Commit 21ee9f39 authored Oct 31, 2011 by Minchan Kim Committed by Linus Torvalds Oct 31, 2011

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 <minchan.kim@gmail.com>
Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Acked-by: Johannes Weiner <jweiner@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 2f1da642

mm/shmem.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -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;

mm/vmscan.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -633,13 +633,14 @@ void putback_lru_page(struct page *page)
		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();
		}