Firstly we have mature bvec/bio iterator helper for iterate each
page in one bio, not necessary to reinvent a wheel to do that.

Secondly the coming multipage bvecs requires this patch.

Also add comments about the direct access to bvec table.

Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 drivers/md/dm-io.c | 34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/drivers/md/dm-io.c b/drivers/md/dm-io.c
index 0bf1a12e35fe..2ef573c220fc 100644
--- a/drivers/md/dm-io.c
+++ b/drivers/md/dm-io.c

@@ -162,7 +162,10 @@ struct dpages {
                         struct page **p, unsigned long *len, unsigned *offset);
        void (*next_page)(struct dpages *dp);

-       unsigned context_u;
+       union {
+               unsigned context_u;
+               struct bvec_iter context_bi;
+       };
        void *context_ptr;

        void *vma_invalidate_address;

@@ -204,25 +207,36 @@ static void list_dp_init(struct dpages *dp, struct page_list *pl, unsigned offse
 static void bio_get_page(struct dpages *dp, struct page **p,
                         unsigned long *len, unsigned *offset)
 {
-       struct bio_vec *bvec = dp->context_ptr;
-       *p = bvec->bv_page;
-       *len = bvec->bv_len - dp->context_u;
-       *offset = bvec->bv_offset + dp->context_u;
+       struct bio_vec bv = bvec_iter_bvec((struct bio_vec *)dp->context_ptr,
+                       dp->context_bi);
+
+       *p = bv.bv_page;
+       *len = bv.bv_len;
+       *offset = bv.bv_offset;
+
+       /* avoid to figure out it in bio_next_page() again */
+       dp->context_bi.bi_sector = (sector_t)bv.bv_len;
 }

 static void bio_next_page(struct dpages *dp)
 {
-       struct bio_vec *bvec = dp->context_ptr;
-       dp->context_ptr = bvec + 1;
-       dp->context_u = 0;
+       unsigned int len = (unsigned int)dp->context_bi.bi_sector;
+
+       bvec_iter_advance((struct bio_vec *)dp->context_ptr,
+                       &dp->context_bi, len);
 }

 static void bio_dp_init(struct dpages *dp, struct bio *bio)
 {
        dp->get_page = bio_get_page;
        dp->next_page = bio_next_page;
-       dp->context_ptr = __bvec_iter_bvec(bio->bi_io_vec, bio->bi_iter);
-       dp->context_u = bio->bi_iter.bi_bvec_done;
+
+       /*
+        * We just use bvec iterator to retrieve pages, so it is ok to
+        * access the bvec table directly here
+        */
+       dp->context_ptr = bio->bi_io_vec;
+       dp->context_bi = bio->bi_iter;
 }