When racing between putback_lru_page and shmem_unlock 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.

side-note: I didn't meet this problem but just found during review.

Cc: Johannes Weiner <redacted>
Cc: KOSAKI Motohiro <redacted>
Cc: Mel Gorman <redacted>
Cc: Rik van Riel <redacted>
Cc: Lee Schermerhorn <redacted>
Signed-off-by: Minchan Kim <redacted>
---
 mm/shmem.c  |    1 +
 mm/vmscan.c |   11 ++++++-----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 2d35772..22cb349 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -1068,6 +1068,7 @@ 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);
+		smp_mb__after_clear_bit();
 		scan_mapping_unevictable_pages(file->f_mapping);