block: Explicitly handle discard/write same segments

Immutable biovecs changed the way biovecs are interpreted - drivers no
longer use bi_vcnt, they have to go by bi_iter.bi_size (to allow for
using part of an existing segment without modifying it).

This breaks with discards and write_same bios, since for those bi_size
has nothing to do with segments in the biovec. So for now, we need a
fairly gross hack - we fortunately know that there will never be more
than one segment for the entire request, so we can special case
discard/write_same.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Tested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 62 insertions, 29 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8f8adaa95466..6c583f9c5b65 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -21,6 +21,16 @@ static unsigned int __blk_recalc_rq_segments(struct request_queue *q,
         if (!bio)
                 return 0;
 
+        /*
+         * This should probably be returning 0, but blk_add_request_payload()
+         * (Christoph!!!!)
+         */
+        if (bio->bi_rw & REQ_DISCARD)
+                return 1;
+
+        if (bio->bi_rw & REQ_WRITE_SAME)
+                return 1;
+
         fbio = bio;
         cluster = blk_queue_cluster(q);
         seg_size = 0;
@@ -161,30 +171,60 @@ new_segment:
         *bvprv = *bvec;
 }
 
-/*
- * map a request to scatterlist, return number of sg entries setup. Caller
- * must make sure sg can hold rq->nr_phys_segments entries
- */
-int blk_rq_map_sg(struct request_queue *q, struct request *rq,
-                  struct scatterlist *sglist)
+static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
+                             struct scatterlist *sglist,
+                             struct scatterlist **sg)
 {
         struct bio_vec bvec, bvprv = { NULL };
-        struct req_iterator iter;
-        struct scatterlist *sg;
+        struct bvec_iter iter;
         int nsegs, cluster;
 
         nsegs = 0;
         cluster = blk_queue_cluster(q);
 
-        /*
-         * for each bio in rq
-         */
-        sg = NULL;
-        rq_for_each_segment(bvec, rq, iter) {
-                __blk_segment_map_sg(q, &bvec, sglist, &bvprv, &sg,
-                                     &nsegs, &cluster);
-        } /* segments in rq */
+        if (bio->bi_rw & REQ_DISCARD) {
+                /*
+                 * This is a hack - drivers should be neither modifying the
+                 * biovec, nor relying on bi_vcnt - but because of
+                 * blk_add_request_payload(), a discard bio may or may not have
+                 * a payload we need to set up here (thank you Christoph) and
+                 * bi_vcnt is really the only way of telling if we need to.
+                 */
+
+                if (bio->bi_vcnt)
+                        goto single_segment;
+
+                return 0;
+        }
+
+        if (bio->bi_rw & REQ_WRITE_SAME) {
+single_segment:
+                *sg = sglist;
+                bvec = bio_iovec(bio);
+                sg_set_page(*sg, bvec.bv_page, bvec.bv_len, bvec.bv_offset);
+                return 1;
+        }
+
+        for_each_bio(bio)
+                bio_for_each_segment(bvec, bio, iter)
+                        __blk_segment_map_sg(q, &bvec, sglist, &bvprv, sg,
+                                             &nsegs, &cluster);
 
+        return nsegs;
+}
+
+/*
+ * map a request to scatterlist, return number of sg entries setup. Caller
+ * must make sure sg can hold rq->nr_phys_segments entries
+ */
+int blk_rq_map_sg(struct request_queue *q, struct request *rq,
+                  struct scatterlist *sglist)
+{
+        struct scatterlist *sg = NULL;
+        int nsegs = 0;
+
+        if (rq->bio)
+                nsegs = __blk_bios_map_sg(q, rq->bio, sglist, &sg);
 
         if (unlikely(rq->cmd_flags & REQ_COPY_USER) &&
             (blk_rq_bytes(rq) & q->dma_pad_mask)) {
@@ -230,20 +270,13 @@ EXPORT_SYMBOL(blk_rq_map_sg);
 int blk_bio_map_sg(struct request_queue *q, struct bio *bio,
                    struct scatterlist *sglist)
 {
-        struct bio_vec bvec, bvprv = { NULL };
-        struct scatterlist *sg;
-        int nsegs, cluster;
-        struct bvec_iter iter;
-
-        nsegs = 0;
-        cluster = blk_queue_cluster(q);
-
-        sg = NULL;
-        bio_for_each_segment(bvec, bio, iter) {
-                __blk_segment_map_sg(q, &bvec, sglist, &bvprv, &sg,
-                                     &nsegs, &cluster);
-        } /* segments in bio */
+        struct scatterlist *sg = NULL;
+        int nsegs;
+        struct bio *next = bio->bi_next;
+        bio->bi_next = NULL;
 
+        nsegs = __blk_bios_map_sg(q, bio, sglist, &sg);
+        bio->bi_next = next;
         if (sg)
                 sg_mark_end(sg);