From 1c94984396dc7bc40b4f6899674eaa41f29a4f6e Mon Sep 17 00:00:00 2001 From: Rasmus Villemoes Date: Thu, 1 Mar 2018 00:19:21 +0100 Subject: 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 Signed-off-by: Al Viro --- include/linux/fs.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include') diff --git a/include/linux/fs.h b/include/linux/fs.h index 2a815560fda0..d7b2caadb292 100644 --- a/include/linux/fs.h +++ b/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[]; }; -- cgit v1.2.3