On Wed, Jun 10, 2015 at 11:32:29AM +0200, Vlastimil Babka wrote:
> Compaction should finish when the migration and free scanner meet, i.e. they
> reach the same pageblock. Currently however, the test in compact_finished()
> simply just compares the exact pfns, which may yield a false negative when the
> free scanner position is in the middle of a pageblock and the migration
> scanner
> reaches the beginning of the same pageblock.
>
> This hasn't been a problem until commit e14c720efdd7 ("mm, compaction:
> remember
> position within pageblock in free pages scanner") allowed the free scanner
> position to be in the middle of a pageblock between invocations. The hot-fix
> 1d5bfe1ffb5b ("mm, compaction: prevent infinite loop in compact_zone")
> prevented the issue by adding a special check in the migration scanner to
> satisfy the current detection of scanners meeting.
>
> However, the proper fix is to make the detection more robust. This patch
> introduces the compact_scanners_met() function that returns true when the free
> scanner position is in the same or lower pageblock than the migration scanner.
> The special case in isolate_migratepages() introduced by 1d5bfe1ffb5b is
> removed.
>
> Suggested-by: Joonsoo Kim <[email protected]>
> Signed-off-by: Vlastimil Babka <[email protected]>
> Cc: Minchan Kim <[email protected]>
> Cc: Mel Gorman <[email protected]>
> Cc: Joonsoo Kim <[email protected]>
> Cc: Michal Nazarewicz <[email protected]>
> Cc: Naoya Horiguchi <[email protected]>
> Cc: Christoph Lameter <[email protected]>
> Cc: Rik van Riel <[email protected]>
> Cc: David Rientjes <[email protected]>
Acked-by: Mel Gorman <[email protected]>
--
Mel Gorman
SUSE Labs
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