linux-kernel - Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim and use a

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Date:	Tue, 8 Jun 2010 10:28:14 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Nick Piggin <npiggin@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [RFC PATCH 0/6] Do not call ->writepage[s] from direct reclaim
	and use a_ops->writepages() where possible

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
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