On Wed, 14 Dec 2011 15:41:27 +0000
Mel Gorman <mgorman@xxxxxxx> 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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