bio_vec-backed iov_iter

New variant of iov_iter - ITER_BVEC in iter->type, backed with
bio_vec array instead of iovec one.  Primitives taught to deal
with such beasts, __swap_write() switched to using that kind
of iov_iter.

Note that bio_vec is just a <page, offset, length> triple - there's
nothing block-specific about it.  I've left the definition where it
was, but took it from under ifdef CONFIG_BLOCK.

Next target: ->splice_write()...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/fuse/file.c

@@ -1288,7 +1288,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 	size_t nbytes = 0;  /* # bytes already packed in req */
 
 	/* Special case for kernel I/O: can copy directly into the buffer */
-	if (ii->type & REQ_KERNEL) {
+	if (ii->type & ITER_KVEC) {
 		unsigned long user_addr = fuse_get_user_addr(ii);
 		size_t frag_size = fuse_get_frag_size(ii, *nbytesp);
 

include/linux/blk_types.h

@@ -5,8 +5,6 @@
 #ifndef __LINUX_BLK_TYPES_H
 #define __LINUX_BLK_TYPES_H
 
-#ifdef CONFIG_BLOCK
-
 #include <linux/types.h>
 
 struct bio_set;
@@ -28,6 +26,8 @@ struct bio_vec {
 	unsigned int	bv_offset;
 };
 
+#ifdef CONFIG_BLOCK
+
 struct bvec_iter {
 	sector_t		bi_sector;	/* device address in 512 byte
 						   sectors */

include/linux/uio.h

@@ -19,12 +19,21 @@ struct kvec {
 	size_t iov_len;
 };
 
+enum {
+	ITER_IOVEC = 0,
+	ITER_KVEC = 2,
+	ITER_BVEC = 4,
+};
+
 struct iov_iter {
 	int type;
-	const struct iovec *iov;
-	unsigned long nr_segs;
 	size_t iov_offset;
 	size_t count;
+	union {
+		const struct iovec *iov;
+		const struct bio_vec *bvec;
+	};
+	unsigned long nr_segs;
 };
 
 /*
@@ -54,6 +63,7 @@ static inline struct iovec iov_iter_iovec(const struct iov_iter *iter)
 }
 
 #define iov_for_each(iov, iter, start)				\
+	if (!((start).type & ITER_BVEC))			\
 	for (iter = (start);					\
 	     (iter).count &&					\
 	     ((iov = iov_iter_iovec(&(iter))), 1);		\

mm/page_io.c

@@ -259,23 +259,28 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
 		struct kiocb kiocb;
 		struct file *swap_file = sis->swap_file;
 		struct address_space *mapping = swap_file->f_mapping;
-		struct iovec iov = {
-			.iov_base = kmap(page),
-			.iov_len  = PAGE_SIZE,
+		struct bio_vec bv = {
+			.bv_page = page,
+			.bv_len  = PAGE_SIZE,
+			.bv_offset = 0
+		};
+		struct iov_iter from = {
+			.type = ITER_BVEC | WRITE,
+			.count = PAGE_SIZE,
+			.iov_offset = 0,
+			.nr_segs = 1,
+			.bvec = &bv
 		};
-		struct iov_iter from;
 
 		init_sync_kiocb(&kiocb, swap_file);
 		kiocb.ki_pos = page_file_offset(page);
 		kiocb.ki_nbytes = PAGE_SIZE;
-		iov_iter_init(&from, KERNEL_WRITE, &iov, 1, PAGE_SIZE);
 
 		set_page_writeback(page);
 		unlock_page(page);
-		ret = mapping->a_ops->direct_IO(KERNEL_WRITE,
+		ret = mapping->a_ops->direct_IO(ITER_BVEC | WRITE,
 						&kiocb, &from,
 						kiocb.ki_pos);
-		kunmap(page);
 		if (ret == PAGE_SIZE) {
 			count_vm_event(PSWPOUT);
 			ret = 0;