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

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

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.

Thanks,
Ming