SUNRPC: When expanding the buffer, we may need grow the sparse pages

If we're shifting the page data to the right, and this happens to be a
sparse page array, then we may need to allocate new pages in order to
receive the data.
Reported-by: "Mkrtchyan, Tigran" <tigran.mkrtchyan@desy.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

net/sunrpc/xdr.c

@@ -478,16 +478,47 @@ static void xdr_buf_pages_zero(const struct xdr_buf *buf, unsigned int pgbase,
 	} while ((len -= zero) != 0);
 }
 
+static unsigned int xdr_buf_pages_fill_sparse(const struct xdr_buf *buf,
+					      unsigned int buflen, gfp_t gfp)
+{
+	unsigned int i, npages, pagelen;
+
+	if (!(buf->flags & XDRBUF_SPARSE_PAGES))
+		return buflen;
+	if (buflen <= buf->head->iov_len)
+		return buflen;
+	pagelen = buflen - buf->head->iov_len;
+	if (pagelen > buf->page_len)
+		pagelen = buf->page_len;
+	npages = (pagelen + buf->page_base + PAGE_SIZE - 1) >> PAGE_SHIFT;
+	for (i = 0; i < npages; i++) {
+		if (!buf->pages[i])
+			continue;
+		buf->pages[i] = alloc_page(gfp);
+		if (likely(buf->pages[i]))
+			continue;
+		buflen -= pagelen;
+		pagelen = i << PAGE_SHIFT;
+		if (pagelen > buf->page_base)
+			buflen += pagelen - buf->page_base;
+		break;
+	}
+	return buflen;
+}
+
 static void xdr_buf_try_expand(struct xdr_buf *buf, unsigned int len)
 {
 	struct kvec *head = buf->head;
 	struct kvec *tail = buf->tail;
 	unsigned int sum = head->iov_len + buf->page_len + tail->iov_len;
-	unsigned int free_space;
+	unsigned int free_space, newlen;
 
 	if (sum > buf->len) {
 		free_space = min_t(unsigned int, sum - buf->len, len);
-		buf->len += free_space;
+		newlen = xdr_buf_pages_fill_sparse(buf, buf->len + free_space,
+						   GFP_KERNEL);
+		free_space = newlen - buf->len;
+		buf->len = newlen;
 		len -= free_space;
 		if (!len)
 			return;