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Commit 1ce82b69 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds
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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 f1819427
("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: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarMel Gorman <mel@csn.ul.ie>
Reviewed-by: default avatarRik 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: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2919bfd0

mm/migrate.c

+19 −29
Original line number Diff line number Diff line
@@ -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);