Allow elevators to sort/merge discard requests

But blkdev_issue_discard() still emits requests which are interpreted as
soft barriers, because naïve callers might otherwise issue subsequent
writes to those same sectors, which might cross on the queue (if they're
reallocated quickly enough).

Callers still _can_ issue non-barrier discard requests, but they have to
take care of queue ordering for themselves.

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

1 files changed, 17 insertions, 10 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5efc9e7a68b7..6cf8f0c70a51 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -11,7 +11,7 @@
 
 void blk_recalc_rq_sectors(struct request *rq, int nsect)
 {
-        if (blk_fs_request(rq)) {
+        if (blk_fs_request(rq) || blk_discard_rq(rq)) {
                 rq->hard_sector += nsect;
                 rq->hard_nr_sectors -= nsect;
 
@@ -131,13 +131,17 @@ static int blk_phys_contig_segment(struct request_queue *q, struct bio *bio,
         if (!test_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags))
                 return 0;
 
-        if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
-                return 0;
         if (bio->bi_size + nxt->bi_size > q->max_segment_size)
                 return 0;
 
+        if (!bio_has_data(bio))
+                return 1;
+
+        if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
+                return 0;
+
         /*
-         * bio and nxt are contigous in memory, check if the queue allows
+         * bio and nxt are contiguous in memory; check if the queue allows
          * these two to be merged into one
          */
         if (BIO_SEG_BOUNDARY(q, bio, nxt))
@@ -153,8 +157,9 @@ static int blk_hw_contig_segment(struct request_queue *q, struct bio *bio,
                 blk_recount_segments(q, bio);
         if (!bio_flagged(nxt, BIO_SEG_VALID))
                 blk_recount_segments(q, nxt);
-        if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
-            BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size))
+        if (bio_has_data(bio) &&
+            (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
+             BIOVEC_VIRT_OVERSIZE(bio->bi_hw_back_size + nxt->bi_hw_front_size)))
                 return 0;
         if (bio->bi_hw_back_size + nxt->bi_hw_front_size > q->max_segment_size)
                 return 0;
@@ -317,8 +322,9 @@ int ll_back_merge_fn(struct request_queue *q, struct request *req,
         if (!bio_flagged(bio, BIO_SEG_VALID))
                 blk_recount_segments(q, bio);
         len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
-        if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio))
-            && !BIOVEC_VIRT_OVERSIZE(len)) {
+        if (!bio_has_data(bio) || 
+            (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio))
+             && !BIOVEC_VIRT_OVERSIZE(len))) {
                 int mergeable =  ll_new_mergeable(q, req, bio);
 
                 if (mergeable) {
@@ -356,8 +362,9 @@ int ll_front_merge_fn(struct request_queue *q, struct request *req,
                 blk_recount_segments(q, bio);
         if (!bio_flagged(req->bio, BIO_SEG_VALID))
                 blk_recount_segments(q, req->bio);
-        if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
-            !BIOVEC_VIRT_OVERSIZE(len)) {
+        if (!bio_has_data(bio) || 
+            (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
+             !BIOVEC_VIRT_OVERSIZE(len))) {
                 int mergeable =  ll_new_mergeable(q, req, bio);
 
                 if (mergeable) {