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author | Rasmus Villemoes <linux@rasmusvillemoes.dk> | 2018-03-01 00:19:21 +0100 |
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committer | Al Viro <viro@zeniv.linux.org.uk> | 2018-03-19 01:07:04 -0400 |
commit | 1c94984396dc7bc40b4f6899674eaa41f29a4f6e (patch) | |
tree | a111ee94fffcd116066eb92572e2658bd2a9d101 /README | |
parent | 304ec482f562885b178b370cd50340447585d1c0 (diff) |
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>
Diffstat (limited to 'README')
0 files changed, 0 insertions, 0 deletions