vfs: make sure struct filename->iname is word-aligned

I noticed that offsetof(struct filename, iname) is actually 28 on 64
bit platforms, so we always pass an unaligned pointer to
strncpy_from_user. This is mostly a problem for those 64 bit platforms
without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned
accesses carry a penalty.

A user-space microbenchmark doing nothing but strncpy_from_user from the
same (aligned) source string runs about 5% faster when the destination
is aligned. That number increases to 20% when the string is long
enough (~32 bytes) that we cross a cache line boundary - that's for
example the case for about half the files a "git status" in a kernel
tree ends up stat'ing.

This won't make any real-life workloads 5%, or even 1%, faster, but path
lookup is common enough that cutting even a few cycles should be
worthwhile. So ensure we always pass an aligned destination pointer to
strncpy_from_user. Instead of explicit padding, simply swap the refcnt
and aname members, as suggested by Al Viro.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -39,6 +39,7 @@
 #include <linux/bitops.h>
 #include <linux/init_task.h>
 #include <linux/uaccess.h>
+#include <linux/build_bug.h>
 
 #include "internal.h"
 #include "mount.h"
@@ -130,6 +131,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
 	struct filename *result;
 	char *kname;
 	int len;
+	BUILD_BUG_ON(offsetof(struct filename, iname) % sizeof(long) != 0);
 
 	result = audit_reusename(filename);
 	if (result)

include/linux/fs.h

@@ -2380,8 +2380,8 @@ struct audit_names;
 struct filename {
 	const char		*name;	/* pointer to actual string */
 	const __user char	*uptr;	/* original userland pointer */
-	struct audit_names	*aname;
 	int			refcnt;
+	struct audit_names	*aname;
 	const char		iname[];
 };