[PATCH] filemap_getpage can block when MAP_NONBLOCK specified

We will return NULL from filemap_getpage when a page does not exist in the
page cache and MAP_NONBLOCK is specified, here:

	page = find_get_page(mapping, pgoff);
	if (!page) {
		if (nonblock)
			return NULL;
		goto no_cached_page;
	}

But we forget to do so when the page in the cache is not uptodate.  The
following could result in a blocking call:

	/*
	 * Ok, found a page in the page cache, now we need to check
	 * that it's up-to-date.
	 */
	if (!PageUptodate(page))
		goto page_not_uptodate;
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/filemap.c

@@ -1379,8 +1379,13 @@ static struct page * filemap_getpage(struct file *file, unsigned long pgoff,
 	 * Ok, found a page in the page cache, now we need to check
 	 * that it's up-to-date.
 	 */
-	if (!PageUptodate(page))
+	if (!PageUptodate(page)) {
+		if (nonblock) {
+			page_cache_release(page);
+			return NULL;
+		}
 		goto page_not_uptodate;
+	}
 
 success:
 	/*