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

[Kevin Wolf]

Am 01.03.2012 17:22, schrieb Stefan Hajnoczi:
> On Thu, Mar 01, 2012 at 05:10:57PM +0100, Kevin Wolf wrote:
>> 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 <address@hidden> for extensive testing
>>> and debugging which lead to discovery of this bug.
>>>
>>> Reported-by: Benoît Canet <address@hidden>
>>> Signed-off-by: Stefan Hajnoczi <address@hidden>
>>
>> Thanks, applied to the block branch.
>>
>> How about a qemu-iotests case?
> 
> The test case is not ready yet.  I started writing one but it is racy
> because I haven't introduced a way of controlling AIO issue/complete for
> tests.  My next step is to add that.

Will it be specific to image formats using AIO then or is it generic
enough that coroutine-based drivers work with it as well?

Kevin