[Qemu-devel] [PATCH v2] nbd: Don't trim unrequested bytes

[Qemu-devel] [PATCH v2] 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>
---

v2: don't underflow request.len on sub-sector requests

 nbd/server.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index fa862cd..b2cfeb9 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1153,12 +1153,20 @@ 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,
-                             request.len / BDRV_SECTOR_SIZE);
-        if (ret < 0) {
-            LOG("discard failed");
-            reply.error = -ret;
+        /* Ignore unaligned head or tail, until block layer adds byte
+         * interface */
+        if (request.len >= BDRV_SECTOR_SIZE) {
+            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");
+                reply.error = -ret;
+            }
+        } else {
+            TRACE("trim request too small, ignoring");
         }
         if (nbd_co_send_reply(req, &reply, 0) < 0) {
             goto out;
-- 
2.5.5

Re: [Qemu-devel] [PATCH v2] nbd: Don't trim unrequested bytes

[Eric Blake]

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On 05/25/2016 04:59 AM, 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>
> ---
> 
> v2: don't underflow request.len on sub-sector requests
> 
>  nbd/server.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/nbd/server.c b/nbd/server.c
> index fa862cd..b2cfeb9 100644
> --- a/nbd/server.c
> +++ b/nbd/server.c
> @@ -1153,12 +1153,20 @@ 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,
> -                             request.len / BDRV_SECTOR_SIZE);
> -        if (ret < 0) {
> -            LOG("discard failed");
> -            reply.error = -ret;
> +        /* Ignore unaligned head or tail, until block layer adds byte
> +         * interface */
> +        if (request.len >= BDRV_SECTOR_SIZE) {
> +            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);

This can end up calling blk_co_discard(, , 0) - do we need to audit
whether all underlying drivers handle a 0-length request, and/or
special-case this in blk_co_discard() to be an explicit no-op?

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


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Re: [Qemu-devel] [PATCH v2] nbd: Don't trim unrequested bytes

[Kevin Wolf]

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Am 25.05.2016 um 13:25 hat Eric Blake geschrieben:
> On 05/25/2016 04:59 AM, Eric Blake wrote:
> > +        /* Ignore unaligned head or tail, until block layer adds byte
> > +         * interface */
> > +        if (request.len >= BDRV_SECTOR_SIZE) {
> > +            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);
> 
> This can end up calling blk_co_discard(, , 0) - do we need to audit
> whether all underlying drivers handle a 0-length request, and/or
> special-case this in blk_co_discard() to be an explicit no-op?

Auditing shouldn't be too hard as we don't have many drivers that
support the operation in the first place. In any case, it might be
useful to add a qemu-iotest case that tries all kinds of operations with
a zero length.

Kevin

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