Re: [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache entries

[Kevin Wolf <kwolf@redhat.com>]

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 multiple
> requests.
> 
> There is a potential data corruption issue when an in-use L2 table is
> evicted from the cache because the following situation occurs:
> 
>   1. An allocating write performs an update to L2 table "A".
> 
>   2. Another request needs L2 table "B" and causes table "A" to be
>      evicted.
> 
>   3. A new read request needs L2 table "A" but it is not cached.
> 
> 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 and
> lose data clusters!
> 
> Thanks to Benoît Canet <benoit.canet@gmail.com> for extensive testing
> and debugging which lead to discovery of this bug.
> 
> Reported-by: Benoît 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