mm: numa: Cleanup flow of transhuge page migration

When correcting commit 04fa5d6a ("mm: migrate: check page_count of THP
before migrating") Hugh Dickins noted that the control flow for transhuge
migration was difficult to follow.  Unconditionally calling put_page() in
numamigrate_isolate_page() made the failure paths of both
migrate_misplaced_transhuge_page() and migrate_misplaced_page() more
complex that they should be.  Further, he was extremely wary that an
unlock_page() should ever happen after a put_page() even if the put_page()
should never be the final put_page.

Hugh implemented the following cleanup to simplify the path by calling
putback_lru_page() inside numamigrate_isolate_page() if it failed to
isolate and always calling unlock_page() within
migrate_misplaced_transhuge_page().  There is no functional change after
this patch is applied but the code is easier to follow and unlock_page()
always happens before put_page().

[mgorman@suse.de: changelog only]
Signed-off-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Simon Jeons <simon.jeons@gmail.com>
Cc: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9db521fa8e8a4d84b3fd4e2fabc9ed5215ffd180..f40b2ce23d60a047f518743483b3f08c60ec4fee 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1296,7 +1296,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
        int target_nid;
        int current_nid = -1;
        bool migrated;
-       bool page_locked = false;
 
        spin_lock(&mm->page_table_lock);
        if (unlikely(!pmd_same(pmd, *pmdp)))
@@ -1318,7 +1317,6 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
        /* Acquire the page lock to serialise THP migrations */
        spin_unlock(&mm->page_table_lock);
        lock_page(page);
-       page_locked = true;
 
        /* Confirm the PTE did not while locked */
        spin_lock(&mm->page_table_lock);
@@ -1331,34 +1329,26 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
 
        /* Migrate the THP to the requested node */
        migrated = migrate_misplaced_transhuge_page(mm, vma,
-                               pmdp, pmd, addr,
-                               page, target_nid);
-       if (migrated)
-               current_nid = target_nid;
-       else {
-               spin_lock(&mm->page_table_lock);
-               if (unlikely(!pmd_same(pmd, *pmdp))) {
-                       unlock_page(page);
-                       goto out_unlock;
-               }
-               goto clear_pmdnuma;
-       }
+                               pmdp, pmd, addr, page, target_nid);
+       if (!migrated)
+               goto check_same;
 
-       task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);
+       task_numa_fault(target_nid, HPAGE_PMD_NR, true);
        return 0;
 
+check_same:
+       spin_lock(&mm->page_table_lock);
+       if (unlikely(!pmd_same(pmd, *pmdp)))
+               goto out_unlock;
 clear_pmdnuma:
        pmd = pmd_mknonnuma(pmd);
        set_pmd_at(mm, haddr, pmdp, pmd);
        VM_BUG_ON(pmd_numa(*pmdp));
        update_mmu_cache_pmd(vma, addr, pmdp);
-       if (page_locked)
-               unlock_page(page);
-
 out_unlock:
        spin_unlock(&mm->page_table_lock);
        if (current_nid != -1)
-               task_numa_fault(current_nid, HPAGE_PMD_NR, migrated);
+               task_numa_fault(current_nid, HPAGE_PMD_NR, false);
        return 0;
 }
 

diff --git a/mm/migrate.c b/mm/migrate.c
index 77f4e70df24d0e9eb3c07a67724630e58eaeac9b..f560071e89c50e6c9caeae7e4f0bec63a0de705c 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1557,41 +1557,40 @@ bool numamigrate_update_ratelimit(pg_data_t *pgdat, unsigned long nr_pages)
 
 int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 {
-       int ret = 0;
+       int page_lru;
 
        VM_BUG_ON(compound_order(page) && !PageTransHuge(page));
 
        /* Avoid migrating to a node that is nearly full */
-       if (migrate_balanced_pgdat(pgdat, 1UL << compound_order(page))) {
-               int page_lru;
+       if (!migrate_balanced_pgdat(pgdat, 1UL << compound_order(page)))
+               return 0;
 
-               if (isolate_lru_page(page)) {
-                       put_page(page);
-                       return 0;
-               }
+       if (isolate_lru_page(page))
+               return 0;
 
-               /* Page is isolated */
-               ret = 1;
-               page_lru = page_is_file_cache(page);
-               if (!PageTransHuge(page))
-                       inc_zone_page_state(page, NR_ISOLATED_ANON + page_lru);
-               else
-                       mod_zone_page_state(page_zone(page),
-                                       NR_ISOLATED_ANON + page_lru,
-                                       HPAGE_PMD_NR);
+       /*
+        * migrate_misplaced_transhuge_page() skips page migration's usual
+        * check on page_count(), so we must do it here, now that the page
+        * has been isolated: a GUP pin, or any other pin, prevents migration.
+        * The expected page count is 3: 1 for page's mapcount and 1 for the
+        * caller's pin and 1 for the reference taken by isolate_lru_page().
+        */
+       if (PageTransHuge(page) && page_count(page) != 3) {
+               putback_lru_page(page);
+               return 0;
        }
 
+       page_lru = page_is_file_cache(page);
+       mod_zone_page_state(page_zone(page), NR_ISOLATED_ANON + page_lru,
+                               hpage_nr_pages(page));
+
        /*
-        * Page is either isolated or there is not enough space on the target
-        * node. If isolated, then it has taken a reference count and the
-        * callers reference can be safely dropped without the page
-        * disappearing underneath us during migration. Otherwise the page is
-        * not to be migrated but the callers reference should still be
-        * dropped so it does not leak.
+        * Isolating the page has taken another reference, so the
+        * caller's reference can be safely dropped without the page
+        * disappearing underneath us during migration.
         */
        put_page(page);
-
-       return ret;
+       return 1;
 }
 
 /*
@@ -1602,7 +1601,7 @@ int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)
 int migrate_misplaced_page(struct page *page, int node)
 {
        pg_data_t *pgdat = NODE_DATA(node);
-       int isolated = 0;
+       int isolated;
        int nr_remaining;
        LIST_HEAD(migratepages);
 
@@ -1610,20 +1609,16 @@ int migrate_misplaced_page(struct page *page, int node)
         * Don't migrate pages that are mapped in multiple processes.
         * TODO: Handle false sharing detection instead of this hammer
         */
-       if (page_mapcount(page) != 1) {
-               put_page(page);
+       if (page_mapcount(page) != 1)
                goto out;
-       }
 
        /*
         * Rate-limit the amount of data that is being migrated to a node.
         * Optimal placement is no good if the memory bus is saturated and
         * all the time is being spent migrating!
         */
-       if (numamigrate_update_ratelimit(pgdat, 1)) {
-               put_page(page);
+       if (numamigrate_update_ratelimit(pgdat, 1))
                goto out;
-       }
 
        isolated = numamigrate_isolate_page(pgdat, page);
        if (!isolated)
@@ -1640,12 +1635,19 @@ int migrate_misplaced_page(struct page *page, int node)
        } else
                count_vm_numa_event(NUMA_PAGE_MIGRATE);
        BUG_ON(!list_empty(&migratepages));
-out:
        return isolated;
+
+out:
+       put_page(page);
+       return 0;
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
 #if defined(CONFIG_NUMA_BALANCING) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
+/*
+ * Migrates a THP to a given target node. page must be locked and is unlocked
+ * before returning.
+ */
 int migrate_misplaced_transhuge_page(struct mm_struct *mm,
                                struct vm_area_struct *vma,
                                pmd_t *pmd, pmd_t entry,
@@ -1676,29 +1678,15 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
 
        new_page = alloc_pages_node(node,
                (GFP_TRANSHUGE | GFP_THISNODE) & ~__GFP_WAIT, HPAGE_PMD_ORDER);
-       if (!new_page) {
-               count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
-               goto out_dropref;
-       }
+       if (!new_page)
+               goto out_fail;
+
        page_xchg_last_nid(new_page, page_last_nid(page));
 
        isolated = numamigrate_isolate_page(pgdat, page);
-
-       /*
-        * Failing to isolate or a GUP pin prevents migration. The expected
-        * page count is 2. 1 for anonymous pages without a mapping and 1
-        * for the callers pin. If the page was isolated, the page will
-        * need to be put back on the LRU.
-        */
-       if (!isolated || page_count(page) != 2) {
-               count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
+       if (!isolated) {
                put_page(new_page);
-               if (isolated) {
-                       putback_lru_page(page);
-                       isolated = 0;
-                       goto out;
-               }
-               goto out_keep_locked;
+               goto out_fail;
        }
 
        /* Prepare a page as a migration target */
@@ -1730,6 +1718,7 @@ int migrate_misplaced_transhuge_page(struct mm_struct *mm,
                putback_lru_page(page);
 
                count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
+               isolated = 0;
                goto out;
        }
 
@@ -1774,9 +1763,11 @@ out:
                        -HPAGE_PMD_NR);
        return isolated;
 
+out_fail:
+       count_vm_events(PGMIGRATE_FAIL, HPAGE_PMD_NR);
 out_dropref:
+       unlock_page(page);
        put_page(page);
-out_keep_locked:
        return 0;
 }
 #endif /* CONFIG_NUMA_BALANCING */