Re: [patch v3 2/6] mm, compaction: return failed migration target pages back to freelist

Andrew Morton
2014-05-07T21:15:40Z

On Tue, 6 May 2014 19:22:43 -0700 (PDT) David Rientjes <rientjes@google.com> wrote:

> Memory compaction works by having a "freeing scanner" scan from one end of a
> zone which isolates pages as migration targets while another "migrating scanner"
> scans from the other end of the same zone which isolates pages for migration.
>
> When page migration fails for an isolated page, the target page is returned to
> the system rather than the freelist built by the freeing scanner. This may
> require the freeing scanner to continue scanning memory after suitable migration
> targets have already been returned to the system needlessly.
>
> This patch returns destination pages to the freeing scanner freelist when page
> migration fails. This prevents unnecessary work done by the freeing scanner but
> also encourages memory to be as compacted as possible at the end of the zone.
>
> Reported-by: Greg Thelen <gthelen@google.com>

What did Greg actually report? IOW, what if any observable problem is
being fixed here?