When racing 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.
side-note: I didn't meet this problem but just found during review.
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: Mel Gorman <mel@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Lee Schermerhorn <lee.schermerhorn@xxxxxx>
Acked-by: Johannes Weiner <jweiner@xxxxxxxxxx>
Signed-off-by: Minchan Kim <minchan.kim@xxxxxxxxx>
---
mm/shmem.c | 6 ++++++
mm/vmscan.c | 11 ++++++-----
2 files changed, 12 insertions(+), 5 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c
index 2d35772..fa4fa6c 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 23256e8..4480f67 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -634,13 +634,14 @@ 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();
}
--
1.7.4.1
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