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SubjectRe: [Xen-devel] Re: [PATCH 2/3] xen/blkback: Fix the inhibition to map pages when discarding sector ranges.
>>> On 11.10.11 at 22:50, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> On Tue, Oct 11, 2011 at 02:39:09PM -0400, Konrad Rzeszutek Wilk wrote:
>> On Tue, Oct 11, 2011 at 03:33:11PM +0800, Li Dongyang wrote:
>> > On Tue, Oct 11, 2011 at 3:25 PM, Jan Beulich <JBeulich@suse.com> wrote:
>> > >>>> On 10.10.11 at 17:28, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>> > >> The 'operation' parameters are the ones provided to the bio layer while
>> > >> the req->operation are the ones passed in between the backend and
>> > >> frontend. We used the wrong 'operation' value to squash the
>> > >> call to map pages when processing the discard operation resulting
>> > >> in mapping the pages unnecessarily.
>> > >>
>> > >> CC: Li Dongyang <lidongyang@novell.com>
>> > >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> > >> ---
>> > >> drivers/block/xen-blkback/blkback.c | 2 +-
>> > >> 1 files changed, 1 insertions(+), 1 deletions(-)
>> > >>
>> > >> diff --git a/drivers/block/xen-blkback/blkback.c
>> > >> b/drivers/block/xen-blkback/blkback.c
>> > >> index 184b133..3da9a40 100644
>> > >> --- a/drivers/block/xen-blkback/blkback.c
>> > >> +++ b/drivers/block/xen-blkback/blkback.c
>> > >> @@ -707,7 +707,7 @@ static int dispatch_rw_block_io(struct xen_blkif
> *blkif,
>> > >> * the hypercall to unmap the grants - that is all done in
>> > >> * xen_blkbk_unmap.
>> > >> */
>> > >> - if (operation != BLKIF_OP_DISCARD &&
>> > >> + if (operation != REQ_DISCARD &&
>> > >
>> > > Why is that check necessary in the first place? xen_blkbk_map() doesn't
>> > > do any harm when req->nr_segments is zero (as could also be the case
>> > > on WRITE_FLUSH ones).
>> > >
>> > Ah, you are right, we could remove this check then, Thanks
>>
>> Except that req->nr_segments for blkif__request_discard is actually
>> the reserved field.
>>
>> See:
>>
>> struct blkif_request {
>> uint8_t operation; /* BLKIF_OP_??? */
>> uint8_t nr_segments; /* number of segments */
>> blkif_vdev_t handle; /* only for read/write requests */
>> .. snip..
>>
>> and:
>> struct blkif_request_discard {
>> uint8_t operation; /* BLKIF_OP_DISCARD */
>> /* ignored if 'discard-secure=0' */
>> #define BLKIF_OP_DISCARD_FLAG_SECURE (1<<0)
>> uint8_t flag; /* BLKIF_OP_DISCARD_FLAG_SECURE or 0 */
>> blkif_vdev_t handle; /* same as for read/write requests */
>>
>> which will throw off the logic for nr_segments all wrong since for some
>> discard operations it would read the nr_segments as 1.
>>
>> So we do need some logic in there to work with this.
>
>
> So a patch like this (and there is another on top that moves the setting
> of nseg) should do it.

With that later patch, you should then probably also check that none
of the so far unassigned bits in u1.flag are being set.

Jan

> commit 12679b29b2f828454f833e17e9090ed576c63afc
> Author: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Date: Mon Oct 10 00:47:49 2011 -0400
>
> xen/blkback: Fix the inhibition to map pages when discarding sector
> ranges.
>
> The 'operation' parameters are the ones provided to the bio layer while
> the req->operation are the ones passed in between the backend and
> frontend. We used the wrong 'operation' value to squash the
> call to map pages when processing the discard operation resulting
> in an hypercall that did nothing. Lets guard against going in the
> mapping function by checking for the amount of segments.
>
> CC: Li Dongyang <lidongyang@novell.com>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>
> diff --git a/drivers/block/xen-blkback/blkback.c
> b/drivers/block/xen-blkback/blkback.c
> index c15c559..94e659d 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -707,8 +707,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> * the hypercall to unmap the grants - that is all done in
> * xen_blkbk_unmap.
> */
> - if (operation != BLKIF_OP_DISCARD &&
> - xen_blkbk_map(req, pending_req, seg))
> + if (nseg && xen_blkbk_map(req, pending_req, seg))
> goto fail_flush;
>
> /*




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