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

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Mateusz Guzik <mjguzik@gmail.com>
Cc: 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
Subject: Re: [PATCH v3 2/2] vfs: avoid duplicating creds in faccessat if possible
Date: Fri, 3 Mar 2023 12:08:01 -0800	[thread overview]
Message-ID: <CAHk-=wjy_q9t4APgug9q-EBMRKAybXt9DQbyM9Egsh=F+0k2Mg@mail.gmail.com> (raw)
In-Reply-To: <CAGudoHG+anGcO1XePmLjb+Hatr4VQMiZ2FufXs8hT3JrHyGMAw@mail.gmail.com>

On Fri, Mar 3, 2023 at 11:37 AM Mateusz Guzik <mjguzik@gmail.com> wrote:
>
> I mentioned in the previous e-mail that memset is used a lot even
> without the problematic opt and even have shown size distribution of
> what's getting passed there.

Well, I *have* been pushing Intel to try to fix memcpy and memset for
over two decades by now, but with FSRM I haven't actually seen the
huge problems any more.

It may just be that the loads I look at don't have issues (or the
machines I've done profiles on don't tend to show them as much).

Hmm. Just re-did my usual kernel profile. It may also be that
something has changed. I do see "clear_page" at the top, but yes,
memset is higher up than I remembered.

I actually used to have the reverse of your hack for this - I've had
various hacks over the year that made memcpy and memset be inlined
"rep movs/stos", which (along with inlined spinlocks) is a great way
to see the _culprit_ (without having to deal with the call chains -
which always get done the wrong way around imnsho).

            Linus