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

[Greg Thelen]

On Wed, May 07 2014, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

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