[Ocfs2-devel] ocfs2: serialize unaligned aio

[Joel Becker <jlbec@evilplan.org>]

On Wed, Jun 22, 2011 at 02:23:38PM -0700, Mark Fasheh wrote:
> Fix a corruption that can happen when we have (two or more) outstanding
> aio's to an overlapping unaligned region.  Ext4
> (e9e3bcecf44c04b9e6b505fd8e2eb9cea58fb94d) and xfs recently had to fix
> similar issues.
> 
> In our case what happens is that we can have an outstanding aio on a region
> and if a write comes in with some bytes overlapping the original aio we may
> decide to read that region into a page before continuing (typically because
> of buffered-io fallback).  Since we have no ordering guarantees with the
> aio, we can read stale or bad data into the page and then write it back out.
> 
> If the i/o is page and block aligned, then we avoid this issue as there
> won't be any need to read data from disk.
> 
> I took the same approach as Eric in the ext4 patch and introduced some
> serialization of unaligned async direct i/o.  I don't expect this to have an
> effect on the most common cases of AIO.  Unaligned aio will be slower
> though, but that's far more acceptable than data corruption.
> 
> Signed-off-by: Mark Fasheh <mfasheh@suse.com>

This patch is now in the fixes branch of ocfs2.git.

Joel


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