Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Skip to content
Commit 1c949843 authored by Rasmus Villemoes's avatar Rasmus Villemoes Committed by Al Viro
Browse files

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: default avatarRasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 304ec482
Loading
Loading
Loading
Loading
0% Loading or .
You are about to add 0 people to the discussion. Proceed with caution.
Please register or to comment