block: rewrite blk_bvec_map_sg to avoid a nth_page call

The offset in scatterlists is allowed to be larger than the page size,
so don't go to great length to avoid that case and simplify the
arithmetics.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 6 insertions, 15 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 895795cdb145..247b17f2a0f6 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -469,26 +469,17 @@ static unsigned blk_bvec_map_sg(struct request_queue *q,
                 struct scatterlist **sg)
 {
         unsigned nbytes = bvec->bv_len;
-        unsigned nsegs = 0, total = 0, offset = 0;
+        unsigned nsegs = 0, total = 0;
 
         while (nbytes > 0) {
-                unsigned seg_size;
-                struct page *pg;
-                unsigned idx;
+                unsigned offset = bvec->bv_offset + total;
+                unsigned len = min(get_max_segment_size(q, offset), nbytes);
 
                 *sg = blk_next_sg(sg, sglist);
+                sg_set_page(*sg, bvec->bv_page, len, offset);
 
-                seg_size = get_max_segment_size(q, bvec->bv_offset + total);
-                seg_size = min(nbytes, seg_size);
-
-                offset = (total + bvec->bv_offset) % PAGE_SIZE;
-                idx = (total + bvec->bv_offset) / PAGE_SIZE;
-                pg = bvec_nth_page(bvec->bv_page, idx);
-
-                sg_set_page(*sg, pg, seg_size, offset);
-
-                total += seg_size;
-                nbytes -= seg_size;
+                total += len;
+                nbytes -= len;
                 nsegs++;
         }