From eee0f252c6537da2e883f75d22cff1427515eaf3 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sat, 28 May 2011 13:20:21 -0700 Subject: [PATCH] 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 Signed-off-by: Linus Torvalds --- mm/rmap.c | 13 ++++++++----- 1 file 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; -- 2.20.1