RE: [PATCH 16/22] xen-blkfront: Make use of the new sg_map helper function

[David Laight <David.Laight@ACULAB.COM>]

From: Logan Gunthorpe
> Sent: 13 April 2017 23:05
> Straightforward conversion to the new helper, except due to
> the lack of error path, we have to warn if unmapable memory
> is ever present in the sgl.
> 
> Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
> ---
>  drivers/block/xen-blkfront.c | 33 +++++++++++++++++++++++++++------
>  1 file changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 5067a0a..7dcf41d 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -807,8 +807,19 @@ 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, SG_KMAP_ATOMIC);
> +			if (IS_ERR(setup.bvec_data)) {
> +				/*
> +				 * This should really never happen unless
> +				 * the code is changed to use memory that is
> +				 * not mappable in the sg. Seeing there is a
> +				 * questionable error path out of here,
> +				 * we WARN.
> +				 */
> +				WARN(1, "Non-mappable memory used in sg!");
> +				return 1;
> +			}
...

Perhaps add a flag to mark failure as 'unexpected' and trace (and panic?)
inside sg_map().

	David