ublk: support to copy any part of request pages

Add 'offset' to 'struct ublk_map_data', so that ublk_copy_user_pages()
can be used to copy any sub-buffer(linear mapped) of the request.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20230519065030.351216-6-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/ublk_drv.c

@@ -505,19 +505,36 @@ static void ublk_copy_io_pages(struct ublk_io_iter *data,
 	}
 }
 
+static bool ublk_advance_io_iter(const struct request *req,
+		struct ublk_io_iter *iter, unsigned int offset)
+{
+	struct bio *bio = req->bio;
+
+	for_each_bio(bio) {
+		if (bio->bi_iter.bi_size > offset) {
+			iter->bio = bio;
+			iter->iter = bio->bi_iter;
+			bio_advance_iter(iter->bio, &iter->iter, offset);
+			return true;
+		}
+		offset -= bio->bi_iter.bi_size;
+	}
+	return false;
+}
+
 /*
  * Copy data between request pages and io_iter, and 'offset'
  * is the start point of linear offset of request.
  */
 static size_t ublk_copy_user_pages(const struct request *req,
-		struct iov_iter *uiter, int dir)
+		unsigned offset, struct iov_iter *uiter, int dir)
 {
-	struct ublk_io_iter iter = {
-		.bio	= req->bio,
-		.iter	= req->bio->bi_iter,
-	};
+	struct ublk_io_iter iter;
 	size_t done = 0;
 
+	if (!ublk_advance_io_iter(req, &iter, offset))
+		return 0;
+
 	while (iov_iter_count(uiter) && iter.bio) {
 		unsigned nr_pages;
 		size_t len, off;
@@ -570,7 +587,7 @@ static int ublk_map_io(const struct ublk_queue *ubq, const struct request *req,
 		import_single_range(dir, u64_to_user_ptr(io->addr), rq_bytes,
 				&iov, &iter);
 
-		return ublk_copy_user_pages(req, &iter, dir);
+		return ublk_copy_user_pages(req, 0, &iter, dir);
 	}
 	return rq_bytes;
 }
@@ -590,7 +607,7 @@ static int ublk_unmap_io(const struct ublk_queue *ubq,
 
 		import_single_range(dir, u64_to_user_ptr(io->addr), io->res,
 				&iov, &iter);
-		return ublk_copy_user_pages(req, &iter, dir);
+		return ublk_copy_user_pages(req, 0, &iter, dir);
 	}
 	return rq_bytes;
 }