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

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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
>> --- a/drivers/block/xen-blkfront.c
>> +++ 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.

Logan

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[PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper function Logan Gunthorpe <logang@deltatee.com> - 2017-04-25 20:30 +0200
  Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Roger Pau Monné <roger.pau@citrix.com> - 2017-04-26 09:40 +0200
    Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Logan Gunthorpe <logang@deltatee.com> - 2017-04-27 22:30 +0200
      Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2017-04-27 23:00 +0200
        Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Logan Gunthorpe <logang@deltatee.com> - 2017-04-28 00:00 +0200
          Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2017-04-28 00:20 +0200
            Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Logan Gunthorpe <logang@deltatee.com> - 2017-04-28 01:10 +0200
              Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Jason Gunthorpe <jgunthorpe@obsidianresearch.com> - 2017-04-28 01:30 +0200
                Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper  function Logan Gunthorpe <logang@deltatee.com> - 2017-04-28 01:40 +0200

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