From 21ee9f398be209ccbb62929d35961ca1ed48eec3 Mon Sep 17 00:00:00 2001 From: Minchan Kim Date: Mon, 31 Oct 2011 17:09:28 -0700 Subject: vmscan: add barrier to prevent evictable page in unevictable list When a race between putback_lru_page() and shmem_lock with lock=0 happens, progrom execution order is as follows, but clear_bit in processor #1 could be reordered right before spin_unlock of processor #1. Then, the page would be stranded on the unevictable list. spin_lock SetPageLRU spin_unlock clear_bit(AS_UNEVICTABLE) spin_lock if PageLRU() if !test_bit(AS_UNEVICTABLE) move evictable list smp_mb if !test_bit(AS_UNEVICTABLE) move evictable list spin_unlock But, pagevec_lookup() in scan_mapping_unevictable_pages() has rcu_read_[un]lock() so it could protect reordering before reaching test_bit(AS_UNEVICTABLE) on processor #1 so this problem never happens. But it's a unexpected side effect and we should solve this problem properly. This patch adds a barrier after mapping_clear_unevictable. I didn't meet this problem but just found during review. Signed-off-by: Minchan Kim Acked-by: KOSAKI Motohiro Cc: Mel Gorman Cc: Rik van Riel Cc: Lee Schermerhorn Acked-by: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/shmem.c | 6 ++++++ mm/vmscan.c | 11 ++++++----- 2 files changed, 12 insertions(+), 5 deletions(-) (limited to 'mm') diff --git a/mm/shmem.c b/mm/shmem.c index 2d357729529..fa4fa6ce13b 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -1068,6 +1068,12 @@ int shmem_lock(struct file *file, int lock, struct user_struct *user) user_shm_unlock(inode->i_size, user); info->flags &= ~VM_LOCKED; mapping_clear_unevictable(file->f_mapping); + /* + * Ensure that a racing putback_lru_page() can see + * the pages of this mapping are evictable when we + * skip them due to !PageLRU during the scan. + */ + smp_mb__after_clear_bit(); scan_mapping_unevictable_pages(file->f_mapping); } retval = 0; diff --git a/mm/vmscan.c b/mm/vmscan.c index 3886b0bd786..f51a33e8ed8 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -633,13 +633,14 @@ redo: lru = LRU_UNEVICTABLE; add_page_to_unevictable_list(page); /* - * When racing with an mlock clearing (page is - * unlocked), make sure that if the other thread does - * not observe our setting of PG_lru and fails - * isolation, we see PG_mlocked cleared below and move + * When racing with an mlock or AS_UNEVICTABLE clearing + * (page is unlocked) make sure that if the other thread + * does not observe our setting of PG_lru and fails + * isolation/check_move_unevictable_page, + * we see PG_mlocked/AS_UNEVICTABLE cleared below and move * the page back to the evictable list. * - * The other side is TestClearPageMlocked(). + * The other side is TestClearPageMlocked() or shmem_lock(). */ smp_mb(); } -- cgit v1.2.3-70-g09d2