linux-kernel - Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg

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Message-ID: <20170427205339.GB26330@obsidianresearch.com>
Date:   Thu, 27 Apr 2017 14:53:39 -0600
From:   Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     Roger Pau Monné <roger.pau@...rix.com>,
        linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, linux-raid@...r.kernel.org,
        linux-mmc@...r.kernel.org, linux-nvdimm@...1.01.org,
        linux-scsi@...r.kernel.org, open-iscsi@...glegroups.com,
        megaraidlinux.pdl@...adcom.com, sparmaintainer@...sys.com,
        devel@...verdev.osuosl.org, target-devel@...r.kernel.org,
        netdev@...r.kernel.org, linux-rdma@...r.kernel.org,
        dm-devel@...hat.com, Christoph Hellwig <hch@....de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        Jens Axboe <axboe@...nel.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Stephen Bates <sbates@...thlin.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Julien Grall <julien.grall@....com>
Subject: Re: [PATCH v2 15/21] xen-blkfront: Make use of the new sg_map helper
 function

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@...tatee.com>
> >> Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> >> Cc: Juergen Gross <jgross@...e.com>
> >> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> >> Cc: "Roger Pau Monné" <roger.pau@...rix.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