libceph: fix breakage caused by multipage bvecs

A bvec can now consist of multiple physically contiguous pages.
This means that bvec_iter_advance() can move to a different page while
staying in the same bvec (i.e. ->bi_bvec_done != 0).

The messenger works in terms of segments which can now be defined as
the smaller of a bvec and a page.  The "more bytes to process in this
segment" condition holds only if bvec_iter_advance() leaves us in the
same bvec _and_ in the same page.  On next bvec (possibly in the same
page) and on next page (possibly in the same bvec) we may need to set
->last_piece.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger.c

@@ -840,6 +840,7 @@ static bool ceph_msg_data_bio_advance(struct ceph_msg_data_cursor *cursor,
 					size_t bytes)
 {
 	struct ceph_bio_iter *it = &cursor->bio_iter;
+	struct page *page = bio_iter_page(it->bio, it->iter);
 
 	BUG_ON(bytes > cursor->resid);
 	BUG_ON(bytes > bio_iter_len(it->bio, it->iter));
@@ -851,7 +852,8 @@ static bool ceph_msg_data_bio_advance(struct ceph_msg_data_cursor *cursor,
 		return false;   /* no more data */
 	}
 
-	if (!bytes || (it->iter.bi_size && it->iter.bi_bvec_done))
+	if (!bytes || (it->iter.bi_size && it->iter.bi_bvec_done &&
+		       page == bio_iter_page(it->bio, it->iter)))
 		return false;	/* more bytes to process in this segment */
 
 	if (!it->iter.bi_size) {
@@ -899,6 +901,7 @@ static bool ceph_msg_data_bvecs_advance(struct ceph_msg_data_cursor *cursor,
 					size_t bytes)
 {
 	struct bio_vec *bvecs = cursor->data->bvec_pos.bvecs;
+	struct page *page = bvec_iter_page(bvecs, cursor->bvec_iter);
 
 	BUG_ON(bytes > cursor->resid);
 	BUG_ON(bytes > bvec_iter_len(bvecs, cursor->bvec_iter));
@@ -910,7 +913,8 @@ static bool ceph_msg_data_bvecs_advance(struct ceph_msg_data_cursor *cursor,
 		return false;   /* no more data */
 	}
 
-	if (!bytes || cursor->bvec_iter.bi_bvec_done)
+	if (!bytes || (cursor->bvec_iter.bi_bvec_done &&
+		       page == bvec_iter_page(bvecs, cursor->bvec_iter)))
 		return false;	/* more bytes to process in this segment */
 
 	BUG_ON(cursor->last_piece);