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Re: [Qemu-stable] [Qemu-devel] [PATCH] qed: do not evict in-use L2 table
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-stable] [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache entries |
Date: |
Thu, 1 Mar 2012 11:04:25 +0000 |
On Mon, Feb 27, 2012 at 1:16 PM, Stefan Hajnoczi
<address@hidden> wrote:
> 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>
> ---
> Please include this in -stable once it has been merged into qemu.git/master.
>
> block/qed-l2-cache.c | 22 ++++++++++++++++++----
> 1 files changed, 18 insertions(+), 4 deletions(-)
Thanks for testing this fix and confirming it works, Benoît. Feel
free to reply with your Tested-by: line.
Stefan
- Re: [Qemu-stable] [Qemu-devel] [PATCH] qed: do not evict in-use L2 table cache entries,
Stefan Hajnoczi <=