Re: [PATCH 3/4] dm: fix missing bio_split() pattern code in __split_and_process_bio()

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jan 21 2019 at  9:46pm -0500,
Ming Lei <ming.lei@redhat.com> wrote:

> On Mon, Jan 21, 2019 at 11:02:04AM -0500, Mike Snitzer wrote:
> > On Sun, Jan 20 2019 at 10:21P -0500,
> > Ming Lei <ming.lei@redhat.com> wrote:
> > 
> > > On Sat, Jan 19, 2019 at 01:05:05PM -0500, Mike Snitzer wrote:
> > > > Use the same BIO_QUEUE_ENTERED pattern that was established by commit
> > > > cd4a4ae4683dc ("block: don't use blocking queue entered for recursive
> > > > bio submits") by setting BIO_QUEUE_ENTERED after bio_split() and before
> > > > recursing via generic_make_request().
> > > > 
> > > > Also add trace_block_split() because it provides useful context about
> > > > bio splits in blktrace.
> > > > 
> > > > Depends-on: cd4a4ae4683dc ("block: don't use blocking queue entered for recursive bio submits")
> > > > Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk")
> > > > Cc: stable@vger.kernel.org # 4.16+
> > > > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > > > ---
> > > >  drivers/md/dm.c | 2 ++
> > > >  1 file changed, 2 insertions(+)
> > > > 
> > > > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > > > index fbadda68e23b..6e29c2d99b99 100644
> > > > --- a/drivers/md/dm.c
> > > > +++ b/drivers/md/dm.c
> > > > @@ -1654,7 +1654,9 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md,
> > > >  						   sectors[op_stat_group(bio_op(bio))], ci.sector_count);
> > > >  				part_stat_unlock();
> > > >  
> > > > +				bio_set_flag(bio, BIO_QUEUE_ENTERED);
> > > >  				bio_chain(b, bio);
> > > > +				trace_block_split(md->queue, b, bio->bi_iter.bi_sector);
> > > >  				ret = generic_make_request(bio);
> > > >  				break;
> > > >  			}
> > > 
> > > In theory, BIO_QUEUE_ENTERED is only required when __split_and_process_bio() is
> > > called from generic_make_request(). However, it may be called from dm_wq_work(),
> > > this way might cause trouble on operation to q->q_usage_counter.
> > 
> > Good point, I've tweaked this patch to clear BIO_QUEUE_ENTERED in
> > dm_make_request().
> > 
> > And to Neil's point: yes, these changes really do need to made
> > common since it appears all bio_split() callers do go on to call
> > generic_make_request().
> > 
> > Anyway, here is the updated patch that is now staged in linux-next:
> > 
> > From: Mike Snitzer <snitzer@redhat.com>
> > Date: Fri, 18 Jan 2019 01:21:11 -0500
> > Subject: [PATCH v2] dm: fix missing bio_split() pattern code in __split_and_process_bio()
> > 
> > Use the same BIO_QUEUE_ENTERED pattern that was established by commit
> > cd4a4ae4683dc ("block: don't use blocking queue entered for recursive
> > bio submits") by setting BIO_QUEUE_ENTERED after bio_split() and before
> > recursing via generic_make_request().
> > 
> > Also add trace_block_split() because it provides useful context about
> > bio splits in blktrace.
> > 
> > Depends-on: cd4a4ae4683dc ("block: don't use blocking queue entered for recursive bio submits")
> > Fixes: 18a25da84354 ("dm: ensure bio submission follows a depth-first tree walk")
> > Cc: stable@vger.kernel.org # 4.16+
> > Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> > ---
> >  drivers/md/dm.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> > index fbadda68e23b..25884f833a32 100644
> > --- a/drivers/md/dm.c
> > +++ b/drivers/md/dm.c
> > @@ -1654,7 +1654,9 @@ static blk_qc_t __split_and_process_bio(struct mapped_device *md,
> >  						   sectors[op_stat_group(bio_op(bio))], ci.sector_count);
> >  				part_stat_unlock();
> >  
> > +				bio_set_flag(bio, BIO_QUEUE_ENTERED);
> >  				bio_chain(b, bio);
> > +				trace_block_split(md->queue, b, bio->bi_iter.bi_sector);
> >  				ret = generic_make_request(bio);
> >  				break;
> >  			}
> > @@ -1734,6 +1736,13 @@ static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
> >  
> >  	map = dm_get_live_table(md, &srcu_idx);
> >  
> > +	/*
> > +	 * Clear the bio-reentered-generic_make_request() flag,
> > +	 * will be set again as needed if bio needs to be split.
> > +	 */
> > +	if (bio_flagged(bio, BIO_QUEUE_ENTERED))
> > +		bio_clear_flag(bio, BIO_QUEUE_ENTERED);
> > +
> >  	/* if we're suspended, we have to queue this io for later */
> >  	if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
> >  		dm_put_live_table(md, srcu_idx);
> > -- 
> > 2.15.0
> > 
> 
> Hi Mike,
> 
> I'd suggest to fix this kind issue in the following way, then we
> can avoid to touch this flag from drivers:
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 3c5f61ceeb67..e70103560ac2 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1024,6 +1024,8 @@ blk_qc_t generic_make_request(struct bio *bio)
>  		else
>  			bio_io_error(bio);
>  		return ret;
> +	} else {
> +		bio_set_flag(bio, BIO_QUEUE_ENTERED);
>  	}
>  
>  	if (!generic_make_request_checks(bio))
> @@ -1074,6 +1076,8 @@ blk_qc_t generic_make_request(struct bio *bio)
>  			if (blk_queue_enter(q, flags) < 0) {
>  				enter_succeeded = false;
>  				q = NULL;
> +			} else {
> +				bio_set_flag(bio, BIO_QUEUE_ENTERED);
>  			}
>  		}
>  
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index b990853f6de7..8777e286bd3f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -339,16 +339,6 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
>  		/* there isn't chance to merge the splitted bio */
>  		split->bi_opf |= REQ_NOMERGE;
>  
> -		/*
> -		 * Since we're recursing into make_request here, ensure
> -		 * that we mark this bio as already having entered the queue.
> -		 * If not, and the queue is going away, we can get stuck
> -		 * forever on waiting for the queue reference to drop. But
> -		 * that will never happen, as we're already holding a
> -		 * reference to it.
> -		 */
> -		bio_set_flag(*bio, BIO_QUEUE_ENTERED);
> -
>  		bio_chain(split, *bio);
>  		trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
>  		generic_make_request(*bio);
> 

Not opposed to this.

Thanks,
Mike