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

[Vlastimil Babka <vbabka@suse.cz>]

On 05/07/2014 11:39 PM, Greg Thelen wrote:
>
> 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.

That sounds like something the "mm, compaction: add per-zone migration 
pfn cache for async compaction" patch would fix, not this one, though.