mm: Close races between THP migration and PMD numa clearing

THP migration uses the page lock to guard against parallel allocations
but there are cases like this still open

  Task A					Task B
  ---------------------				---------------------
  do_huge_pmd_numa_page				do_huge_pmd_numa_page
  lock_page
  mpol_misplaced == -1
  unlock_page
  goto clear_pmdnuma
						lock_page
						mpol_misplaced == 2
						migrate_misplaced_transhuge
  pmd = pmd_mknonnuma
  set_pmd_at

During hours of testing, one crashed with weird errors and while I have
no direct evidence, I suspect something like the race above happened.
This patch extends the page lock to being held until the pmd_numa is
cleared to prevent migration starting in parallel while the pmd_numa is
being cleared. It also flushes the old pmd entry and orders pagetable
insertion before rmap insertion.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-9-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 15 insertions, 18 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index c3bb65f284d5..d4928769680f 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1304,24 +1304,25 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
         target_nid = mpol_misplaced(page, vma, haddr);
         if (target_nid == -1) {
                 /* If the page was locked, there are no parallel migrations */
-                if (page_locked) {
-                        unlock_page(page);
+                if (page_locked)
                         goto clear_pmdnuma;
-                }
 
-                /* Otherwise wait for potential migrations and retry fault */
+                /*
+                 * Otherwise wait for potential migrations and retry. We do
+                 * relock and check_same as the page may no longer be mapped.
+                 * As the fault is being retried, do not account for it.
+                 */
                 spin_unlock(&mm->page_table_lock);
                 wait_on_page_locked(page);
+                page_nid = -1;
                 goto out;
         }
 
         /* Page is misplaced, serialise migrations and parallel THP splits */
         get_page(page);
         spin_unlock(&mm->page_table_lock);
-        if (!page_locked) {
+        if (!page_locked)
                 lock_page(page);
-                page_locked = true;
-        }
         anon_vma = page_lock_anon_vma_read(page);
 
         /* Confirm the PMD did not change while page_table_lock was released */
@@ -1329,32 +1330,28 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
         if (unlikely(!pmd_same(pmd, *pmdp))) {
                 unlock_page(page);
                 put_page(page);
+                page_nid = -1;
                 goto out_unlock;
         }
 
-        /* Migrate the THP to the requested node */
+        /*
+         * Migrate the THP to the requested node, returns with page unlocked
+         * and pmd_numa cleared.
+         */
         spin_unlock(&mm->page_table_lock);
         migrated = migrate_misplaced_transhuge_page(mm, vma,
                                 pmdp, pmd, addr, page, target_nid);
         if (migrated)
                 page_nid = target_nid;
-        else
-                goto check_same;
 
         goto out;
-
-check_same:
-        spin_lock(&mm->page_table_lock);
-        if (unlikely(!pmd_same(pmd, *pmdp))) {
-                /* Someone else took our fault */
-                page_nid = -1;
-                goto out_unlock;
-        }
 clear_pmdnuma:
+        BUG_ON(!PageLocked(page));
         pmd = pmd_mknonnuma(pmd);
         set_pmd_at(mm, haddr, pmdp, pmd);
         VM_BUG_ON(pmd_numa(*pmdp));
         update_mmu_cache_pmd(vma, addr, pmdp);
+        unlock_page(page);
 out_unlock:
         spin_unlock(&mm->page_table_lock);