dm mpath: reinstate bio-based support

Add "multipath-bio" target that offers a bio-based multipath target as
an alternative to the request-based "multipath" target -- but in a
following commit "multipath-bio" will immediately be replaced by a new
"queue_mode" feature for the "multipath" target which will allow
bio-based mode to be selected.

When DM multipath was originally converted from bio-based to
request-based the motivation for the change was better dynamic load
balancing (by leveraging block core's request-based IO schedulers, for
merging and sorting, _before_ DM multipath would make the decision on
where to steer the IO -- based on path load and/or availability).

More background is available in this "Request-based Device-mapper
multipath and Dynamic load balancing" paper:
https://www.kernel.org/doc/ols/2007/ols2007v2-pages-235-244.pdf

But we've now come full circle where significantly faster storage
devices no longer need IOs to be made larger to drive optimal IO
performance.  And even if they do there have been changes to the block
and filesystem layers that help ensure upper layers are constructing
larger IOs.  In addition, SCSI's differentiated IO errors will propagate
through to bio-based IO completion hooks -- so that eliminates another
historic justiciation for request-based DM multipath.  Lastly, the block
layer's immutable biovec changes have made bio cloning cheaper than it
has ever been; whereas request cloning is still relatively expensive
(both on a CPU usage and memory footprint level).

As such, bio-based DM multipath offers the promise of a more efficient
IO path for high IOPs devices that are, or will be, emerging.
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -8,6 +8,7 @@
 #include <linux/device-mapper.h>
 
 #include "dm-rq.h"
+#include "dm-bio-record.h"
 #include "dm-path-selector.h"
 #include "dm-uevent.h"
 
@@ -97,14 +98,22 @@ struct multipath {
 
 	struct mutex work_mutex;
 	struct work_struct trigger_event;
+
+	struct work_struct process_queued_bios;
+	struct bio_list queued_bios;
 };
 
 /*
- * Context information attached to each bio we process.
+ * Context information attached to each io we process.
  */
 struct dm_mpath_io {
 	struct pgpath *pgpath;
 	size_t nr_bytes;
+
+	/*
+	 * FIXME: make request-based code _not_ include this member.
+	 */
+	struct dm_bio_details bio_details;
 };
 
 typedef int (*action_fn) (struct pgpath *pgpath);
@@ -114,6 +123,7 @@ static struct kmem_cache *_mpio_cache;
 static struct workqueue_struct *kmultipathd, *kmpath_handlerd;
 static void trigger_event(struct work_struct *work);
 static void activate_path(struct work_struct *work);
+static void process_queued_bios(struct work_struct *work);
 
 /*-----------------------------------------------
  * Multipath state flags.
@@ -126,6 +136,7 @@ static void activate_path(struct work_struct *work);
 #define MPATHF_PG_INIT_DISABLED 4		/* pg_init is not currently allowed */
 #define MPATHF_PG_INIT_REQUIRED 5		/* pg_init needs calling? */
 #define MPATHF_PG_INIT_DELAY_RETRY 6		/* Delay pg_init retry? */
+#define MPATHF_BIO_BASED 7			/* Device is bio-based? */
 
 /*-----------------------------------------------
  * Allocation routines
@@ -185,7 +196,8 @@ static void free_priority_group(struct priority_group *pg,
 	kfree(pg);
 }
 
-static struct multipath *alloc_multipath(struct dm_target *ti, bool use_blk_mq)
+static struct multipath *alloc_multipath(struct dm_target *ti, bool use_blk_mq,
+					 bool bio_based)
 {
 	struct multipath *m;
 
@@ -203,7 +215,7 @@ static struct multipath *alloc_multipath(struct dm_target *ti, bool use_blk_mq)
 		mutex_init(&m->work_mutex);
 
 		m->mpio_pool = NULL;
-		if (!use_blk_mq) {
+		if (!use_blk_mq && !bio_based) {
 			unsigned min_ios = dm_get_reserved_rq_based_ios();
 
 			m->mpio_pool = mempool_create_slab_pool(min_ios, _mpio_cache);
@@ -213,6 +225,16 @@ static struct multipath *alloc_multipath(struct dm_target *ti, bool use_blk_mq)
 			}
 		}
 
+		if (bio_based) {
+			INIT_WORK(&m->process_queued_bios, process_queued_bios);
+			set_bit(MPATHF_BIO_BASED, &m->flags);
+			/*
+			 * bio-based doesn't support any direct scsi_dh management;
+			 * it just discovers if a scsi_dh is attached.
+			 */
+			set_bit(MPATHF_RETAIN_ATTACHED_HW_HANDLER, &m->flags);
+		}
+
 		m->ti = ti;
 		ti->private = m;
 	}
@@ -272,6 +294,21 @@ static void clear_request_fn_mpio(struct multipath *m, union map_info *info)
 	}
 }
 
+static struct dm_mpath_io *get_mpio_from_bio(struct bio *bio)
+{
+	return dm_per_bio_data(bio, sizeof(struct dm_mpath_io));
+}
+
+static struct dm_mpath_io *set_mpio_bio(struct multipath *m, struct bio *bio)
+{
+	struct dm_mpath_io *mpio = get_mpio_from_bio(bio);
+
+	memset(mpio, 0, sizeof(*mpio));
+	dm_bio_record(&mpio->bio_details, bio);
+
+	return mpio;
+}
+
 /*-----------------------------------------------
  * Path selection
  *-----------------------------------------------*/
@@ -431,16 +468,26 @@ static struct pgpath *choose_pgpath(struct multipath *m, size_t nr_bytes)
  * and multipath_resume() calls and we have no need to check
  * for the DMF_NOFLUSH_SUSPENDING flag.
  */
-static int must_push_back(struct multipath *m)
+static bool __must_push_back(struct multipath *m)
+{
+	return ((test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) !=
+		 test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags)) &&
+		dm_noflush_suspending(m->ti));
+}
+
+static bool must_push_back_rq(struct multipath *m)
 {
 	return (test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) ||
-		((test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags) !=
-		  test_bit(MPATHF_SAVED_QUEUE_IF_NO_PATH, &m->flags)) &&
-		 dm_noflush_suspending(m->ti)));
+		__must_push_back(m));
+}
+
+static bool must_push_back_bio(struct multipath *m)
+{
+	return __must_push_back(m);
 }
 
 /*
- * Map cloned requests
+ * Map cloned requests (request-based multipath)
  */
 static int __multipath_map(struct dm_target *ti, struct request *clone,
 			   union map_info *map_context,
@@ -459,7 +506,7 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
 		pgpath = choose_pgpath(m, nr_bytes);
 
 	if (!pgpath) {
-		if (!must_push_back(m))
+		if (!must_push_back_rq(m))
 			r = -EIO;	/* Failed */
 		return r;
 	} else if (test_bit(MPATHF_QUEUE_IO, &m->flags) ||
@@ -529,6 +576,106 @@ static void multipath_release_clone(struct request *clone)
 	blk_mq_free_request(clone);
 }
 
+/*
+ * Map cloned bios (bio-based multipath)
+ */
+static int __multipath_map_bio(struct multipath *m, struct bio *bio, struct dm_mpath_io *mpio)
+{
+	size_t nr_bytes = bio->bi_iter.bi_size;
+	struct pgpath *pgpath;
+	unsigned long flags;
+	bool queue_io;
+
+	/* Do we need to select a new pgpath? */
+	pgpath = lockless_dereference(m->current_pgpath);
+	queue_io = test_bit(MPATHF_QUEUE_IO, &m->flags);
+	if (!pgpath || !queue_io)
+		pgpath = choose_pgpath(m, nr_bytes);
+
+	if ((pgpath && queue_io) ||
+	    (!pgpath && test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags))) {
+		/* Queue for the daemon to resubmit */
+		spin_lock_irqsave(&m->lock, flags);
+		bio_list_add(&m->queued_bios, bio);
+		spin_unlock_irqrestore(&m->lock, flags);
+		/* PG_INIT_REQUIRED cannot be set without QUEUE_IO */
+		if (queue_io || test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
+			pg_init_all_paths(m);
+		else if (!queue_io)
+			queue_work(kmultipathd, &m->process_queued_bios);
+		return DM_MAPIO_SUBMITTED;
+	}
+
+	if (!pgpath) {
+		if (!must_push_back_bio(m))
+			return -EIO;
+		return DM_MAPIO_REQUEUE;
+	}
+
+	mpio->pgpath = pgpath;
+	mpio->nr_bytes = nr_bytes;
+
+	bio->bi_error = 0;
+	bio->bi_bdev = pgpath->path.dev->bdev;
+	bio->bi_rw |= REQ_FAILFAST_TRANSPORT;
+
+	if (pgpath->pg->ps.type->start_io)
+		pgpath->pg->ps.type->start_io(&pgpath->pg->ps,
+					      &pgpath->path,
+					      nr_bytes);
+	return DM_MAPIO_REMAPPED;
+}
+
+static int multipath_map_bio(struct dm_target *ti, struct bio *bio)
+{
+	struct multipath *m = ti->private;
+	struct dm_mpath_io *mpio = set_mpio_bio(m, bio);
+
+	return __multipath_map_bio(m, bio, mpio);
+}
+
+static void process_queued_bios_list(struct multipath *m)
+{
+	if (test_bit(MPATHF_BIO_BASED, &m->flags))
+		queue_work(kmultipathd, &m->process_queued_bios);
+}
+
+static void process_queued_bios(struct work_struct *work)
+{
+	int r;
+	unsigned long flags;
+	struct bio *bio;
+	struct bio_list bios;
+	struct blk_plug plug;
+	struct multipath *m =
+		container_of(work, struct multipath, process_queued_bios);
+
+	bio_list_init(&bios);
+
+	spin_lock_irqsave(&m->lock, flags);
+
+	if (bio_list_empty(&m->queued_bios)) {
+		spin_unlock_irqrestore(&m->lock, flags);
+		return;
+	}
+
+	bio_list_merge(&bios, &m->queued_bios);
+	bio_list_init(&m->queued_bios);
+
+	spin_unlock_irqrestore(&m->lock, flags);
+
+	blk_start_plug(&plug);
+	while ((bio = bio_list_pop(&bios))) {
+		r = __multipath_map_bio(m, bio, get_mpio_from_bio(bio));
+		if (r < 0 || r == DM_MAPIO_REQUEUE) {
+			bio->bi_error = r;
+			bio_endio(bio);
+		} else if (r == DM_MAPIO_REMAPPED)
+			generic_make_request(bio);
+	}
+	blk_finish_plug(&plug);
+}
+
 /*
  * If we run out of usable paths, should we queue I/O or error it?
  */
@@ -557,8 +704,10 @@ static int queue_if_no_path(struct multipath *m, bool queue_if_no_path,
 
 	spin_unlock_irqrestore(&m->lock, flags);
 
-	if (!queue_if_no_path)
+	if (!queue_if_no_path) {
 		dm_table_run_md_queue_async(m->ti->table);
+		process_queued_bios_list(m);
+	}
 
 	return 0;
 }
@@ -798,6 +947,12 @@ static int parse_hw_handler(struct dm_arg_set *as, struct multipath *m)
 	if (!hw_argc)
 		return 0;
 
+	if (test_bit(MPATHF_BIO_BASED, &m->flags)) {
+		dm_consume_args(as, hw_argc);
+		DMERR("bio-based multipath doesn't allow hardware handler args");
+		return 0;
+	}
+
 	m->hw_handler_name = kstrdup(dm_shift_arg(as), GFP_KERNEL);
 
 	if (hw_argc > 1) {
@@ -880,8 +1035,8 @@ static int parse_features(struct dm_arg_set *as, struct multipath *m)
 	return r;
 }
 
-static int multipath_ctr(struct dm_target *ti, unsigned int argc,
-			 char **argv)
+static int __multipath_ctr(struct dm_target *ti, unsigned int argc,
+			   char **argv, bool bio_based)
 {
 	/* target arguments */
 	static struct dm_arg _args[] = {
@@ -899,7 +1054,7 @@ static int multipath_ctr(struct dm_target *ti, unsigned int argc,
 	as.argc = argc;
 	as.argv = argv;
 
-	m = alloc_multipath(ti, use_blk_mq);
+	m = alloc_multipath(ti, use_blk_mq, bio_based);
 	if (!m) {
 		ti->error = "can't allocate multipath";
 		return -EINVAL;
@@ -958,7 +1113,7 @@ static int multipath_ctr(struct dm_target *ti, unsigned int argc,
 	ti->num_flush_bios = 1;
 	ti->num_discard_bios = 1;
 	ti->num_write_same_bios = 1;
-	if (use_blk_mq)
+	if (use_blk_mq || bio_based)
 		ti->per_io_data_size = sizeof(struct dm_mpath_io);
 
 	return 0;
@@ -968,6 +1123,16 @@ static int multipath_ctr(struct dm_target *ti, unsigned int argc,
 	return r;
 }
 
+static int multipath_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+	return __multipath_ctr(ti, argc, argv, false);
+}
+
+static int multipath_bio_ctr(struct dm_target *ti, unsigned argc, char **argv)
+{
+	return __multipath_ctr(ti, argc, argv, true);
+}
+
 static void multipath_wait_for_pg_init_completion(struct multipath *m)
 {
 	DECLARE_WAITQUEUE(wait, current);
@@ -1083,8 +1248,10 @@ static int reinstate_path(struct pgpath *pgpath)
 
 out:
 	spin_unlock_irqrestore(&m->lock, flags);
-	if (run_queue)
+	if (run_queue) {
 		dm_table_run_md_queue_async(m->ti->table);
+		process_queued_bios_list(m);
+	}
 
 	return r;
 }
@@ -1281,6 +1448,8 @@ static void pg_init_done(void *data, int errors)
 	}
 	clear_bit(MPATHF_QUEUE_IO, &m->flags);
 
+	process_queued_bios_list(m);
+
 	/*
 	 * Wake up any thread waiting to suspend.
 	 */
@@ -1347,7 +1516,7 @@ static int do_end_io(struct multipath *m, struct request *clone,
 
 	if (!atomic_read(&m->nr_valid_paths)) {
 		if (!test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
-			if (!must_push_back(m))
+			if (!must_push_back_rq(m))
 				r = -EIO;
 		} else {
 			if (error == -EBADE)
@@ -1381,6 +1550,64 @@ static int multipath_end_io(struct dm_target *ti, struct request *clone,
 	return r;
 }
 
+static int do_end_io_bio(struct multipath *m, struct bio *clone,
+			 int error, struct dm_mpath_io *mpio)
+{
+	unsigned long flags;
+
+	if (!error)
+		return 0;	/* I/O complete */
+
+	if (noretry_error(error))
+		return error;
+
+	if (mpio->pgpath)
+		fail_path(mpio->pgpath);
+
+	if (!atomic_read(&m->nr_valid_paths)) {
+		if (!test_bit(MPATHF_QUEUE_IF_NO_PATH, &m->flags)) {
+			if (!must_push_back_bio(m))
+				return -EIO;
+			return DM_ENDIO_REQUEUE;
+		} else {
+			if (error == -EBADE)
+				return error;
+		}
+	}
+
+	/* Queue for the daemon to resubmit */
+	dm_bio_restore(&mpio->bio_details, clone);
+
+	spin_lock_irqsave(&m->lock, flags);
+	bio_list_add(&m->queued_bios, clone);
+	spin_unlock_irqrestore(&m->lock, flags);
+	if (!test_bit(MPATHF_QUEUE_IO, &m->flags))
+		queue_work(kmultipathd, &m->process_queued_bios);
+
+	return DM_ENDIO_INCOMPLETE;
+}
+
+static int multipath_end_io_bio(struct dm_target *ti, struct bio *clone, int error)
+{
+	struct multipath *m = ti->private;
+	struct dm_mpath_io *mpio = get_mpio_from_bio(clone);
+	struct pgpath *pgpath;
+	struct path_selector *ps;
+	int r;
+
+	BUG_ON(!mpio);
+
+	r = do_end_io_bio(m, clone, error, mpio);
+	pgpath = mpio->pgpath;
+	if (pgpath) {
+		ps = &pgpath->pg->ps;
+		if (ps->type->end_io)
+			ps->type->end_io(ps, &pgpath->path, mpio->nr_bytes);
+	}
+
+	return r;
+}
+
 /*
  * Suspend can't complete until all the I/O is processed so if
  * the last path fails we must error any remaining I/O.
@@ -1642,6 +1869,7 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
 		if (test_bit(MPATHF_PG_INIT_REQUIRED, &m->flags))
 			pg_init_all_paths(m);
 		dm_table_run_md_queue_async(m->ti->table);
+		process_queued_bios_list(m);
 	}
 
 	/*
@@ -1767,22 +1995,47 @@ static struct target_type multipath_target = {
 	.busy = multipath_busy,
 };
 
+static struct target_type multipath_bio_target = {
+	.name = "multipath-bio",
+	.version = {1, 0, 0},
+	.module = THIS_MODULE,
+	.ctr = multipath_bio_ctr,
+	.dtr = multipath_dtr,
+	.map = multipath_map_bio,
+	.end_io = multipath_end_io_bio,
+	.presuspend = multipath_presuspend,
+	.postsuspend = multipath_postsuspend,
+	.resume = multipath_resume,
+	.status = multipath_status,
+	.message = multipath_message,
+	.prepare_ioctl = multipath_prepare_ioctl,
+	.iterate_devices = multipath_iterate_devices,
+	.busy = multipath_busy,
+};
+
 static int __init dm_multipath_init(void)
 {
 	int r;
 
-	/* allocate a slab for the dm_ios */
+	/* allocate a slab for the dm_mpath_ios */
 	_mpio_cache = KMEM_CACHE(dm_mpath_io, 0);
 	if (!_mpio_cache)
 		return -ENOMEM;
 
 	r = dm_register_target(&multipath_target);
 	if (r < 0) {
-		DMERR("register failed %d", r);
+		DMERR("request-based register failed %d", r);
 		r = -EINVAL;
 		goto bad_register_target;
 	}
 
+	r = dm_register_target(&multipath_bio_target);
+	if (r < 0) {
+		DMERR("bio-based register failed %d", r);
+		r = -EINVAL;
+		goto bad_register_bio_based_target;
+	}
+
 	kmultipathd = alloc_workqueue("kmpathd", WQ_MEM_RECLAIM, 0);
 	if (!kmultipathd) {
 		DMERR("failed to create workqueue kmpathd");
@@ -1804,15 +2057,13 @@ static int __init dm_multipath_init(void)
 		goto bad_alloc_kmpath_handlerd;
 	}
 
-	DMINFO("version %u.%u.%u loaded",
-	       multipath_target.version[0], multipath_target.version[1],
-	       multipath_target.version[2]);
-
 	return 0;
 
 bad_alloc_kmpath_handlerd:
 	destroy_workqueue(kmultipathd);
 bad_alloc_kmultipathd:
+	dm_unregister_target(&multipath_bio_target);
+bad_register_bio_based_target:
 	dm_unregister_target(&multipath_target);
 bad_register_target:
 	kmem_cache_destroy(_mpio_cache);
@@ -1826,6 +2077,7 @@ static void __exit dm_multipath_exit(void)
 	destroy_workqueue(kmultipathd);
 
 	dm_unregister_target(&multipath_target);
+	dm_unregister_target(&multipath_bio_target);
 	kmem_cache_destroy(_mpio_cache);
 }