linux-kernel - Re: [PATCH 14/14] fs,xfs: Allow kswapd to writeback pages

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Message-ID: <20100629125143.GB31561@csn.ul.ie>
Date:	Tue, 29 Jun 2010 13:51:43 +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>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 14/14] fs,xfs: Allow kswapd to writeback pages

On Tue, Jun 29, 2010 at 08:37:22AM -0400, Christoph Hellwig wrote:
> I don't see a patch in this set which refuses writeback from the memcg
> context, which we identified as having large stack footprint in hte
> discussion of the last patch set.
> 

It wasn't clear to me what the right approach was there and should
have noted that in the intro. The last note I have on it is this message
http://kerneltrap.org/mailarchive/linux-kernel/2010/6/17/4584087 which might
avoid the deep stack usage but I wasn't 100% sure. As kswapd doesn't clean
pages for memcg, I left memcg being able to direct writeback to see what
the memcg people preferred.

> Meanwhile I've submitted a patch to xfs to allow reclaim from kswapd,
> and just prevent it from direct and memcg reclaim.
> 

Good stuff.

> Btw, it might be worth to also allow kswap to all writeout on ext4,
> but doing that will be a bit more complicated than the btrfs and xfs
> variants as the code is rather convoluted.
> 

Fully agreed. I looked into it and got caught in its twisty web so
postponed it until this much can be finalised, agreed upon or rejected -
all pre-requisities to making the ext4 work worthwhile.

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