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@xxxxxxxxxx>
---
mm/rmap.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
--- linux.orig/mm/rmap.c 2011-05-27 20:07:44.000000000 -0700
+++ linux/mm/rmap.c 2011-05-27 20:31:04.596303434 -0700
@@ -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(stru
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;
--
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