Commit 21ee9f39 authored by Minchan Kim's avatar Minchan Kim Committed by Linus Torvalds

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.

                                if PageLRU()
                                        if !test_bit(AS_UNEVICTABLE)
                                        	move evictable list
if !test_bit(AS_UNEVICTABLE)
        move evictable list

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

This patch adds a barrier after mapping_clear_unevictable.

I didn't meet this problem but just found during review.
Signed-off-by: default avatarMinchan Kim <>
Acked-by: default avatarKOSAKI Motohiro <>
Cc: Mel Gorman <>
Cc: Rik van Riel <>
Cc: Lee Schermerhorn <>
Acked-by: default avatarJohannes Weiner <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 2f1da642
......@@ -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;
* 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.
retval = 0;
......@@ -633,13 +633,14 @@ void putback_lru_page(struct page *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().
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment