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

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Date:	Tue, 11 Oct 2011 14:39:09 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Li Dongyang <lidongyang@...ell.com>
Cc:	Jan Beulich <JBeulich@...e.com>, hch@...radead.org,
	Dong Yang Li <lidongyang@...e.com>,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] Re: [PATCH 2/3] xen/blkback: Fix the inhibition to
 map pages when discarding sector ranges.

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@...e.com> wrote:
> >>>> On 10.10.11 at 17:28, Konrad Rzeszutek Wilk <konrad.wilk@...cle.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@...ell.com>
> >> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.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.
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