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authorHugh Dickins <hughd@google.com>2011-05-28 13:20:21 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2011-05-28 16:55:32 -0700
commiteee0f252c6537da2e883f75d22cff1427515eaf3 (patch)
tree87ca6a49dd2d299b6cb4146ff3295bb625519b05
parent5dbe0af47f8a8f968bac2991c3ec974c6e3eaabc (diff)
mm: fix page_lock_anon_vma leaving mutex locked
On one machine I've been getting hangs, a page fault's anon_vma_prepare() waiting in anon_vma_lock(), other processes waiting for that page's lock. This is a replay of last year's f18194275c39 "mm: fix hang on anon_vma->root->lock". The new page_lock_anon_vma() places too much faith in its refcount: when it has acquired the mutex_trylock(), it's possible that a racing task in anon_vma_alloc() has just reallocated the struct anon_vma, set refcount to 1, and is about to reset its anon_vma->root. Fix this by saving anon_vma->root, and relying on the usual page_mapped() check instead of a refcount check: if page is still mapped, the anon_vma is still ours; if page is not still mapped, we're no longer interested. Signed-off-by: Hugh Dickins <hughd@google.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/rmap.c13
1 files changed, 8 insertions, 5 deletions
diff --git a/mm/rmap.c b/mm/rmap.c
index ba58ca36fc90..6bada99cd61c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -405,6 +405,7 @@ out:
struct anon_vma *page_lock_anon_vma(struct page *page)
{
struct anon_vma *anon_vma = NULL;
+ struct anon_vma *root_anon_vma;
unsigned long anon_mapping;
rcu_read_lock();
@@ -415,13 +416,15 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
goto out;
anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON);
- if (mutex_trylock(&anon_vma->root->mutex)) {
+ root_anon_vma = ACCESS_ONCE(anon_vma->root);
+ if (mutex_trylock(&root_anon_vma->mutex)) {
/*
- * If we observe a !0 refcount, then holding the lock ensures
- * the anon_vma will not go away, see __put_anon_vma().
+ * If the page is still mapped, then this anon_vma is still
+ * its anon_vma, and holding the mutex ensures that it will
+ * not go away, see __put_anon_vma().
*/
- if (!atomic_read(&anon_vma->refcount)) {
- anon_vma_unlock(anon_vma);
+ if (!page_mapped(page)) {
+ mutex_unlock(&root_anon_vma->mutex);
anon_vma = NULL;
}
goto out;