Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a_ops->writepages() where possible

Mel Gorman
2010-06-08T09:28:34Z

On Tue, Jun 08, 2010 at 05:08:11AM -0400, Christoph Hellwig wrote:
> On Tue, Jun 08, 2010 at 10:02:19AM +0100, Mel Gorman wrote:
> > seeky patterns. The second is that direct reclaim calling the filesystem
> > splices two potentially deep call paths together and potentially overflows
> > the stack on complex storage or filesystems. This series is an early draft
> > at tackling both of these problems and is in three stages.
>
> Btw, one more thing came up when I discussed the issue again with Dave
> recently:
>
> - we also need to care about ->releasepage. At least for XFS it
> can end up in the same deep allocator chain as ->writepage because
> it does all the extent state conversions, even if it doesn't
> start I/O.

Dang.

> I haven't managed yet to decode the ext4/btrfs codepaths
> for ->releasepage yet to figure out how they release a page that
> covers a delayed allocated or unwritten range.
>

If ext4/btrfs are also very deep call-chains and this series is going more
or less the right direction, then avoiding calling ->releasepage from direct
reclaim is one, somewhat unfortunate, option. The second is to avoid it on
a per-filesystem basis for direct reclaim using PF_MEMALLOC to detect
reclaimers and PF_KSWAPD to tell the difference between direct
reclaimers and kswapd.

Either way, these pages could be treated similar to dirty pages on the
dirty_pages list.

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab