Commit 1ce82b69 authored by Hugh Dickins's avatar Hugh Dickins Committed by Linus Torvalds

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
......@@ -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);
......
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