Re: [Qemu-devel] [PATCH] nbd: Don't trim unrequested bytes

[Eric Blake]

On 05/25/2016 02:31 AM, Paolo Bonzini wrote:
> 
> 
> On 25/05/2016 06:37, Eric Blake wrote:
>> Similar to commit df7b97ff, we are mishandling clients that
>> give an unaligned NBD_CMD_TRIM request, and potentially
>> trimming bytes that occur before their request; which in turn
>> can cause potential unintended data loss (unlikely in
>> practice, since most clients are sane and issue aligned trim
>> requests).  However, while we fixed read and write by switching
>> to the byte interfaces of blk_, we don't yet have a byte
>> interface for discard.  On the other hand, trim is advisory, so
>> rounding the user's request to simply ignore the first and last
>> unaligned sectors (or the entire request, if it is sub-sector
>> in length) is just fine.
>>
>> CC: address@hidden
>> Signed-off-by: Eric Blake <address@hidden>
>> ---
>>  nbd/server.c | 8 ++++++--
>>  1 file changed, 6 insertions(+), 2 deletions(-)
>>
>> diff --git a/nbd/server.c b/nbd/server.c
>> index fa862cd..5aed8db 100644
>> --- a/nbd/server.c
>> +++ b/nbd/server.c
>> @@ -1153,8 +1153,12 @@ static void nbd_trip(void *opaque)
>>          break;
>>      case NBD_CMD_TRIM:
>>          TRACE("Request type is TRIM");
>> -        ret = blk_co_discard(exp->blk, (request.from + exp->dev_offset)
>> -                                       / BDRV_SECTOR_SIZE,
>> +        /* Ignore unaligned head or tail, until block layer adds byte
>> +         * interface */
>> +        request.len -= (request.from + request.len) % BDRV_SECTOR_SIZE;

Gaaah - this version can underflow request.len.  v2 coming up.

> Thanks, queued for 2.7.

Fortunately, rebasing a staging queue is easier than dealing with an
incorrect pull request.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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