new helper: iov_iter_npages()

counts the pages covered by iov_iter, up to given limit.
do_block_direct_io() and fuse_iter_npages() switched to
it.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/direct-io.c

@@ -1100,7 +1100,6 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	get_block_t get_block, dio_iodone_t end_io,
 	dio_submit_t submit_io,	int flags)
 {
-	int seg;
 	unsigned i_blkbits = ACCESS_ONCE(inode->i_blkbits);
 	unsigned blkbits = i_blkbits;
 	unsigned blocksize_mask = (1 << blkbits) - 1;
@@ -1108,7 +1107,6 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	loff_t end = offset + iov_iter_count(iter);
 	struct dio *dio;
 	struct dio_submit sdio = { 0, };
-	unsigned long user_addr;
 	struct buffer_head map_bh = { 0, };
 	struct blk_plug plug;
 	unsigned long align = offset | iov_iter_alignment(iter);
@@ -1231,12 +1229,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
 	if (unlikely(sdio.blkfactor))
 		sdio.pages_in_io = 2;
 
-	for (seg = 0; seg < iter->nr_segs; seg++) {
-		user_addr = (unsigned long)iter->iov[seg].iov_base;
-		sdio.pages_in_io +=
-			((user_addr + iter->iov[seg].iov_len + PAGE_SIZE-1) /
-				PAGE_SIZE - user_addr / PAGE_SIZE);
-	}
+	sdio.pages_in_io += iov_iter_npages(iter, INT_MAX);
 
 	blk_start_plug(&plug);
 

fs/fuse/file.c

@@ -1310,7 +1310,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 
 	while (nbytes < *nbytesp && req->num_pages < req->max_pages) {
 		unsigned npages;
-		size_t start, end, frag_size;
+		size_t start;
 		unsigned n = req->max_pages - req->num_pages;
 		ssize_t ret = iov_iter_get_pages(ii,
 					&req->pages[req->num_pages],
@@ -1344,19 +1344,7 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 
 static inline int fuse_iter_npages(const struct iov_iter *ii_p)
 {
-	struct iov_iter ii = *ii_p;
-	int npages = 0;
-
-	while (iov_iter_count(&ii) && npages < FUSE_MAX_PAGES_PER_REQ) {
-		unsigned long user_addr = fuse_get_user_addr(&ii);
-		unsigned offset = user_addr & ~PAGE_MASK;
-		size_t frag_size = iov_iter_single_seg_count(&ii);
-
-		npages += (frag_size + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
-		iov_iter_advance(&ii, frag_size);
-	}
-
-	return min(npages, FUSE_MAX_PAGES_PER_REQ);
+	return iov_iter_npages(ii_p, FUSE_MAX_PAGES_PER_REQ);
 }
 
 ssize_t fuse_direct_io(struct fuse_io_priv *io, struct iov_iter *iter,

include/linux/uio.h

@@ -73,6 +73,7 @@ void iov_iter_init(struct iov_iter *i, int direction, const struct iovec *iov,
 			unsigned long nr_segs, size_t count);
 ssize_t iov_iter_get_pages(struct iov_iter *i, struct page **pages,
 			size_t maxsize, size_t *start);
+int iov_iter_npages(const struct iov_iter *i, int maxpages);
 
 static inline size_t iov_iter_count(struct iov_iter *i)
 {

mm/iov_iter.c

@@ -262,3 +262,30 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 	return (res == n ? len : res * PAGE_SIZE) - *start;
 }
 EXPORT_SYMBOL(iov_iter_get_pages);
+
+int iov_iter_npages(const struct iov_iter *i, int maxpages)
+{
+	size_t offset = i->iov_offset;
+	size_t size = i->count;
+	const struct iovec *iov = i->iov;
+	int npages = 0;
+	int n;
+
+	for (n = 0; size && n < i->nr_segs; n++, iov++) {
+		unsigned long addr = (unsigned long)iov->iov_base + offset;
+		size_t len = iov->iov_len - offset;
+		offset = 0;
+		if (unlikely(!len))	/* empty segment */
+			continue;
+		if (len > size)
+			len = size;
+		npages += (addr + len + PAGE_SIZE - 1) / PAGE_SIZE
+			  - addr / PAGE_SIZE;
+		if (npages >= maxpages)	/* don't bother going further */
+			return maxpages;
+		size -= len;
+		offset = 0;
+	}
+	return min(npages, maxpages);
+}
+EXPORT_SYMBOL(iov_iter_npages);