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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>