Re: [PATCH 1/6] mm, compaction: more robust check for scanners meeting

[Joonsoo Kim]

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 <iamjoonsoo.kim@xxxxxxx>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Thanks.
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