Re: Patch "dm: ensure bio submission follows a depth-first tree walk" has been added to the 4.14-stable tree

[Mike Snitzer <snitzer@redhat.com>]

Please see my reply to:
 Patch "dm: ensure bio submission follows a depth-first tree walk" has
 been added to the 4.15-stable tree

NAK

On Thu, Mar 22 2018 at  9:29am -0400,
gregkh@linuxfoundation.org <gregkh@linuxfoundation.org> wrote:

> 
> This is a note to let you know that I've just added the patch titled
> 
>     dm: ensure bio submission follows a depth-first tree walk
> 
> to the 4.14-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      dm-ensure-bio-submission-follows-a-depth-first-tree-walk.patch
> and it can be found in the queue-4.14 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.
> 
> 
> From foo@baz Thu Mar 22 14:26:48 CET 2018
> From: NeilBrown <neilb@suse.com>
> Date: Wed, 6 Sep 2017 09:43:28 +1000
> Subject: dm: ensure bio submission follows a depth-first tree walk
> 
> From: NeilBrown <neilb@suse.com>
> 
> 
> [ Upstream commit 18a25da84354c6bb655320de6072c00eda6eb602 ]
> 
> A dm device can, in general, represent a tree of targets, each of which
> handles a sub-range of the range of blocks handled by the parent.
> 
> The bio sequencing managed by generic_make_request() requires that bios
> are generated and handled in a depth-first manner.  Each call to a
> make_request_fn() may submit bios to a single member device, and may
> submit bios for a reduced region of the same device as the
> make_request_fn.
> 
> In particular, any bios submitted to member devices must be expected to
> be processed in order, so a later one must never wait for an earlier
> one.
> 
> This ordering is usually achieved by using bio_split() to reduce a bio
> to a size that can be completely handled by one target, and resubmitting
> the remainder to the originating device. bio_queue_split() shows the
> canonical approach.
> 
> dm doesn't follow this approach, largely because it has needed to split
> bios since long before bio_split() was available.  It currently can
> submit bios to separate targets within the one dm_make_request() call.
> Dependencies between these targets, as can happen with dm-snap, can
> cause deadlocks if either bios gets stuck behind the other in the queues
> managed by generic_make_request().  This requires the 'rescue'
> functionality provided by dm_offload_{start,end}.
> 
> Some of this requirement can be removed by changing the order of bio
> submission to follow the canonical approach.  That is, if dm finds that
> it needs to split a bio, the remainder should be sent to
> generic_make_request() rather than being handled immediately.  This
> delays the handling until the first part is completely processed, so the
> deadlock problems do not occur.
> 
> __split_and_process_bio() can be called both from dm_make_request() and
> from dm_wq_work().  When called from dm_wq_work() the current approach
> is perfectly satisfactory as each bio will be processed immediately.
> When called from dm_make_request(), current->bio_list will be non-NULL,
> and in this case it is best to create a separate "clone" bio for the
> remainder.
> 
> When we use bio_clone_bioset() to split off the front part of a bio
> and chain the two together and submit the remainder to
> generic_make_request(), it is important that the newly allocated
> bio is used as the head to be processed immediately, and the original
> bio gets "bio_advance()"d and sent to generic_make_request() as the
> remainder.  Otherwise, if the newly allocated bio is used as the
> remainder, and if it then needs to be split again, then the next
> bio_clone_bioset() call will be made while holding a reference a bio
> (result of the first clone) from the same bioset.  This can potentially
> exhaust the bioset mempool and result in a memory allocation deadlock.
> 
> Note that there is no race caused by reassigning cio.io->bio after already
> calling __map_bio().  This bio will only be dereferenced again after
> dec_pending() has found io->io_count to be zero, and this cannot happen
> before the dec_pending() call at the end of __split_and_process_bio().
> 
> To provide the clone bio when splitting, we use q->bio_split.  This
> was previously being freed by bio-based dm to avoid having excess
> rescuer threads.  As bio_split bio sets no longer create rescuer
> threads, there is little cost and much gain from restoring the
> q->bio_split bio set.
> 
> Signed-off-by: NeilBrown <neilb@suse.com>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  drivers/md/dm.c |   33 ++++++++++++++++++++++++---------
>  1 file changed, 24 insertions(+), 9 deletions(-)
> 
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1497,8 +1497,29 @@ static void __split_and_process_bio(stru
>  	} else {
>  		ci.bio = bio;
>  		ci.sector_count = bio_sectors(bio);
> -		while (ci.sector_count && !error)
> +		while (ci.sector_count && !error) {
>  			error = __split_and_process_non_flush(&ci);
> +			if (current->bio_list && ci.sector_count && !error) {
> +				/*
> +				 * Remainder must be passed to generic_make_request()
> +				 * so that it gets handled *after* bios already submitted
> +				 * have been completely processed.
> +				 * We take a clone of the original to store in
> +				 * ci.io->bio to be used by end_io_acct() and
> +				 * for dec_pending to use for completion handling.
> +				 * As this path is not used for REQ_OP_ZONE_REPORT,
> +				 * the usage of io->bio in dm_remap_zone_report()
> +				 * won't be affected by this reassignment.
> +				 */
> +				struct bio *b = bio_clone_bioset(bio, GFP_NOIO,
> +								 md->queue->bio_split);
> +				ci.io->bio = b;
> +				bio_advance(bio, (bio_sectors(bio) - ci.sector_count) << 9);
> +				bio_chain(b, bio);
> +				generic_make_request(bio);
> +				break;
> +			}
> +		}
>  	}
>  
>  	/* drop the extra reference count */
> @@ -1509,8 +1530,8 @@ static void __split_and_process_bio(stru
>   *---------------------------------------------------------------*/
>  
>  /*
> - * The request function that just remaps the bio built up by
> - * dm_merge_bvec.
> + * The request function that remaps the bio to one target and
> + * splits off any remainder.
>   */
>  static blk_qc_t dm_make_request(struct request_queue *q, struct bio *bio)
>  {
> @@ -2044,12 +2065,6 @@ int dm_setup_md_queue(struct mapped_devi
>  	case DM_TYPE_DAX_BIO_BASED:
>  		dm_init_normal_md_queue(md);
>  		blk_queue_make_request(md->queue, dm_make_request);
> -		/*
> -		 * DM handles splitting bios as needed.  Free the bio_split bioset
> -		 * since it won't be used (saves 1 process per bio-based DM device).
> -		 */
> -		bioset_free(md->queue->bio_split);
> -		md->queue->bio_split = NULL;
>  
>  		if (type == DM_TYPE_DAX_BIO_BASED)
>  			queue_flag_set_unlocked(QUEUE_FLAG_DAX, md->queue);
> 
> 
> Patches currently in stable-queue which might be from neilb@suse.com are
> 
> queue-4.14/dm-ensure-bio-submission-follows-a-depth-first-tree-walk.patch