fuse: optimize __fuse_direct_io()

__fuse_direct_io() allocates fuse-requests by calling fuse_get_req(fc, n). The
patch calculates 'n' based on iov[] array. This is useful because allocating
FUSE_MAX_PAGES_PER_REQ page pointers and descriptors for each fuse request
would be waste of memory in case of iov-s of smaller size.

Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -1088,14 +1088,14 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 		return 0;
 	}
 
-	while (nbytes < *nbytesp && req->num_pages < FUSE_MAX_PAGES_PER_REQ) {
+	while (nbytes < *nbytesp && req->num_pages < req->max_pages) {
 		unsigned npages;
 		unsigned long user_addr = fuse_get_user_addr(ii);
 		unsigned offset = user_addr & ~PAGE_MASK;
 		size_t frag_size = fuse_get_frag_size(ii, *nbytesp - nbytes);
 		int ret;
 
-		unsigned n = FUSE_MAX_PAGES_PER_REQ - req->num_pages;
+		unsigned n = req->max_pages - req->num_pages;
 		frag_size = min_t(size_t, frag_size, n << PAGE_SHIFT);
 
 		npages = (frag_size + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
@@ -1131,6 +1131,23 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 	return 0;
 }
 
+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);
+}
+
 static ssize_t __fuse_direct_io(struct file *file, const struct iovec *iov,
 				unsigned long nr_segs, size_t count,
 				loff_t *ppos, int write)
@@ -1145,7 +1162,7 @@ static ssize_t __fuse_direct_io(struct file *file, const struct iovec *iov,
 
 	iov_iter_init(&ii, iov, nr_segs, count, 0);
 
-	req = fuse_get_req(fc, FUSE_MAX_PAGES_PER_REQ);
+	req = fuse_get_req(fc, fuse_iter_npages(&ii));
 	if (IS_ERR(req))
 		return PTR_ERR(req);
 
@@ -1180,7 +1197,7 @@ static ssize_t __fuse_direct_io(struct file *file, const struct iovec *iov,
 			break;
 		if (count) {
 			fuse_put_request(fc, req);
-			req = fuse_get_req(fc, FUSE_MAX_PAGES_PER_REQ);
+			req = fuse_get_req(fc, fuse_iter_npages(&ii));
 			if (IS_ERR(req))
 				break;
 		}