Messages in this thread | | | Date | Mon, 12 May 2014 10:37:16 +0200 | From | Vlastimil Babka <> | Subject | Re: [patch v3 2/6] mm, compaction: return failed migration target pages back to freelist |
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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.
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