Re: [PATCH 05/11] mm: compaction: Determine if dirty pages can be migrated without blocking within ->migratepage

[Andrew Morton <akpm@linux-foundation.org>]

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?