time
         * for this type of request. We can (and should) make this smarter.
         * For instance, if the completion latencies are tight, we can
         * get closer than just half the mean. This is especially
         * important on devices where the completion latencies are longer
         * than ~10 usec.
         */
        if (req_op(rq) == REQ_OP_READ && stat[BLK_STAT_READ].nr_samples)
                ret = (stat[BLK_STAT_READ].mean + 1) / 2;
        else if (req_op(rq) == REQ_OP_WRITE && stat[BLK_STAT_WRITE].nr_samples)
                ret = (stat[BLK_STAT_WRITE].mean + 1) / 2;

        return ret;
}

static bool blk_mq_poll_hybrid_sleep(struct request_queue *q,
                                     struct blk_mq_hw_ctx *hctx,
                                     struct request *rq)
{
        struct hrtimer_sleeper hs;
        enum hrtimer_mode mode;
        unsigned int nsecs;
        ktime_t kt;

        if (test_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags))
                return false;

        /*
         * poll_nsec can be:
         *
         * -1:  don't ever hybrid sleep
         *  0:  use half of prev avg
         * >0:  use this specific value
         */
        if (q->poll_nsec == -1)
                return false;
        else if (q->poll_nsec > 0)
                nsecs = q->poll_nsec;
        else
                nsecs = blk_mq_poll_nsecs(q, hctx, rq);

        if (!nsecs)
                return false;

        set_bit(REQ_ATOM_POLL_SLEPT, &rq->atomic_flags);

        /*
         * This will be replaced with the stats tracking code, using
         * 'avg_completion_time / 2' as the pre-sleep target.
         */
        kt = ktime_set(0, nsecs);

        mode = HRTIMER_MODE_REL;
        hrtimer_init_on_stack(&hs.timer, CLOCK_MONOTONIC, mode);
        hrtimer_set_expires(&hs.timer, kt);

        hrtimer_init_sleeper(&hs, current);
        do {
                if (test_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags))
                        break;
                set_current_state(TASK_UNINTERRUPTIBLE);
                hrtimer_start_expires(&hs.timer, mode);
                if (hs.task)
                        io_schedule();
                hrtimer_cancel(&hs.timer);
                mode = HRTIMER_MODE_ABS;
        } while (hs.task && !signal_pending(current));

        __set_current_state(TASK_RUNNING);
        destroy_hrtimer_on_stack(&hs.timer);
        return true;
}

static bool __blk_mq_poll(struct blk_mq_hw_ctx *hctx, struct request *rq)
{
        struct request_queue *q = hctx->queue;
        long state;

        /*
         * If we sleep, have the caller restart the poll loop to reset
         * the state. Like for the other success return cases, the
         * caller is responsible for checking if the IO completed. If
         * the IO isn't complete, we'll get called again and will go
         * straight to the busy poll loop.
         */
        if (blk_mq_poll_hybrid_sleep(q, hctx, rq))
                return true;

        hctx->poll_considered++;

        state = current->state;
        while (!need_resched()) {
                int ret;

                hctx->poll_invoked++;

                ret = q->mq_ops->poll(hctx, rq->tag);
                if (ret > 0) {
                        hctx->poll_success++;
                        set_current_state(TASK_RUNNING);
                        return true;
                }

                if (signal_pending_state(state, current))
                        set_current_state(TASK_RUNNING);

                if (current->state == TASK_RUNNING)
                        return true;
                if (ret < 0)
                        break;
                cpu_relax();
        }

        return false;
}

bool blk_mq_poll(struct request_queue *q, blk_qc_t cookie)
{
        struct blk_mq_hw_ctx *hctx;
        struct blk_plug *plug;
        struct request *rq;

        if (!q->mq_ops || !q->mq_ops->poll || !blk_qc_t_valid(cookie) ||
            !test_bit(QUEUE_FLAG_POLL, &q->queue_flags))
                return false;

        plug = current->plug;
        if (plug)
                blk_flush_plug_list(plug, false);

        hctx = q->queue_hw_ctx[blk_qc_t_to_queue_num(cookie)];
        rq = blk_mq_tag_to_rq(hctx->tags, blk_qc_t_to_tag(cookie));

        return __blk_mq_poll(hctx, rq);
}
EXPORT_SYMBOL_GPL(blk_mq_poll);

void blk_mq_disable_hotplug(void)
{
        mutex_lock(&all_q_mutex);
}

void blk_mq_enable_hotplug(void)
{
        mutex_unlock(&all_q_mutex);
}

static int __init blk_mq_init(void)
{
        cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL,
                                blk_mq_hctx_notify_dead);

        cpuhp_setup_state_nocalls(CPUHP_BLK_MQ_PREPARE, "block/mq:prepare",
                                  blk_mq_queue_reinit_prepare,
                                  blk_mq_queue_reinit_dead);
        return 0;
}
subsys_initcall(blk_mq_init);