dm: add request based barrier support

This patch adds barrier support for request-based dm.

CORE DESIGN

The design is basically same as bio-based dm, which emulates barrier
by mapping empty barrier bios before/after a barrier I/O.
But request-based dm has been using struct request_queue for I/O
queueing, so the block-layer's barrier mechanism can be used.

o Summary of the block-layer's behavior (which is depended by dm-core)
  Request-based dm uses QUEUE_ORDERED_DRAIN_FLUSH ordered mode for
  I/O barrier.  It means that when an I/O requiring barrier is found
  in the request_queue, the block-layer makes pre-flush request and
  post-flush request just before and just after the I/O respectively.

  After the ordered sequence starts, the block-layer waits for all
  in-flight I/Os to complete, then gives drivers the pre-flush request,
  the barrier I/O and the post-flush request one by one.
  It means that the request_queue is stopped automatically by
  the block-layer until drivers complete each sequence.

o dm-core
  For the barrier I/O, treats it as a normal I/O, so no additional
  code is needed.

  For the pre/post-flush request, flushes caches by the followings:
    1. Make the number of empty barrier requests required by target's
       num_flush_requests, and map them (dm_rq_barrier()).
    2. Waits for the mapped barriers to complete (dm_rq_barrier()).
       If error has occurred, save the error value to md->barrier_error
       (dm_end_request()).
       (*) Basically, the first reported error is taken.
           But -EOPNOTSUPP supersedes any error and DM_ENDIO_REQUEUE
           follows.
    3. Requeue the pre/post-flush request if the error value is
       DM_ENDIO_REQUEUE.  Otherwise, completes with the error value
       (dm_rq_barrier_work()).
  The pre/post-flush work above is done in the kernel thread (kdmflush)
  context, since memory allocation which might sleep is needed in
  dm_rq_barrier() but sleep is not allowed in dm_request_fn(), which is
  an irq-disabled context.
  Also, clones of the pre/post-flush request share an original, so
  such clones can't be completed using the softirq context.
  Instead, complete them in the context of underlying device drivers.
  It should be safe since there is no I/O dispatching during
  the completion of such clones.

  For suspend, the workqueue of kdmflush needs to be flushed after
  the request_queue has been stopped.  Otherwise, the next flush work
  can be kicked even after the suspend completes.

TARGET INTERFACE

No new interface is added.
Just use the existing num_flush_requests in struct target_type
as same as bio-based dm.

Signed-off-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -142,10 +142,20 @@ struct mapped_device {
 	 */
 	int barrier_error;
 
+	/*
+	 * Protect barrier_error from concurrent endio processing
+	 * in request-based dm.
+	 */
+	spinlock_t barrier_error_lock;
+
 	/*
 	 * Processing queue (flush/barriers)
 	 */
 	struct workqueue_struct *wq;
+	struct work_struct barrier_work;
+
+	/* A pointer to the currently processing pre/post flush request */
+	struct request *flush_request;
 
 	/*
 	 * The current mapping.
@@ -722,6 +732,23 @@ static void end_clone_bio(struct bio *clone, int error)
 	blk_update_request(tio->orig, 0, nr_bytes);
 }
 
+static void store_barrier_error(struct mapped_device *md, int error)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&md->barrier_error_lock, flags);
+	/*
+	 * Basically, the first error is taken, but:
+	 *   -EOPNOTSUPP supersedes any I/O error.
+	 *   Requeue request supersedes any I/O error but -EOPNOTSUPP.
+	 */
+	if (!md->barrier_error || error == -EOPNOTSUPP ||
+	    (md->barrier_error != -EOPNOTSUPP &&
+	     error == DM_ENDIO_REQUEUE))
+		md->barrier_error = error;
+	spin_unlock_irqrestore(&md->barrier_error_lock, flags);
+}
+
 /*
  * Don't touch any member of the md after calling this function because
  * the md may be freed in dm_put() at the end of this function.
@@ -759,11 +786,13 @@ static void free_rq_clone(struct request *clone)
 static void dm_end_request(struct request *clone, int error)
 {
 	int rw = rq_data_dir(clone);
+	int run_queue = 1;
+	bool is_barrier = blk_barrier_rq(clone);
 	struct dm_rq_target_io *tio = clone->end_io_data;
 	struct mapped_device *md = tio->md;
 	struct request *rq = tio->orig;
 
-	if (blk_pc_request(rq)) {
+	if (blk_pc_request(rq) && !is_barrier) {
 		rq->errors = clone->errors;
 		rq->resid_len = clone->resid_len;
 
@@ -778,9 +807,14 @@ static void dm_end_request(struct request *clone, int error)
 
 	free_rq_clone(clone);
 
-	blk_end_request_all(rq, error);
+	if (unlikely(is_barrier)) {
+		if (unlikely(error))
+			store_barrier_error(md, error);
+		run_queue = 0;
+	} else
+		blk_end_request_all(rq, error);
 
-	rq_completed(md, rw, 1);
+	rq_completed(md, rw, run_queue);
 }
 
 static void dm_unprep_request(struct request *rq)
@@ -805,6 +839,16 @@ void dm_requeue_unmapped_request(struct request *clone)
 	struct request_queue *q = rq->q;
 	unsigned long flags;
 
+	if (unlikely(blk_barrier_rq(clone))) {
+		/*
+		 * Barrier clones share an original request.
+		 * Leave it to dm_end_request(), which handles this special
+		 * case.
+		 */
+		dm_end_request(clone, DM_ENDIO_REQUEUE);
+		return;
+	}
+
 	dm_unprep_request(rq);
 
 	spin_lock_irqsave(q->queue_lock, flags);
@@ -894,6 +938,19 @@ static void dm_complete_request(struct request *clone, int error)
 	struct dm_rq_target_io *tio = clone->end_io_data;
 	struct request *rq = tio->orig;
 
+	if (unlikely(blk_barrier_rq(clone))) {
+		/*
+		 * Barrier clones share an original request.  So can't use
+		 * softirq_done with the original.
+		 * Pass the clone to dm_done() directly in this special case.
+		 * It is safe (even if clone->q->queue_lock is held here)
+		 * because there is no I/O dispatching during the completion
+		 * of barrier clone.
+		 */
+		dm_done(clone, error, true);
+		return;
+	}
+
 	tio->error = error;
 	rq->completion_data = clone;
 	blk_complete_request(rq);
@@ -910,6 +967,17 @@ void dm_kill_unmapped_request(struct request *clone, int error)
 	struct dm_rq_target_io *tio = clone->end_io_data;
 	struct request *rq = tio->orig;
 
+	if (unlikely(blk_barrier_rq(clone))) {
+		/*
+		 * Barrier clones share an original request.
+		 * Leave it to dm_end_request(), which handles this special
+		 * case.
+		 */
+		BUG_ON(error > 0);
+		dm_end_request(clone, error);
+		return;
+	}
+
 	rq->cmd_flags |= REQ_FAILED;
 	dm_complete_request(clone, error);
 }
@@ -1364,11 +1432,6 @@ static int dm_make_request(struct request_queue *q, struct bio *bio)
 {
 	struct mapped_device *md = q->queuedata;
 
-	if (unlikely(bio_rw_flagged(bio, BIO_RW_BARRIER))) {
-		bio_endio(bio, -EOPNOTSUPP);
-		return 0;
-	}
-
 	return md->saved_make_request_fn(q, bio); /* call __make_request() */
 }
 
@@ -1387,6 +1450,25 @@ static int dm_request(struct request_queue *q, struct bio *bio)
 	return _dm_request(q, bio);
 }
 
+/*
+ * Mark this request as flush request, so that dm_request_fn() can
+ * recognize.
+ */
+static void dm_rq_prepare_flush(struct request_queue *q, struct request *rq)
+{
+	rq->cmd_type = REQ_TYPE_LINUX_BLOCK;
+	rq->cmd[0] = REQ_LB_OP_FLUSH;
+}
+
+static bool dm_rq_is_flush_request(struct request *rq)
+{
+	if (rq->cmd_type == REQ_TYPE_LINUX_BLOCK &&
+	    rq->cmd[0] == REQ_LB_OP_FLUSH)
+		return true;
+	else
+		return false;
+}
+
 void dm_dispatch_request(struct request *rq)
 {
 	int r;
@@ -1432,16 +1514,24 @@ static int dm_rq_bio_constructor(struct bio *bio, struct bio *bio_orig,
 static int setup_clone(struct request *clone, struct request *rq,
 		       struct dm_rq_target_io *tio)
 {
-	int r = blk_rq_prep_clone(clone, rq, tio->md->bs, GFP_ATOMIC,
-				  dm_rq_bio_constructor, tio);
+	int r;
 
-	if (r)
-		return r;
+	if (dm_rq_is_flush_request(rq)) {
+		blk_rq_init(NULL, clone);
+		clone->cmd_type = REQ_TYPE_FS;
+		clone->cmd_flags |= (REQ_HARDBARRIER | WRITE);
+	} else {
+		r = blk_rq_prep_clone(clone, rq, tio->md->bs, GFP_ATOMIC,
+				      dm_rq_bio_constructor, tio);
+		if (r)
+			return r;
+
+		clone->cmd = rq->cmd;
+		clone->cmd_len = rq->cmd_len;
+		clone->sense = rq->sense;
+		clone->buffer = rq->buffer;
+	}
 
-	clone->cmd = rq->cmd;
-	clone->cmd_len = rq->cmd_len;
-	clone->sense = rq->sense;
-	clone->buffer = rq->buffer;
 	clone->end_io = end_clone_request;
 	clone->end_io_data = tio;
 
@@ -1482,6 +1572,9 @@ static int dm_prep_fn(struct request_queue *q, struct request *rq)
 	struct mapped_device *md = q->queuedata;
 	struct request *clone;
 
+	if (unlikely(dm_rq_is_flush_request(rq)))
+		return BLKPREP_OK;
+
 	if (unlikely(rq->special)) {
 		DMWARN("Already has something in rq->special.");
 		return BLKPREP_KILL;
@@ -1560,6 +1653,14 @@ static void dm_request_fn(struct request_queue *q)
 		if (!rq)
 			goto plug_and_out;
 
+		if (unlikely(dm_rq_is_flush_request(rq))) {
+			BUG_ON(md->flush_request);
+			md->flush_request = rq;
+			blk_start_request(rq);
+			queue_work(md->wq, &md->barrier_work);
+			goto out;
+		}
+
 		ti = dm_table_find_target(map, blk_rq_pos(rq));
 		if (ti->type->busy && ti->type->busy(ti))
 			goto plug_and_out;
@@ -1726,6 +1827,7 @@ static int next_free_minor(int *minor)
 static const struct block_device_operations dm_blk_dops;
 
 static void dm_wq_work(struct work_struct *work);
+static void dm_rq_barrier_work(struct work_struct *work);
 
 /*
  * Allocate and initialise a blank device with a given minor.
@@ -1755,6 +1857,7 @@ static struct mapped_device *alloc_dev(int minor)
 	init_rwsem(&md->io_lock);
 	mutex_init(&md->suspend_lock);
 	spin_lock_init(&md->deferred_lock);
+	spin_lock_init(&md->barrier_error_lock);
 	rwlock_init(&md->map_lock);
 	atomic_set(&md->holders, 1);
 	atomic_set(&md->open_count, 0);
@@ -1789,6 +1892,8 @@ static struct mapped_device *alloc_dev(int minor)
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 	blk_queue_lld_busy(md->queue, dm_lld_busy);
+	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH,
+			  dm_rq_prepare_flush);
 
 	md->disk = alloc_disk(1);
 	if (!md->disk)
@@ -1798,6 +1903,7 @@ static struct mapped_device *alloc_dev(int minor)
 	atomic_set(&md->pending[1], 0);
 	init_waitqueue_head(&md->wait);
 	INIT_WORK(&md->work, dm_wq_work);
+	INIT_WORK(&md->barrier_work, dm_rq_barrier_work);
 	init_waitqueue_head(&md->eventq);
 
 	md->disk->major = _major;
@@ -2185,6 +2291,73 @@ static void dm_queue_flush(struct mapped_device *md)
 	queue_work(md->wq, &md->work);
 }
 
+static void dm_rq_set_flush_nr(struct request *clone, unsigned flush_nr)
+{
+	struct dm_rq_target_io *tio = clone->end_io_data;
+
+	tio->info.flush_request = flush_nr;
+}
+
+/* Issue barrier requests to targets and wait for their completion. */
+static int dm_rq_barrier(struct mapped_device *md)
+{
+	int i, j;
+	struct dm_table *map = dm_get_table(md);
+	unsigned num_targets = dm_table_get_num_targets(map);
+	struct dm_target *ti;
+	struct request *clone;
+
+	md->barrier_error = 0;
+
+	for (i = 0; i < num_targets; i++) {
+		ti = dm_table_get_target(map, i);
+		for (j = 0; j < ti->num_flush_requests; j++) {
+			clone = clone_rq(md->flush_request, md, GFP_NOIO);
+			dm_rq_set_flush_nr(clone, j);
+			atomic_inc(&md->pending[rq_data_dir(clone)]);
+			map_request(ti, clone, md);
+		}
+	}
+
+	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE);
+	dm_table_put(map);
+
+	return md->barrier_error;
+}
+
+static void dm_rq_barrier_work(struct work_struct *work)
+{
+	int error;
+	struct mapped_device *md = container_of(work, struct mapped_device,
+						barrier_work);
+	struct request_queue *q = md->queue;
+	struct request *rq;
+	unsigned long flags;
+
+	/*
+	 * Hold the md reference here and leave it at the last part so that
+	 * the md can't be deleted by device opener when the barrier request
+	 * completes.
+	 */
+	dm_get(md);
+
+	error = dm_rq_barrier(md);
+
+	rq = md->flush_request;
+	md->flush_request = NULL;
+
+	if (error == DM_ENDIO_REQUEUE) {
+		spin_lock_irqsave(q->queue_lock, flags);
+		blk_requeue_request(q, rq);
+		spin_unlock_irqrestore(q->queue_lock, flags);
+	} else
+		blk_end_request_all(rq, error);
+
+	blk_run_queue(q);
+
+	dm_put(md);
+}
+
 /*
  * Swap in a new table (destroying old one).
  */
@@ -2325,11 +2498,16 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
 	set_bit(DMF_QUEUE_IO_TO_THREAD, &md->flags);
 	up_write(&md->io_lock);
 
-	flush_workqueue(md->wq);
-
+	/*
+	 * Request-based dm uses md->wq for barrier (dm_rq_barrier_work) which
+	 * can be kicked until md->queue is stopped.  So stop md->queue before
+	 * flushing md->wq.
+	 */
 	if (dm_request_based(md))
 		stop_queue(md->queue);
 
+	flush_workqueue(md->wq);
+
 	/*
 	 * At this point no more requests are entering target request routines.
 	 * We call dm_wait_for_completion to wait for all existing requests