Re: [Xen-devel] Re: [PATCH 2/3] xen/blkback: Fix the inhibition to map pages when discarding sector ranges.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 11.10.11 at 22:50, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 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@xxxxxxxx> wrote:
>> > >>>> On 10.10.11 at 17:28, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
>> > >>>> 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@xxxxxxxxxx>
>> > >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> > >> ---
>> > >>  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@xxxxxxxxxx>
> 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@xxxxxxxxxx>
>     Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> 
> 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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