Re: [PATCH v2 4/4] dm: unconditionally call blk_queue_split() in dm_process_bio()

[Mike Snitzer <snitzer@redhat.com>]

On Tue, Sep 15 2020 at  9:08pm -0400,
Ming Lei <ming.lei@redhat.com> wrote:

> On Tue, Sep 15, 2020 at 01:23:57PM -0400, Mike Snitzer wrote:
> > blk_queue_split() has become compulsory from .submit_bio -- regardless
> > of whether it is recursing.  Update DM core to always call
> > blk_queue_split().
> > 
> > dm_queue_split() is removed because __split_and_process_bio() handles
> > splitting as needed.
> > 
> > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > ---
> >  drivers/md/dm.c | 45 +--------------------------------------------
> >  1 file changed, 1 insertion(+), 44 deletions(-)
> > 
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index fb0255d25e4b..0bae9f26dc8e 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -1530,22 +1530,6 @@ static int __send_write_zeroes(struct clone_info *ci, struct dm_target *ti)
> >  	return __send_changing_extent_only(ci, ti, get_num_write_zeroes_bios(ti));
> >  }
> >  
> > -static bool is_abnormal_io(struct bio *bio)
> > -{
> > -	bool r = false;
> > -
> > -	switch (bio_op(bio)) {
> > -	case REQ_OP_DISCARD:
> > -	case REQ_OP_SECURE_ERASE:
> > -	case REQ_OP_WRITE_SAME:
> > -	case REQ_OP_WRITE_ZEROES:
> > -		r = true;
> > -		break;
> > -	}
> > -
> > -	return r;
> > -}
> > -
> >  static bool __process_abnormal_io(struct clone_info *ci, struct dm_target *ti,
> >  				  int *result)
> >  {
> > @@ -1723,23 +1707,6 @@ static blk_qc_t __process_bio(struct mapped_device *md, struct dm_table *map,
> >  	return ret;
> >  }
> >  
> > -static void dm_queue_split(struct mapped_device *md, struct dm_target *ti, struct bio **bio)
> > -{
> > -	unsigned len, sector_count;
> > -
> > -	sector_count = bio_sectors(*bio);
> > -	len = min_t(sector_t, max_io_len((*bio)->bi_iter.bi_sector, ti), sector_count);
> > -
> > -	if (sector_count > len) {
> > -		struct bio *split = bio_split(*bio, len, GFP_NOIO, &md->queue->bio_split);
> > -
> > -		bio_chain(split, *bio);
> > -		trace_block_split(md->queue, split, (*bio)->bi_iter.bi_sector);
> > -		submit_bio_noacct(*bio);
> > -		*bio = split;
> > -	}
> > -}
> > -
> >  static blk_qc_t dm_process_bio(struct mapped_device *md,
> >  			       struct dm_table *map, struct bio *bio)
> >  {
> > @@ -1759,17 +1726,7 @@ static blk_qc_t dm_process_bio(struct mapped_device *md,
> >  		}
> >  	}
> >  
> > -	/*
> > -	 * If in ->queue_bio we need to use blk_queue_split(), otherwise
> > -	 * queue_limits for abnormal requests (e.g. discard, writesame, etc)
> > -	 * won't be imposed.
> > -	 */
> > -	if (current->bio_list) {
> > -		if (is_abnormal_io(bio))
> > -			blk_queue_split(&bio);
> > -		else
> > -			dm_queue_split(md, ti, &bio);
> > -	}
> > +	blk_queue_split(&bio);
> 
> In max_io_len(), target boundary is taken into account when figuring out
> the max io len. However, this info won't be used any more after
> switching to blk_queue_split(). Is that one potential problem?

Thanks for your review.  But no, as the patch header says:
"dm_queue_split() is removed because __split_and_process_bio() handles
splitting as needed."

(__split_and_process_non_flush calls max_io_len, as does
__process_abnormal_io by calling __send_changing_extent_only)

SO the blk_queue_split() bio will be further split if needed (due to
DM target boundary, etc).

Mike