NFS: Fix a potential data corruption

We must ensure that the subrequests are joined back into the head before
we can retransmit a request. If the head was not on the commit lists,
because the server wrote it synchronously, we still need to add it back
to the retransmission list.
Add a call that mirrors the effect of nfs_cancel_remove_inode() for
O_DIRECT.

Fixes: ed5d588 ("NFS: Try to join page groups before an O_DIRECT retransmission")
Cc: stable@vger.kernel.org
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/direct.c

@@ -472,13 +472,31 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, struct iov_iter *iter,
 	return result;
 }
 
+static void nfs_direct_add_page_head(struct list_head *list,
+				     struct nfs_page *req)
+{
+	struct nfs_page *head = req->wb_head;
+
+	if (!list_empty(&head->wb_list) || !nfs_lock_request(head))
+		return;
+	if (!list_empty(&head->wb_list)) {
+		nfs_unlock_request(head);
+		return;
+	}
+	list_add(&head->wb_list, list);
+	kref_get(&head->wb_kref);
+	kref_get(&head->wb_kref);
+}
+
 static void nfs_direct_join_group(struct list_head *list, struct inode *inode)
 {
 	struct nfs_page *req, *subreq;
 
 	list_for_each_entry(req, list, wb_list) {
-		if (req->wb_head != req)
+		if (req->wb_head != req) {
+			nfs_direct_add_page_head(&req->wb_list, req);
 			continue;
+		}
 		subreq = req->wb_this_page;
 		if (subreq == req)
 			continue;