fuse: optimize fuse_get_user_pages()

Let fuse_get_user_pages() pack as many iov-s to a single fuse_req as
possible. This is very beneficial in case of iov[] consisting of many
iov-s of relatively small sizes (e.g. PAGE_SIZE).
Signed-off-by: Maxim Patlasov <mpatlasov@parallels.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>

fs/fuse/file.c

@@ -1047,29 +1047,37 @@ static void fuse_release_user_pages(struct fuse_req *req, int write)
 	}
 }
 
-static inline void fuse_page_descs_length_init(struct fuse_req *req)
+static inline void fuse_page_descs_length_init(struct fuse_req *req,
+		unsigned index, unsigned nr_pages)
 {
 	int i;
 
-	for (i = 0; i < req->num_pages; i++)
+	for (i = index; i < index + nr_pages; i++)
 		req->page_descs[i].length = PAGE_SIZE -
 			req->page_descs[i].offset;
 }
 
+static inline unsigned long fuse_get_user_addr(const struct iov_iter *ii)
+{
+	return (unsigned long)ii->iov->iov_base + ii->iov_offset;
+}
+
+static inline size_t fuse_get_frag_size(const struct iov_iter *ii,
+					size_t max_size)
+{
+	return min(iov_iter_single_seg_count(ii), max_size);
+}
+
 static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 			       size_t *nbytesp, int write)
 {
-	size_t nbytes = *nbytesp;
-	size_t frag_size = min(iov_iter_single_seg_count(ii), nbytes);
-	unsigned long user_addr;
-	unsigned offset;
-	int npages;
-
-	user_addr = (unsigned long)ii->iov->iov_base + ii->iov_offset;
-	offset = user_addr & ~PAGE_MASK;
+	size_t nbytes = 0;  /* # bytes already packed in req */
 
 	/* Special case for kernel I/O: can copy directly into the buffer */
 	if (segment_eq(get_fs(), KERNEL_DS)) {
+		unsigned long user_addr = fuse_get_user_addr(ii);
+		size_t frag_size = fuse_get_frag_size(ii, *nbytesp);
+
 		if (write)
 			req->in.args[1].value = (void *) user_addr;
 		else
@@ -1080,30 +1088,45 @@ static int fuse_get_user_pages(struct fuse_req *req, struct iov_iter *ii,
 		return 0;
 	}
 
-	nbytes = min_t(size_t, frag_size, FUSE_MAX_PAGES_PER_REQ << PAGE_SHIFT);
-	npages = (nbytes + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
-	npages = clamp(npages, 1, FUSE_MAX_PAGES_PER_REQ);
-	npages = get_user_pages_fast(user_addr, npages, !write, req->pages);
-	if (npages < 0)
-		return npages;
+	while (nbytes < *nbytesp && req->num_pages < FUSE_MAX_PAGES_PER_REQ) {
+		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;
 
-	req->num_pages = npages;
-	req->page_descs[0].offset = offset;
-	fuse_page_descs_length_init(req);
+		unsigned n = FUSE_MAX_PAGES_PER_REQ - req->num_pages;
+		frag_size = min_t(size_t, frag_size, n << PAGE_SHIFT);
+
+		npages = (frag_size + offset + PAGE_SIZE - 1) >> PAGE_SHIFT;
+		npages = clamp(npages, 1U, n);
+
+		ret = get_user_pages_fast(user_addr, npages, !write,
+					  &req->pages[req->num_pages]);
+		if (ret < 0)
+			return ret;
+
+		npages = ret;
+		frag_size = min_t(size_t, frag_size,
+				  (npages << PAGE_SHIFT) - offset);
+		iov_iter_advance(ii, frag_size);
+
+		req->page_descs[req->num_pages].offset = offset;
+		fuse_page_descs_length_init(req, req->num_pages, npages);
+
+		req->num_pages += npages;
+		req->page_descs[req->num_pages - 1].length -=
+			(npages << PAGE_SHIFT) - offset - frag_size;
+
+		nbytes += frag_size;
+	}
 
 	if (write)
 		req->in.argpages = 1;
 	else
 		req->out.argpages = 1;
 
-	nbytes = (req->num_pages << PAGE_SHIFT) - req->page_descs[0].offset;
-
-	if (frag_size < nbytes)
-		req->page_descs[req->num_pages - 1].length -=
-			nbytes - frag_size;
-
-	*nbytesp = min(frag_size, nbytes);
-	iov_iter_advance(ii, *nbytesp);
+	*nbytesp = nbytes;
 
 	return 0;
 }
@@ -1948,7 +1971,7 @@ long fuse_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg,
 	}
 	memcpy(req->pages, pages, sizeof(req->pages[0]) * num_pages);
 	req->num_pages = num_pages;
-	fuse_page_descs_length_init(req);
+	fuse_page_descs_length_init(req, 0, req->num_pages);
 
 	/* okay, let's send it to the client */
 	req->in.h.opcode = FUSE_IOCTL;