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>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8f8adaa95466ccc8335cde7e313b551b375ed9b9..6c583f9c5b65d002a6ca357c53b19799f822f512 100644 (file)
--- 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);