Re: [Qemu-devel] [PATCH] xen_disk: convert discard input to byte ranges

[Eric Blake <eblake@redhat.com>]

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On 11/18/2016 04:24 AM, Olaf Hering wrote:
> The guest sends discard requests as u64 sector/count pairs, but the
> block layer operates internally with s64/s32 pairs. The conversion
> leads to IO errors in the guest, the discard request is not processed.

Doesn't the block layer already split discard requests into 2^31 byte
chunks?

> 
>   domU.cfg:
>   'vdev=xvda, format=qcow2, backendtype=qdisk, target=/x.qcow2'
>   domU:
>   mkfs.ext4 -F /dev/xvda
>   Discarding device blocks: failed - Input/output error
> 
> Fix this by splitting the request into chunks of BDRV_REQUEST_MAX_SECTORS.
> Add input range checking to avoid overflow.
> 
> Signed-off-by: Olaf Hering <olaf@aepfle.de>
> ---
>  hw/block/xen_disk.c | 45 +++++++++++++++++++++++++++++++++++++++------
>  1 file changed, 39 insertions(+), 6 deletions(-)
> 

> @@ -708,12 +743,10 @@ static int ioreq_runio_qemu_aio(struct ioreq *ioreq)
>          break;
>      case BLKIF_OP_DISCARD:
>      {
> -        struct blkif_request_discard *discard_req = (void *)&ioreq->req;
> -        ioreq->aio_inflight++;
> -        blk_aio_pdiscard(blkdev->blk,

That is, blk_aio_pdiscard() calls into bdrv_co_pdiscard() which is
supposed to be fragmenting things as needed.  Can you trace what is
going wrong there?  You shouldn't have to reimplement fragementation if
the block layer is doing it correctly.

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


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