linux-kernel - Re: [PATCH 001 of 2] Fix read/truncate race.

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Message-ID: <20070607071929.GL4735@kernel.dk>
Date:	Thu, 7 Jun 2007 09:19:30 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	NeilBrown <neilb@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 001 of 2] Fix read/truncate race.

On Thu, Jun 07 2007, NeilBrown wrote:
> 
> do_generic_mapping_read currently samples the i_size at the start
> and doesn't do so again unless it needs to call ->readpage to load
> a page.  After ->readpage it has to re-sample i_size as a truncate
> may have caused that page to be filled with zeros, and the read()
> call should not see these.
> 
> However there are other activities that might cause ->readpage to be
> called on a page between the time that do_generic_mapping_read
> samples i_size and when it finds that it has an uptodate page.  These
> include at least read-ahead and possibly another thread performing a
> read.
> 
> So do_generic_mapping_read must sample i_size *after* it has an
> uptodate page.  Thus the current sampling at the start and after a read
> can be replaced with a sampling before the copy-out.
> 
> The same change applied to __generic_file_splice_read.
> 
> Note that this fixes any race with truncate_complete_page, but does
> not fix a possible race with truncate_partial_page.  If a partial
> truncate happens after do_generic_mapping_read samples i_size and
> before the copy_out, the nuls that truncate_partial_page place in the
> page could be copied out incorrectly.
> 
> I think the best fix for that is to *not* zero out parts of the page
> in truncate_partial_page, but rather to zero out the tail of a page
> when increasing i_size.

Thanks Neil, if you don't mind I'll steal the splice bits and add them
to the splice branch, as this particular patch conflicts with other
isize fixes that Hugh and I have been working on.

-- 
Jens Axboe

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