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

[Mike Snitzer <snitzer@redhat.com>]

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