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>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 895795cdb145afdab43456e10257436eed2913cc..247b17f2a0f6c0d445d299ef0bae25b956d99a69 100644 (file)
--- 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++;
        }