linux-kernel - Re: [PATCH] mm fix page writeback accounting to fix oom condition under heavy I/O

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Date:	Tue, 10 Feb 2009 16:56:13 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jens Axboe <jens.axboe@...cle.com>, akpm@...ux-foundation.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, thomas.pi@...or.dea,
	Yuriy Lalym <ylalym@...il.com>, ltt-dev@...ts.casi.polymtl.ca,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm fix page writeback accounting to fix oom condition under heavy I/O

On Tuesday 10 February 2009 16:23:56 Linus Torvalds wrote:
> On Mon, 9 Feb 2009, Mathieu Desnoyers wrote:
> > So this patch fixes this behavior by only decrementing the page
> > accounting _after_ the block I/O writepage has been done.
>
> This makes no sense, really.
>
> Or rather, I don't mind the notion of updating the counters only after IO
> per se, and _that_ part of it probably makes sense. But why is it that you
> only then fix up two of the call-sites. There's a lot more call-sites than
> that for this function.

Well if you do that, then I'd think you also have to change some
calculations that today use dirty+writeback.

In some ways it does make sense, but OTOH it is natural in the
pagecache since it was introduced to treat writeback as basically
equivalent to dirty. So writeback && !dirty pages shouldn't cause
things to blow up, or if it does then hopefully it is a simple
bug somewhere.

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