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Re: [PATCH] dio: track and serialise unaligned direct IO

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH] dio: track and serialise unaligned direct IO
From: Matthew Wilcox <matthew@xxxxxx>
Date: Thu, 29 Jul 2010 20:53:24 -0600
Cc: linux-fsdevel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, sandeen@xxxxxxxxxxx
In-reply-to: <1280443516-14448-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1280443516-14448-1-git-send-email-david@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.18 (2008-05-17)
On Fri, Jul 30, 2010 at 08:45:16AM +1000, Dave Chinner wrote:
> If we get two unaligned direct IO's to the same filesystem block
> that is marked as a new allocation (i.e. buffer_new), then both IOs will
> zero the portion of the block they are not writing data to. As a
> result, when the IOs complete there will be a portion of the block
> that contains zeros from the last IO to complete rather than the
> data that should be there.

Urgh.  Yuck.

> This is easily manifested by qemu using aio+dio with an unaligned
> guest filesystem - every IO is unaligned and fileystem corruption is
> encountered in the guest filesystem. xfstest 240 (from Eric Sandeen)
> is also a simple reproducer.
> To avoid this problem, track unaligned IO that triggers sub-block zeroing and
> check new incoming unaligned IO that require sub-block zeroing against that
> list. If we get an overlap where the start and end of unaligned IOs hit the
> same filesystem block, then we need to block the incoming IOs until the IO 
> that
> is zeroing the block completes. The blocked IO can then continue without
> needing to do any zeroing and hence won't overwrite valid data with zeros.

Urgh.  Yuck.

Could we perhaps handle this by making an IO instantiate a page cache
page for partial writes, and forcing that portion of the IO through the
page cache?  The second IO would hit the same page and use the existing
O_DIRECT vs page cache paths.

Matthew Wilcox                          Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."

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