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>

diff --git a/block/elevator.c b/block/elevator.c
index ec23ca0..952aee0 100644 (file)
--- 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 548f0d8..4fde3a3 100644 (file)
--- 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;

diff --git a/include/linux/bio.h b/include/linux/bio.h
index 089a8bc..4da4413 100644 (file)
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -176,13 +176,28 @@ struct bio {
 #define bio_offset(bio)                bio_iovec((bio))->bv_offset
 #define bio_segments(bio)      ((bio)->bi_vcnt - (bio)->bi_idx)
 #define bio_sectors(bio)       ((bio)->bi_size >> 9)
-#define bio_cur_sectors(bio)   (bio_iovec(bio)->bv_len >> 9)
-#define bio_data(bio)          (page_address(bio_page((bio))) + bio_offset((bio)))
 #define bio_barrier(bio)       ((bio)->bi_rw & (1 << BIO_RW_BARRIER))
 #define bio_sync(bio)          ((bio)->bi_rw & (1 << BIO_RW_SYNC))
 #define bio_failfast(bio)      ((bio)->bi_rw & (1 << BIO_RW_FAILFAST))
 #define bio_rw_ahead(bio)      ((bio)->bi_rw & (1 << BIO_RW_AHEAD))
 #define bio_rw_meta(bio)       ((bio)->bi_rw & (1 << BIO_RW_META))
+#define bio_empty_barrier(bio) (bio_barrier(bio) && !(bio)->bi_size)
+
+static inline unsigned int bio_cur_sectors(struct bio *bio)
+{
+       if (bio->bi_vcnt)
+               return bio_iovec(bio)->bv_len >> 9;
+
+       return 0;
+}
+
+static inline void *bio_data(struct bio *bio)
+{
+       if (bio->bi_vcnt)
+               return page_address(bio_page(bio)) + bio_offset(bio);
+
+       return NULL;
+}
 
 /*
  * will die

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 6109679..fb2ff74 100644 (file)
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -540,6 +540,7 @@ enum {
 #define blk_barrier_rq(rq)     ((rq)->cmd_flags & REQ_HARDBARRIER)
 #define blk_fua_rq(rq)         ((rq)->cmd_flags & REQ_FUA)
 #define blk_bidi_rq(rq)                ((rq)->next_rq != NULL)
+#define blk_empty_barrier(rq)  (blk_barrier_rq(rq) && blk_fs_request(rq) && !(rq)->hard_nr_sectors)
 
 #define list_entry_rq(ptr)     list_entry((ptr), struct request, queuelist)
 

diff --git a/mm/bounce.c b/mm/bounce.c
index 3b549bf..b6d2d0f 100644 (file)
--- a/mm/bounce.c
+++ b/mm/bounce.c
@@ -265,6 +265,12 @@ void blk_queue_bounce(struct request_queue *q, struct bio **bio_orig)
        mempool_t *pool;
 
        /*
+        * Data-less bio, nothing to bounce
+        */
+       if (bio_empty_barrier(*bio_orig))
+               return;
+
+       /*
         * for non-isa bounce case, just check if the bounce pfn is equal
         * to or bigger than the highest pfn in the system -- in that case,
         * don't waste time iterating over bio segments