linux-kernel - Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

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Message-ID: <alpine.LSU.2.00.1011180934400.3210@tigran.mtv.corp.google.com>
Date:	Thu, 18 Nov 2010 09:41:22 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Christoph Hellwig <hch@...radead.org>
cc:	Dave Chinner <david@...morbit.com>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Nick Piggin <npiggin@...nel.dk>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Kosaki Motohiro <kosaki.motohiro@...fujitsu.com>,
	Theodore Tso <tytso@...gle.com>,
	Michael Rubin <mrubin@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering
 writeback

On Thu, 18 Nov 2010, Christoph Hellwig wrote:
> On Thu, Nov 18, 2010 at 10:11:43AM +1100, Dave Chinner wrote:
> > Hence I think that avoiding ->page_mkwrite callouts is likely to
> > break some filesystems in subtle, undetected ways.  IMO, regardless
> > of what is done, it would be really good to start by writing a new
> > regression test to exercise and encode the expected the mlock
> > behaviour so we can detect regressions later on....
> 
> I think it would help if we could drink a bit of the test driven design
> coolaid here. Michel, can you write some testcases where pages on a
> shared mapping are mlocked, then dirtied and then munlocked, and then
> written out using msync/fsync.  Anything that fails this test on
> btrfs/ext4/gfs/xfs/etc obviously doesn't work.

Whilst it's hard to argue against a request for testing, Dave's worries
just sprang from a misunderstanding of all the talk about "avoiding ->
page_mkwrite".  There's nothing strange or risky about Michel's patch,
it does not avoid ->page_mkwrite when there is a write: it just stops
pretending that there was a write when locking down the shared area.

Hugh
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