Re: [PATCH v2 4/4] mm, compaction: direct freepage allocation for async direct compaction

[Mel Gorman]

On Thu, Mar 31, 2016 at 10:50:36AM +0200, Vlastimil Babka wrote:
> The goal of direct compaction is to quickly make a high-order page available
> for the pending allocation. The free page scanner can add significant latency
> when searching for migration targets, although to succeed the compaction, the
> only important limit on the target free pages is that they must not come from
> the same order-aligned block as the migrated pages.
> 

What prevents the free pages being allocated from behind the migration
scanner? Having compaction abort when the scanners meet misses
compaction opportunities but it avoids the problem of Compactor A using
pageblock X as a migration target and Compactor B using pageblock X as a
migration source.

-- 
Mel Gorman
SUSE Labs