NFS: Don't deadlock when cookie hashes collide

In the very rare case where the readdir reply contains multiple cookies
that map to the same hash value, we can end up deadlocking waiting for a
page lock that we already hold. In this case we should fail the page
lock by using grab_cache_page_nowait().
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/dir.c

@@ -381,23 +381,28 @@ static void nfs_readdir_page_unlock_and_put(struct page *page)
 	put_page(page);
 }
 
+static void nfs_readdir_page_init_and_validate(struct page *page, u64 cookie,
+					       u64 change_attr)
+{
+	if (PageUptodate(page)) {
+		if (nfs_readdir_page_validate(page, cookie, change_attr))
+			return;
+		nfs_readdir_clear_array(page);
+	}
+	nfs_readdir_page_init_array(page, cookie, change_attr);
+	SetPageUptodate(page);
+}
+
 static struct page *nfs_readdir_page_get_locked(struct address_space *mapping,
-						u64 last_cookie,
-						u64 change_attr)
+						u64 cookie, u64 change_attr)
 {
-	pgoff_t index = nfs_readdir_page_cookie_hash(last_cookie);
+	pgoff_t index = nfs_readdir_page_cookie_hash(cookie);
 	struct page *page;
 
 	page = grab_cache_page(mapping, index);
 	if (!page)
 		return NULL;
-	if (PageUptodate(page)) {
-		if (nfs_readdir_page_validate(page, last_cookie, change_attr))
-			return page;
-		nfs_readdir_clear_array(page);
-	}
-	nfs_readdir_page_init_array(page, last_cookie, change_attr);
-	SetPageUptodate(page);
+	nfs_readdir_page_init_and_validate(page, cookie, change_attr);
 	return page;
 }
 
@@ -435,11 +440,13 @@ static void nfs_readdir_page_set_eof(struct page *page)
 static struct page *nfs_readdir_page_get_next(struct address_space *mapping,
 					      u64 cookie, u64 change_attr)
 {
+	pgoff_t index = nfs_readdir_page_cookie_hash(cookie);
 	struct page *page;
 
-	page = nfs_readdir_page_get_locked(mapping, cookie, change_attr);
+	page = grab_cache_page_nowait(mapping, index);
 	if (!page)
 		return NULL;
+	nfs_readdir_page_init_and_validate(page, cookie, change_attr);
 	if (nfs_readdir_page_last_cookie(page) != cookie)
 		nfs_readdir_page_reinit_array(page, cookie, change_attr);
 	return page;