Re: [PATCH] block: recalculate segment count for multi-segment discard requests correctly

[Ming Lei <ming.lei@redhat.com>]

On Mon, Feb 01, 2021 at 11:48:50AM -0500, David Jeffery wrote:
> When a stacked block device inserts a request into another block device
> using blk_insert_cloned_request, the request's nr_phys_segments field gets
> recalculated by a call to blk_recalc_rq_segments in
> blk_cloned_rq_check_limits. But blk_recalc_rq_segments does not know how to
> handle multi-segment discards. For disk types which can handle
> multi-segment discards like nvme, this results in discard requests which
> claim a single segment when it should report several, triggering a warning
> in nvme and causing nvme to fail the discard from the invalid state.
> 
>  WARNING: CPU: 5 PID: 191 at drivers/nvme/host/core.c:700 nvme_setup_discard+0x170/0x1e0 [nvme_core]
>  ...
>  nvme_setup_cmd+0x217/0x270 [nvme_core]
>  nvme_loop_queue_rq+0x51/0x1b0 [nvme_loop]
>  __blk_mq_try_issue_directly+0xe7/0x1b0
>  blk_mq_request_issue_directly+0x41/0x70
>  ? blk_account_io_start+0x40/0x50
>  dm_mq_queue_rq+0x200/0x3e0
>  blk_mq_dispatch_rq_list+0x10a/0x7d0
>  ? __sbitmap_queue_get+0x25/0x90
>  ? elv_rb_del+0x1f/0x30
>  ? deadline_remove_request+0x55/0xb0
>  ? dd_dispatch_request+0x181/0x210
>  __blk_mq_do_dispatch_sched+0x144/0x290
>  ? bio_attempt_discard_merge+0x134/0x1f0
>  __blk_mq_sched_dispatch_requests+0x129/0x180
>  blk_mq_sched_dispatch_requests+0x30/0x60
>  __blk_mq_run_hw_queue+0x47/0xe0
>  __blk_mq_delay_run_hw_queue+0x15b/0x170
>  blk_mq_sched_insert_requests+0x68/0xe0
>  blk_mq_flush_plug_list+0xf0/0x170
>  blk_finish_plug+0x36/0x50
>  xlog_cil_committed+0x19f/0x290 [xfs]
>  xlog_cil_process_committed+0x57/0x80 [xfs]
>  xlog_state_do_callback+0x1e0/0x2a0 [xfs]
>  xlog_ioend_work+0x2f/0x80 [xfs]
>  process_one_work+0x1b6/0x350
>  worker_thread+0x53/0x3e0
>  ? process_one_work+0x350/0x350
>  kthread+0x11b/0x140
>  ? __kthread_bind_mask+0x60/0x60
>  ret_from_fork+0x22/0x30
> 
> This patch fixes blk_recalc_rq_segments to be aware of devices which can
> have multi-segment discards. It calculates the correct discard segment
> count by counting the number of bio as each discard bio is considered its
> own segment.
> 
> Signed-off-by: David Jeffery <djeffery@redhat.com>
> Tested-by: Laurence Oberman <loberman@redhat.com>
> ---
>  block/blk-merge.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 808768f6b174..fe7358bd5d09 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -382,6 +382,13 @@ unsigned int blk_recalc_rq_segments(struct request *rq)
>  
>  	switch (bio_op(rq->bio)) {
>  	case REQ_OP_DISCARD:
> +		if (queue_max_discard_segments(rq->q) > 1) {
> +			struct bio *bio = rq->bio;
> +			for_each_bio(bio)
> +				nr_phys_segs++;
> +			return nr_phys_segs;
> +		}
> +		/* fall through */
>  	case REQ_OP_SECURE_ERASE:
>  	case REQ_OP_WRITE_ZEROES:
>  		return 0;

blk_rq_nr_discard_segments() always returns >=1 segments, so no similar
issue in case of single range discard.

Reviewed-by: Ming Lei <ming.lei@redhat.com>

And it can be thought as:

Fixes: 1e739730c5b9 ("block: optionally merge discontiguous discard bios into a single request")


-- 
Ming