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

[Kevin Wolf]

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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