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

[Vlastimil Babka]

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

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