Messages in this thread Patch in this message | | | Date | Fri, 16 Dec 2011 15:20:54 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH 05/11] mm: compaction: Determine if dirty pages can be migrated without blocking within ->migratepage |
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On Wed, 14 Dec 2011 15:41:27 +0000 Mel Gorman <mgorman@suse.de> wrote:
> Asynchronous compaction is used when allocating transparent hugepages > to avoid blocking for long periods of time. Due to reports of > stalling, there was a debate on disabling synchronous compaction > but this severely impacted allocation success rates. Part of the > reason was that many dirty pages are skipped in asynchronous compaction > by the following check; > > if (PageDirty(page) && !sync && > mapping->a_ops->migratepage != migrate_page) > rc = -EBUSY; > > This skips over all mapping aops using buffer_migrate_page() > even though it is possible to migrate some of these pages without > blocking. This patch updates the ->migratepage callback with a "sync" > parameter. It is the responsibility of the callback to fail gracefully > if migration would block. > > ... > > @@ -259,6 +309,19 @@ static int migrate_page_move_mapping(struct address_space *mapping, > } > > /* > + * In the async migration case of moving a page with buffers, lock the > + * buffers using trylock before the mapping is moved. If the mapping > + * was moved, we later failed to lock the buffers and could not move > + * the mapping back due to an elevated page count, we would have to > + * block waiting on other references to be dropped. > + */ > + if (!sync && head && !buffer_migrate_lock_buffers(head, sync)) {
Once it has been established that "sync" is true, I find it clearer to pass in plain old "true" to buffer_migrate_lock_buffers(). Minor point.
I hadn't paid a lot of attention to buffer_migrate_page() before. Scary function. I'm rather worried about its interactions with ext3 journal commit which locks buffers then plays with them while leaving the page unlocked. How vigorously has this been whitebox-tested?
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