Re: [patch 02/14] tmpfs: fix regressions from wider use of ZERO_PAGE

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, May 4, 2022 at 11:56 AM Borislav Petkov <bp@alien8.de> wrote:
>
> Just to update folks here: I haven't forgotten about this - Mel and I
> are running some benchmarks first and staring at results to see whether
> all the hoopla is even worth it.

Side note: the "do FSRM inline" would likely be a really good thing
for "copy_to_user()", more so than the silly "clear_user()" that we
realistically do almost nowhere.

I doubt you can find "clear_user()" outside of benchmarks (but hey,
people do odd things).

But "copy_to_user()" is everywhere, and the I$ advantage of inlining
it might be noticeable on some real loads.

I remember some git profiles having copy_to_user very high due to
fstat(), for example - cp_new_stat64 and friends.

Of course, I haven't profiled git in ages, but I doubt that has
changed. Many of those kinds of loads are all about name lookup and
stat (basic things like "make" would be that too, if it weren't for
the fact that it spends a _lot_ of its time in user space string
handling).

The inlining advantage would obviously only show up on CPUs that
actually do FSRM. Which I think is currently only Ice Lake. I don't
have access to one.

                Linus