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

Greg Thelen
2014-05-07T21:39:27Z

On Wed, May 07 2014, Andrew Morton <akpm@linux-foundation.org> wrote:

> 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?

I detected the problem at runtime seeing that ext4 metadata pages (esp
the ones read by "sbi->s_group_desc[i] = sb_bread(sb, block)") were
constantly visited by compaction calls of migrate_pages(). These pages
had a non-zero b_count which caused fallback_migrate_page() ->
try_to_release_page() -> try_to_free_buffers() to fail.