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Subject[123/272] mm: fix migration hangs on anon_vma lock
2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Hugh Dickins <hughd@google.com>

commit 1ce82b69e96c838d007f316b8347b911fdfa9842 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
mm/migrate.c | 48 +++++++++++++++++++-----------------------------
1 file changed, 19 insertions(+), 29 deletions(-)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -620,7 +620,6 @@ static int unmap_and_move(new_page_t get
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;
@@ -672,20 +671,26 @@ static int unmap_and_move(new_page_t get
/*
* 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
@@ -700,13 +705,7 @@ static int unmap_and_move(new_page_t get
*/
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;
}
}

@@ -723,16 +722,10 @@ static int unmap_and_move(new_page_t get
* 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;
}
@@ -746,14 +739,11 @@ 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);



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