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

[David Jeffery <djeffery@redhat.com>]

On Tue, Feb 02, 2021 at 11:33:43AM +0800, Ming Lei wrote:
> 
> 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:
> 
> REQ_OP_SECURE_ERASE needs to be covered since block layer treats
> the two in very similar way from discard viewpoint.
> 
> Also single range discard should be fixed too, since block layer
> thinks single-range discard req segment is 1. Otherwise, the warning in
> virtblk_setup_discard_write_zeroes() still may be triggered, at least.
> 
> 
> -- 
> Ming
>

The return 0 does seem to be an old relic that does not make sense anymore.
Moving REQ_OP_SECURE_ERASE to be with discard and removing the old return 0,
is this what you had in mind?

 
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 808768f6b174..68458aa01b05 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -383,8 +383,14 @@ unsigned int blk_recalc_rq_segments(struct request *rq)
 	switch (bio_op(rq->bio)) {
 	case REQ_OP_DISCARD:
 	case REQ_OP_SECURE_ERASE:
+		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_WRITE_ZEROES:
-		return 0;
 	case REQ_OP_WRITE_SAME:
 		return 1;
 	}

--
David Jeffery