Introduce a new iterator type for dmabuf maps. The map in an opaque object with internals and format specific to the subsystem / driver, and only it can use that subsystem / driver for issuing IO. The task of the middle layers is to pass the map / iterator further down, maybe doing basic splitting and length checking. The iterator can only be used by operations of the file the associated map was created for. Suggested-by: Keith Busch <kbusch@kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com> Link: https://patch.msgid.link/6aedb1c606f20ae5bd7fa358829966384c6e738f.1791975048.git.asml.silence@gmail.com Signed-off-by: Jens Axboe <axboe@kernel.dk>