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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 4 Mar 2023 13:10:32 -0800
Message-ID: <CAHk-=whEwe1H1_YXki1aYwGnVwazY+z0=6deU-Zd855ogvLgww@mail.gmail.com>
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Mateusz Guzik <mjguzik@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Eric Biggers <ebiggers@google.com>,
        Christian Brauner <brauner@kernel.org>, serge@hallyn.com,
        paul@paul-moore.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-security-module@vger.kernel.org
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On Sat, Mar 4, 2023 at 1:01=E2=80=AFPM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> It's still completely untested. Treat this very much as a "Let's make
> the common cases faster, at least for !MAXSMP".

Ok, so I started "testing" it in the sense that I actually looked at
the code it generated, and went all "why didn't it make any
difference".

And that's because the patch had the

  #ifdef CONFIG_CPUMASK_OFFSTACK

condition exactly the wrong way around.

So if somebody actually wants to play with that patch, you need to
change that to be

  #ifndef CONFIG_CPUMASK_OFFSTACK

(and then you obviously need to have a kernel config that does *not*
have MAXSMP set).

That at least simplifies some of the code generation when I look at
it. Whether the end result _works_ or not, I still haven't checked.
That patch is still very much meant as a "look, something like this
should make our cpumask handling much more efficient"

              Linus