Re: [PATCH] block: return ELEVATOR_DISCARD_MERGE if possible

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

On Thu, Jul 29, 2021 at 09:17:28AM +0200, Oleksandr Natalenko wrote:
> On čtvrtek 29. července 2021 5:42:26 CEST Ming Lei wrote:
> > When merging one bio to request, if they are discard IO and the queue
> > supports multi-range discard, we need to return ELEVATOR_DISCARD_MERGE
> > because both block core and related drivers(nvme, virtio-blk) doesn't
> > handle mixed discard io merge(traditional IO merge together with
> > discard merge) well.
> > 
> > Fix the issue by returning ELEVATOR_DISCARD_MERGE in this situation,
> > so both blk-mq and drivers just need to handle multi-range discard.
> > 
> > Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/bfq-iosched.c      |  3 +++
> >  block/blk-merge.c        | 16 ----------------
> >  block/elevator.c         |  3 +++
> >  block/mq-deadline-main.c |  2 ++
> >  include/linux/blkdev.h   | 16 ++++++++++++++++
> >  5 files changed, 24 insertions(+), 16 deletions(-)
> > 
> > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > index 727955918563..673a634eadd9 100644
> > --- a/block/bfq-iosched.c
> > +++ b/block/bfq-iosched.c
> > @@ -2361,6 +2361,9 @@ static int bfq_request_merge(struct request_queue *q,
> > struct request **req, __rq = bfq_find_rq_fmerge(bfqd, bio, q);
> >  	if (__rq && elv_bio_merge_ok(__rq, bio)) {
> >  		*req = __rq;
> > +
> > +		if (blk_discard_mergable(__rq))
> > +			return ELEVATOR_DISCARD_MERGE;
> >  		return ELEVATOR_FRONT_MERGE;
> >  	}
> > 
> > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > index a11b3b53717e..f8707ff7e2fc 100644
> > --- a/block/blk-merge.c
> > +++ b/block/blk-merge.c
> > @@ -705,22 +705,6 @@ static void blk_account_io_merge_request(struct request
> > *req) }
> >  }
> > 
> > -/*
> > - * Two cases of handling DISCARD merge:
> > - * If max_discard_segments > 1, the driver takes every bio
> > - * as a range and send them to controller together. The ranges
> > - * needn't to be contiguous.
> > - * Otherwise, the bios/requests will be handled as same as
> > - * others which should be contiguous.
> > - */
> > -static inline bool blk_discard_mergable(struct request *req)
> > -{
> > -	if (req_op(req) == REQ_OP_DISCARD &&
> > -	    queue_max_discard_segments(req->q) > 1)
> > -		return true;
> > -	return false;
> > -}
> > -
> >  static enum elv_merge blk_try_req_merge(struct request *req,
> >  					struct request *next)
> >  {
> > diff --git a/block/elevator.c b/block/elevator.c
> > index 52ada14cfe45..a5fe2615ec0f 100644
> > --- a/block/elevator.c
> > +++ b/block/elevator.c
> > @@ -336,6 +336,9 @@ enum elv_merge elv_merge(struct request_queue *q, struct
> > request **req, __rq = elv_rqhash_find(q, bio->bi_iter.bi_sector);
> >  	if (__rq && elv_bio_merge_ok(__rq, bio)) {
> >  		*req = __rq;
> > +
> > +		if (blk_discard_mergable(__rq))
> > +			return ELEVATOR_DISCARD_MERGE;
> >  		return ELEVATOR_BACK_MERGE;
> >  	}
> > 
> > diff --git a/block/mq-deadline-main.c b/block/mq-deadline-main.c
> > index 6f612e6dc82b..294be0c0db65 100644
> > --- a/block/mq-deadline-main.c
> > +++ b/block/mq-deadline-main.c
> > @@ -677,6 +677,8 @@ static int dd_request_merge(struct request_queue *q,
> > struct request **rq,
> > 
> >  		if (elv_bio_merge_ok(__rq, bio)) {
> >  			*rq = __rq;
> > +			if (blk_discard_mergable(__rq))
> > +				return ELEVATOR_DISCARD_MERGE;
> >  			return ELEVATOR_FRONT_MERGE;
> >  		}
> >  	}
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 3177181c4326..87f00292fd7a 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -1521,6 +1521,22 @@ static inline int
> > queue_limit_discard_alignment(struct queue_limits *lim, sector return
> > offset << SECTOR_SHIFT;
> >  }
> > 
> > +/*
> > + * Two cases of handling DISCARD merge:
> > + * If max_discard_segments > 1, the driver takes every bio
> > + * as a range and send them to controller together. The ranges
> > + * needn't to be contiguous.
> > + * Otherwise, the bios/requests will be handled as same as
> > + * others which should be contiguous.
> > + */
> > +static inline bool blk_discard_mergable(struct request *req)
> > +{
> > +	if (req_op(req) == REQ_OP_DISCARD &&
> > +	    queue_max_discard_segments(req->q) > 1)
> > +		return true;
> > +	return false;
> > +}
> > +
> >  static inline int bdev_discard_alignment(struct block_device *bdev)
> >  {
> >  	struct request_queue *q = bdev_get_queue(bdev);
> 
> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> 
> Also,
> 
> Fixes: ?

Fixes: 2705dfb20947 ("block: fix discard request merge")

In theory, the fixes should be 1e739730c5b9 ("block: optionally merge
discontiguous discard bios into a single request"). The reason why it
isn't be triggered before 2705dfb20947 is that the nr_segment of each
bio is exactly 1 for multi-range device.

> 
> and possibly:
> 
> CC: stable@ # v5.x?

CC: stable@ # v5.13


Thanks,
Ming