kvm tools, qcow: Delayed L2 table writeout

This patch delays writeout for new L2 tables like we do for L1 tables. If a L2
table has non-allocated clusters, we mark that in the in-memory L2 table but
don't actually write it to disk until the L2 table is thrown out of LRU cache
or when qcow_disk_flush() is called. That makes writes to new clusters volatile
before VIRTIO_BLK_T_FLUSH is issued without corrupting the QCOW image on I/O
error.

Cc: Asias He <asias.hejun@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>

diff --git a/tools/kvm/disk/qcow.c b/tools/kvm/disk/qcow.c
index c851e7f382700d4d079db2a07a481854719dae47..b71762f27d3ee976568c871c1af14ab577391339 100644 (file)
--- a/tools/kvm/disk/qcow.c
+++ b/tools/kvm/disk/qcow.c
@@ -88,6 +88,16 @@ static void free_cache(struct qcow *q)
        }
 }
 
+static int qcow_l2_cache_write(struct qcow *q, struct qcow_l2_table *c)
+{
+       struct qcow_header *header = q->header;
+       u64 size;
+
+       size = 1 << header->l2_bits;
+
+       return pwrite_in_full(q->fd, c->table, size * sizeof(u64), c->offset);
+}
+
 static int cache_table(struct qcow *q, struct qcow_l2_table *c)
 {
        struct rb_root *r = &q->root;
@@ -100,6 +110,9 @@ static int cache_table(struct qcow *q, struct qcow_l2_table *c)
                 */
                lru = list_first_entry(&q->lru_list, struct qcow_l2_table, list);
 
+               if (qcow_l2_cache_write(q, lru) < 0)
+                       goto error;
+
                /* Remove the node from the cache */
                rb_erase(&lru->node, r);
                list_del_init(&lru->list);
@@ -361,7 +374,6 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
        struct qcow_header *header = q->header;
        struct qcow_table  *table  = &q->table;
        struct qcow_l2_table *l2t;
-       bool update_meta;
        u64 clust_start;
        u64 clust_off;
        u64 clust_sz;
@@ -371,7 +383,6 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
        u64 l2t_sz;
        u64 f_sz;
        u64 len;
-       u64 t;
 
        l2t             = NULL;
        l2t_sz          = 1 << header->l2_bits;
@@ -434,31 +445,16 @@ static ssize_t qcow_write_cluster(struct qcow *q, u64 offset, void *buf, u32 src
 
        clust_start     = be64_to_cpu(l2t->table[l2t_idx]) & ~header->oflag_mask;
        if (!clust_start) {
-               clust_start     = ALIGN(f_sz, clust_sz);
-               update_meta     = true;
-       } else
-               update_meta     = false;
-
-       /* Write actual data */
-       if (pwrite_in_full(q->fd, buf, len, clust_start + clust_off) < 0)
-               goto error;
-
-       if (update_meta) {
-               t = cpu_to_be64(clust_start);
-               if (qcow_pwrite_sync(q->fd, &t, sizeof(t), l2t_off + l2t_idx * sizeof(u64)) < 0) {
-                       /* Restore the file to consistent state */
-                       if (ftruncate(q->fd, f_sz) < 0)
-                               goto error;
-
-                       goto error;
-               }
-
-               /* Update the cached level2 entry */
-               l2t->table[l2t_idx] = cpu_to_be64(clust_start);
+               clust_start             = ALIGN(f_sz, clust_sz);
+               l2t->table[l2t_idx]     = cpu_to_be64(clust_start);
        }
 
        mutex_unlock(&q->mutex);
 
+       /* Write actual data */
+       if (pwrite_in_full(q->fd, buf, len, clust_start + clust_off) < 0)
+               return -1;
+
        return len;
 
 free_cache:
@@ -508,18 +504,34 @@ static int qcow_disk_flush(struct disk_image *disk)
 {
        struct qcow *q = disk->priv;
        struct qcow_header *header;
+       struct list_head *pos, *n;
        struct qcow_table *table;
 
-       if (fdatasync(disk->fd) < 0)
-               return -1;
-
        header  = q->header;
        table   = &q->table;
 
+       mutex_lock(&q->mutex);
+
+       list_for_each_safe(pos, n, &q->lru_list) {
+               struct qcow_l2_table *c = list_entry(pos, struct qcow_l2_table, list);
+
+               if (qcow_l2_cache_write(q, c) < 0)
+                       goto error_unlock;
+       }
+
+       if (fdatasync(disk->fd) < 0)
+               goto error_unlock;
+
        if (pwrite_in_full(disk->fd, table->l1_table, table->table_size * sizeof(u64), header->l1_table_offset) < 0)
-               return -1;
+               goto error_unlock;
+
+       mutex_unlock(&q->mutex);
 
        return fsync(disk->fd);
+
+error_unlock:
+       mutex_unlock(&q->mutex);
+       return -1;
 }
 
 static int qcow_disk_close(struct disk_image *disk)