mm: fix migration hangs on anon_vma lock

Increased usage of page migration in mmotm reveals that the anon_vma
locking in unmap_and_move() has been deficient since 2.6.36 (or even
earlier).  Review at the time of f18194275c39835cb84563500995e0d503a32d9a
("mm: fix hang on anon_vma->root->lock") missed the issue here: the
anon_vma to which we get a reference may already have been freed back to
its slab (it is in use when we check page_mapped, but that can change),
and so its anon_vma->root may be switched at any moment by reuse in
anon_vma_prepare.

Perhaps we could fix that with a get_anon_vma_unless_zero(), but let's
not: just rely on page_lock_anon_vma() to do all the hard thinking for us,
then we don't need any rcu read locking over here.

In removing the rcu_unlock label: since PageAnon is a bit in
page->mapping, it's impossible for a !page->mapping page to be anon; but
insert VM_BUG_ON in case the implementation ever changes.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: "Jun'ichi Nomura" <j-nomura@ce.jp.nec.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: <stable@kernel.org> [2.6.37, 2.6.36]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 19 insertions, 29 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 89a6bc8cd307..a20cf12edede 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -622,7 +622,6 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
         int *result = NULL;
         struct page *newpage = get_new_page(page, private, &result);
         int remap_swapcache = 1;
-        int rcu_locked = 0;
         int charge = 0;
         struct mem_cgroup *mem = NULL;
         struct anon_vma *anon_vma = NULL;
@@ -694,20 +693,26 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
         /*
          * By try_to_unmap(), page->mapcount goes down to 0 here. In this case,
          * we cannot notice that anon_vma is freed while we migrates a page.
-         * This rcu_read_lock() delays freeing anon_vma pointer until the end
+         * This get_anon_vma() delays freeing anon_vma pointer until the end
          * of migration. File cache pages are no problem because of page_lock()
          * File Caches may use write_page() or lock_page() in migration, then,
          * just care Anon page here.
          */
         if (PageAnon(page)) {
-                rcu_read_lock();
-                rcu_locked = 1;
-
-                /* Determine how to safely use anon_vma */
-                if (!page_mapped(page)) {
-                        if (!PageSwapCache(page))
-                                goto rcu_unlock;
-
+                /*
+                 * Only page_lock_anon_vma() understands the subtleties of
+                 * getting a hold on an anon_vma from outside one of its mms.
+                 */
+                anon_vma = page_lock_anon_vma(page);
+                if (anon_vma) {
+                        /*
+                         * Take a reference count on the anon_vma if the
+                         * page is mapped so that it is guaranteed to
+                         * exist when the page is remapped later
+                         */
+                        get_anon_vma(anon_vma);
+                        page_unlock_anon_vma(anon_vma);
+                } else if (PageSwapCache(page)) {
                         /*
                          * We cannot be sure that the anon_vma of an unmapped
                          * swapcache page is safe to use because we don't
@@ -722,13 +727,7 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
                          */
                         remap_swapcache = 0;
                 } else {
-                        /*
-                         * Take a reference count on the anon_vma if the
-                         * page is mapped so that it is guaranteed to
-                         * exist when the page is remapped later
-                         */
-                        anon_vma = page_anon_vma(page);
-                        get_anon_vma(anon_vma);
+                        goto uncharge;
                 }
         }
 
@@ -745,16 +744,10 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
          * free the metadata, so the page can be freed.
          */
         if (!page->mapping) {
-                if (!PageAnon(page) && page_has_private(page)) {
-                        /*
-                         * Go direct to try_to_free_buffers() here because
-                         * a) that's what try_to_release_page() would do anyway
-                         * b) we may be under rcu_read_lock() here, so we can't
-                         *    use GFP_KERNEL which is what try_to_release_page()
-                         *    needs to be effective.
-                         */
+                VM_BUG_ON(PageAnon(page));
+                if (page_has_private(page)) {
                         try_to_free_buffers(page);
-                        goto rcu_unlock;
+                        goto uncharge;
                 }
                 goto skip_unmap;
         }
@@ -768,14 +761,11 @@ skip_unmap:
 
         if (rc && remap_swapcache)
                 remove_migration_ptes(page, page);
-rcu_unlock:
 
         /* Drop an anon_vma reference if we took one */
         if (anon_vma)
                 drop_anon_vma(anon_vma);
 
-        if (rcu_locked)
-                rcu_read_unlock();
 uncharge:
         if (!charge)
                 mem_cgroup_end_migration(mem, page, newpage);