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

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

[Eric Blake]

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: qemu-stable@nongnu.org
Signed-off-by: Eric Blake <eblake@redhat.com>
---
 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;
+        ret = blk_co_discard(exp->blk,
+                             DIV_ROUND_UP(request.from + exp->dev_offset,
+                                          BDRV_SECTOR_SIZE),
                              request.len / BDRV_SECTOR_SIZE);
         if (ret < 0) {
             LOG("discard failed");
-- 
2.5.5

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

[Paolo Bonzini]

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: qemu-stable@nongnu.org
> Signed-off-by: Eric Blake <eblake@redhat.com>
> ---
>  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;
> +        ret = blk_co_discard(exp->blk,
> +                             DIV_ROUND_UP(request.from + exp->dev_offset,
> +                                          BDRV_SECTOR_SIZE),
>                               request.len / BDRV_SECTOR_SIZE);
>          if (ret < 0) {
>              LOG("discard failed");
> 

Thanks, queued for 2.7.

Paolo

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

[Eric Blake]

[-- Attachment #1: Type: text/plain, Size: 1854 bytes --]

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: qemu-stable@nongnu.org
>> Signed-off-by: Eric Blake <eblake@redhat.com>
>> ---
>>  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


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 604 bytes --]