linux-ext4 - Re: [patch 12/17] vfs: pagecache usage optimization for pagesize!=blocksize

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Message-Id: <200808041719.43293.nickpiggin@yahoo.com.au>
Date:	Mon, 4 Aug 2008 17:19:42 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	hifumi.hisashi@....ntt.co.jp, jack@....cz,
	linux-ext4@...r.kernel.org
Subject: Re: [patch 12/17] vfs: pagecache usage optimization for pagesize!=blocksize

On Tuesday 29 July 2008 09:00, Christoph Hellwig wrote:
> On Mon, Jul 28, 2008 at 03:46:36PM -0700, akpm@...ux-foundation.org wrote:
> > From: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
> >
> > When we read some part of a file through pagecache, if there is a
> > pagecache of corresponding index but this page is not uptodate, read IO
> > is issued and this page will be uptodate.
>
> I was under the impression we wanted to do this in a nicer way than
> the hacky method?

This patch unfortunately appears like it may introduce an
uninitialized memory leak due to a data race between one
thread initializing a buffer then marking it uptodate, and
the other testing buffer uptodate then reading from the
buffer (buffer, read as: page memory covered by buffer head).

For reference, this is basically the same class of data race
that I fixed 0ed361dec36945f3116ee1338638ada9a8920905

I should have picked up on this before it was merged, but I
was kind of rushed to review other things before they got
merged.

I don't think this patch got quite enough justification to
warrant just blindly putting barriers in the buffer bitops.
The best-case numbers for it were reasonable enough when the
downside was only an extra branch or two in a relatively slow
path. I don't really know how best to go from here (maybe
someone can argue it is not a problem or come up with a better
fix?).

Thanks,
Nick
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