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

Ming Lei
2019-01-22T02:47:05Z

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);

Thanks,
Ming