fs: Provide empty .set_page_dirty() aop for anon inodes

.set_page_dirty() is one of those a_ops that defaults to the
buffer implementation when not set. Therefore provide a dummy
function to make it do nothing.

(Uncovered by perfcounters fd's which can now be writable-mmap-ed.)

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Davide Libenzi <davidel@xmailserver.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

fs/anon_inodes.c

@@ -52,6 +52,19 @@ static const struct dentry_operations anon_inodefs_dentry_operations = {
 	.d_delete	= anon_inodefs_delete_dentry,
 };
 
+/*
+ * nop .set_page_dirty method so that people can use .page_mkwrite on
+ * anon inodes.
+ */
+static int anon_set_page_dirty(struct page *page)
+{
+	return 0;
+};
+
+static const struct address_space_operations anon_aops = {
+	.set_page_dirty = anon_set_page_dirty,
+};
+
 /**
  * anon_inode_getfd - creates a new file instance by hooking it up to an
  *                    anonymous inode, and a dentry that describe the "class"
@@ -151,6 +164,8 @@ static struct inode *anon_inode_mkinode(void)
 
 	inode->i_fop = &anon_inode_fops;
 
+	inode->i_mapping->a_ops = &anon_aops;
+
 	/*
 	 * Mark the inode dirty from the very beginning,
 	 * that way it will never be moved to the dirty