linux-kernel - Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible

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Date:   Fri, 3 Mar 2023 19:25:02 -0800
From:   Yury Norov <yury.norov@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mateusz Guzik <mjguzik@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Eric Biggers <ebiggers@...gle.com>,
        Christian Brauner <brauner@...nel.org>, serge@...lyn.com,
        paul@...l-moore.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if
 possible

[...]

> In particular, 'cpumask_clear()' should just zero the cpumask, and on
> the config I use, I have
> 
>     CONFIG_NR_CPUS=64
> 
> so it should literally just be a single "store zero to cpumask word".
> And that's what it used to be.
> 
> But then we had commit aa47a7c215e7 ("lib/cpumask: deprecate
> nr_cpumask_bits") and suddenly 'nr_cpumask_bits' isn't a simple
> constant any more for the "small mask that fits on stack" case, and
> instead you end up with code like
> 
>         movl    nr_cpu_ids(%rip), %edx
>         addq    $63, %rdx
>         shrq    $3, %rdx
>         andl    $-8, %edx
>         ..
>         callq   memset@PLT
> 
> that does a 8-byte memset because I have 32 cores and 64 threads.

Did you enable CONFIG_FORCE_NR_CPUS? If you pick it, the kernel will
bind nr_cpu_ids to NR_CPUS at compile time, and the memset() call
should disappear.

Depending on your compiler you might want to apply this patch as well:

https://lore.kernel.org/lkml/20221027043810.350460-2-yury.norov@gmail.com/

> Now, at least some distro kernels seem to be built with CONFIG_MAXSMP,
> so CONFIG_NR_CPUS is something insane (namely 8192), and then it is
> indeed better to calculate some minimum size instead of doing a 1kB
> memset().

Ubuntu too. That was one of the reasons for the patch.

Thanks,
Yury