[PATCH] nvme-pci: fix single segment detection

[ming.lei@redhat.com (Ming Lei)]

On Thu, May 09, 2019@01:04:09PM +0200, Christoph Hellwig wrote:
> blk_rq_nr_phys_segments isn't exactly accurate at the request/bio
> level, so work around that fact with a few crude sanity checks.  To
> be fixed for real in the block layer soon.
> 
> Fixes: 297910571f08 ("nvme-pci: optimize mapping single segment requests using SGLs")
> Signed-off-by: Christoph Hellwig <hch at lst.de>
> ---
>  drivers/nvme/host/pci.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/nvme/host/pci.c b/drivers/nvme/host/pci.c
> index 2a8708c9ac18..9a4253be2723 100644
> --- a/drivers/nvme/host/pci.c
> +++ b/drivers/nvme/host/pci.c
> @@ -824,7 +824,15 @@ static blk_status_t nvme_map_data(struct nvme_dev *dev, struct request *req,
>  	blk_status_t ret = BLK_STS_RESOURCE;
>  	int nr_mapped;
>  
> -	if (blk_rq_nr_phys_segments(req) == 1) {
> +	/*
> +	 * Crude hack to work around the block layer accounting for the fact
> +	 * that the SG mapping can merge multiple physically contigous segments
> +	 * in blk_rq_nr_phys_segments() despite the fact that not everyone is
> +	 * mapping the request to a scatterlist.  To be fixed for real in the
> +	 * block layer eventually..
> +	 */
> +	if (blk_rq_nr_phys_segments(req) == 1 &&
> +	    !req->bio->bi_next && req->bio->bi_vcnt == 1) {
>  		struct bio_vec bv = req_bvec(req);

I'd suggest to fix block layer instead of working around the issue here,
then any driver may benefit from the fix.

Especially checking bio->bi_vcnt is just a hack, and drivers should
never use .bi_vcnt.


Thanks,
Ming