[PATCH] ramfs o_sync oops fix

Writing to ramfs files with O_SYNC dereferences null in mpage_writepages(),
due to null a_ops->writepage.  The call path is

	generic_file_aio_write_nolock
	->generic_osync_inode
	  ->write_inode_now
	    ->__writeback_single_inode
	      ->__sync_single_inode
	        ->do_writepages
	          ->mpage_writepages

The best fix for this is to visit all ram-backed filesystems and give them a
no-op a_ops.writepages.  But baling out if the file is memory-backed is a
sufficient coverall and is how we handle this in __filemap_fdatawrite().
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/fs-writeback.c

@@ -535,6 +535,9 @@ void write_inode_now(struct inode *inode, int sync)
 		.sync_mode = WB_SYNC_ALL,
 	};
 
+	if (inode->i_mapping->backing_dev_info->memory_backed)
+		return;
+
 	spin_lock(&inode_lock);
 	__writeback_single_inode(inode, &wbc);
 	spin_unlock(&inode_lock);