struct iov_iter

https://elixir.bootlin.com/linux/v5.7/source/include/linux/uio.h

struct iov_iter {
	/*
	 * Bit 0 is the read/write bit, set if we're writing.
	 * Bit 1 is the BVEC_FLAG_NO_REF bit, set if type is a bvec and
	 * the caller isn't expecting to drop a page reference when done.
	 */
	unsigned int type;
	size_t iov_offset;
	size_t count;
	union {
		const struct iovec *iov;
		const struct kvec *kvec;
		const struct bio_vec *bvec;
		struct pipe_inode_info *pipe;
	};
	union {
		unsigned long nr_segs;
		struct {
			unsigned int head;
			unsigned int start_head;
		};
	};
};