Re: [PATCH 2/2] block: support bio merge for multi-range discard

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

On Wed, Jun 09, 2021 at 11:05:59AM +0800, Wang Shanker wrote:
> 
> 
> > 2021年06月09日 08:45，Ming Lei <ming.lei@redhat.com> 写道：
> > 
> > So far multi-range discard treats each bio as one segment(range) of single
> > discard request. This way becomes not efficient if lots of small sized
> > discard bios are submitted, and one example is raid456.
> > 
> > Support bio merge for multi-range discard for improving lots of small
> > sized discard bios.
> > 
> > Turns out it is easy to support it:
> > 
> > 1) always try to merge bio first
> > 
> > 2) run into multi-range discard only if bio merge can't be done
> > 
> > 3) add rq_for_each_discard_range() for retrieving each range(segment)
> > of discard request
> > 
> > Reported-by: Wang Shanker <shankerwangmiao@gmail.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> > block/blk-merge.c          | 12 ++++-----
> > drivers/block/virtio_blk.c |  9 ++++---
> > drivers/nvme/host/core.c   |  8 +++---
> > include/linux/blkdev.h     | 51 ++++++++++++++++++++++++++++++++++++++
> > 4 files changed, 66 insertions(+), 14 deletions(-)
> > 
> > diff --git a/block/blk-merge.c b/block/blk-merge.c
> > index bcdff1879c34..65210e9a8efa 100644
> > --- a/block/blk-merge.c
> > +++ b/block/blk-merge.c
> > @@ -724,10 +724,10 @@ static inline bool blk_discard_mergable(struct request *req)
> > static enum elv_merge blk_try_req_merge(struct request *req,
> > 					struct request *next)
> > {
> > -	if (blk_discard_mergable(req))
> > -		return ELEVATOR_DISCARD_MERGE;
> > -	else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
> > +	if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
> > 		return ELEVATOR_BACK_MERGE;
> > +	else if (blk_discard_mergable(req))
> 
> Shall we adjust how req->nr_phys_segments is calculated in 
> bio_attempt_discard_merge() so that multiple contiguous bio's can
> be seen as one segment?

I think it isn't necessary, because we try to merge discard IOs first
just like plain IO. So when bio_attempt_discard_merge() is reached, it
means that IOs can't be merged, so req->nr_phys_segments should be
increased by 1.


Thanks,
Ming