Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper function

[Jason Gunthorpe <jgunthorpe@obsidianresearch.com>]

On Thu, Apr 27, 2017 at 02:19:24PM -0600, Logan Gunthorpe wrote:
> 
> 
> On 26/04/17 01:37 AM, Roger Pau Monné wrote:
> > On Tue, Apr 25, 2017 at 12:21:02PM -0600, Logan Gunthorpe wrote:
> >> Straightforward conversion to the new helper, except due to the lack
> >> of error path, we have to use SG_MAP_MUST_NOT_FAIL which may BUG_ON in
> >> certain cases in the future.
> >>
> >> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> >> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> >> Cc: Juergen Gross <jgross@suse.com>
> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> >> Cc: "Roger Pau Monné" <roger.pau@citrix.com>
> >>  drivers/block/xen-blkfront.c | 20 +++++++++++---------
> >>  1 file changed, 11 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> >> index 3945963..ed62175 100644
> >> +++ b/drivers/block/xen-blkfront.c
> >> @@ -816,8 +816,9 @@ static int blkif_queue_rw_req(struct request *req, struct blkfront_ring_info *ri
> >>  		BUG_ON(sg->offset + sg->length > PAGE_SIZE);
> >>  
> >>  		if (setup.need_copy) {
> >> -			setup.bvec_off = sg->offset;
> >> -			setup.bvec_data = kmap_atomic(sg_page(sg));
> >> +			setup.bvec_off = 0;
> >> +			setup.bvec_data = sg_map(sg, 0, SG_KMAP_ATOMIC |
> >> +						 SG_MAP_MUST_NOT_FAIL);
> > 
> > I assume that sg_map already adds sg->offset to the address?
> 
> Correct.
> 
> > Also wondering whether we can get rid of bvec_off and just increment bvec_data,
> > adding Julien who IIRC added this code.
> 
> bvec_off is used to keep track of the offset within the current mapping
> so it's not a great idea given that you'd want to kunmap_atomic the
> original address and not something with an offset. It would be nice if
> this could be converted to use the sg_miter interface but that's a much
> more invasive change that would require someone who knows this code and
> can properly test it. I'd be very grateful if someone actually took that on.

blkfront is one of the drivers I looked at, and it appears to only be
memcpying with the bvec_data pointer, so I wonder why it does not use
sg_copy_X_buffer instead..

Jason