block: Initial support for data-less (or empty) barrier support

This implements functionality to pass down or insert a barrier
in a queue, without having data attached to it. The ->prepare_flush_fn()
infrastructure from data barriers are reused to provide this
functionality.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

2 files changed, 47 insertions, 19 deletions

diff --git a/block/elevator.c b/block/elevator.c
index ec23ca02f2fe..952aee04a68a 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -712,6 +712,14 @@ struct request *elv_next_request(struct request_queue *q)
         int ret;
 
         while ((rq = __elv_next_request(q)) != NULL) {
+                /*
+                 * Kill the empty barrier place holder, the driver must
+                 * not ever see it.
+                 */
+                if (blk_empty_barrier(rq)) {
+                        end_queued_request(rq, 1);
+                        continue;
+                }
                 if (!(rq->cmd_flags & REQ_STARTED)) {
                         /*
                          * This is the first time the device driver
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 548f0d826679..4fde3a3c92d3 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -458,9 +458,12 @@ static inline struct request *start_ordered(struct request_queue *q,
          * Queue ordered sequence.  As we stack them at the head, we
          * need to queue in reverse order.  Note that we rely on that
          * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
-         * request gets inbetween ordered sequence.
+         * request gets inbetween ordered sequence. If this request is
+         * an empty barrier, we don't need to do a postflush ever since
+         * there will be no data written between the pre and post flush.
+         * Hence a single flush will suffice.
          */
-        if (q->ordered & QUEUE_ORDERED_POSTFLUSH)
+        if ((q->ordered & QUEUE_ORDERED_POSTFLUSH) && !blk_empty_barrier(rq))
                 queue_flush(q, QUEUE_ORDERED_POSTFLUSH);
         else
                 q->ordseq |= QUEUE_ORDSEQ_POSTFLUSH;
@@ -484,7 +487,7 @@ static inline struct request *start_ordered(struct request_queue *q,
 int blk_do_ordered(struct request_queue *q, struct request **rqp)
 {
         struct request *rq = *rqp;
-        int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
+        const int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
 
         if (!q->ordseq) {
                 if (!is_barrier)
@@ -3054,7 +3057,7 @@ static inline void blk_partition_remap(struct bio *bio)
 {
         struct block_device *bdev = bio->bi_bdev;
 
-        if (bdev != bdev->bd_contains) {
+        if (bio_sectors(bio) && bdev != bdev->bd_contains) {
                 struct hd_struct *p = bdev->bd_part;
                 const int rw = bio_data_dir(bio);
 
@@ -3313,23 +3316,32 @@ void submit_bio(int rw, struct bio *bio)
 {
         int count = bio_sectors(bio);
 
-        BIO_BUG_ON(!bio->bi_size);
-        BIO_BUG_ON(!bio->bi_io_vec);
         bio->bi_rw |= rw;
-        if (rw & WRITE) {
-                count_vm_events(PGPGOUT, count);
-        } else {
-                task_io_account_read(bio->bi_size);
-                count_vm_events(PGPGIN, count);
-        }
 
-        if (unlikely(block_dump)) {
-                char b[BDEVNAME_SIZE];
-                printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
-                        current->comm, current->pid,
-                        (rw & WRITE) ? "WRITE" : "READ",
-                        (unsigned long long)bio->bi_sector,
-                        bdevname(bio->bi_bdev,b));
+        /*
+         * If it's a regular read/write or a barrier with data attached,
+         * go through the normal accounting stuff before submission.
+         */
+        if (!bio_empty_barrier(bio)) {
+
+                BIO_BUG_ON(!bio->bi_size);
+                BIO_BUG_ON(!bio->bi_io_vec);
+
+                if (rw & WRITE) {
+                        count_vm_events(PGPGOUT, count);
+                } else {
+                        task_io_account_read(bio->bi_size);
+                        count_vm_events(PGPGIN, count);
+                }
+
+                if (unlikely(block_dump)) {
+                        char b[BDEVNAME_SIZE];
+                        printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
+                                current->comm, current->pid,
+                                (rw & WRITE) ? "WRITE" : "READ",
+                                (unsigned long long)bio->bi_sector,
+                                bdevname(bio->bi_bdev,b));
+                }
         }
 
         generic_make_request(bio);
@@ -3405,6 +3417,14 @@ static int __end_that_request_first(struct request *req, int uptodate,
         while ((bio = req->bio) != NULL) {
                 int nbytes;
 
+                /*
+                 * For an empty barrier request, the low level driver must
+                 * store a potential error location in ->sector. We pass
+                 * that back up in ->bi_sector.
+                 */
+                if (blk_empty_barrier(req))
+                        bio->bi_sector = req->sector;
+
                 if (nr_bytes >= bio->bi_size) {
                         req->bio = bio->bi_next;
                         nbytes = bio->bi_size;