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Date: Thu, 01 Mar 2012 17:10:57 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: qemu-devel@nongnu.org, qemu-stable@nongnu.org

Am 27.02.2012 14:16, schrieb Stefan Hajnoczi:
> The L2 table cache reduces QED metadata reads that would be required
> when translating LBAs to offsets into the image file.  Since requests
> execute in parallel it is possible to share an L2 table between multipl=
e
> requests.
>=20
> There is a potential data corruption issue when an in-use L2 table is
> evicted from the cache because the following situation occurs:
>=20
>   1. An allocating write performs an update to L2 table "A".
>=20
>   2. Another request needs L2 table "B" and causes table "A" to be
>      evicted.
>=20
>   3. A new read request needs L2 table "A" but it is not cached.
>=20
> As a result the L2 update from #1 can overlap with the L2 fetch from #3.
> We must avoid doing overlapping I/O requests here since the worst case
> outcome is that the L2 fetch completes before the L2 update and yields
> stale data.  In that case we would effectively discard the L2 update an=
d
> lose data clusters!
>=20
> Thanks to Beno=C3=AEt Canet <benoit.canet@gmail.com> for extensive test=
ing
> and debugging which lead to discovery of this bug.
>=20
> Reported-by: Beno=C3=AEt Canet <benoit.canet@gmail.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Thanks, applied to the block branch.

How about a qemu-iotests case?

Kevin