Re: WARNING: CPU: 2 PID: 207 at drivers/nvme/host/core.c:527 nvme_setup_cmd+0x3d3

[Keith Busch <keith.busch@intel.com>]

On Wed, Jan 31, 2018 at 08:29:37AM -0700, Jens Axboe wrote:
> 
> How about something like the below?
> 
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 8452fc7164cc..cee102fb060e 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -574,8 +574,13 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
>  	    blk_rq_get_max_sectors(req, blk_rq_pos(req)))
>  		return 0;
>  
> +	/*
> +	 * For DISCARDs, the segment count isn't interesting since
> +	 * the requests have no data attached.
> +	 */
>  	total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
> -	if (blk_phys_contig_segment(q, req->biotail, next->bio)) {
> +	if (total_phys_segments &&
> +	    blk_phys_contig_segment(q, req->biotail, next->bio)) {
>  		if (req->nr_phys_segments == 1)
>  			req->bio->bi_seg_front_size = seg_size;
>  		if (next->nr_phys_segments == 1)

That'll keep it from going to 0xffff, but you'll still hit the warning and
IO error. Even worse, this will corrupt memory: blk_rq_nr_discard_segments
will return 1, and since you really had 2 segments, the nvme driver will
overrun its array.