Re: [PATCH v2] vmscan: add barrier to prevent evictable page in unevictable list

2011/9/29 Minchan Kim [off-list ref]:

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 <redacted>
Cc: Mel Gorman <redacted>
Cc: Rik van Riel <redacted>
Cc: Lee Schermerhorn <redacted>
Acked-by: Johannes Weiner <redacted>
Signed-off-by: Minchan Kim <redacted>

  Acked-by: KOSAKI Motohiro [off-list ref]

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>