linux-kernel - Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy

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Message-ID: <CAHk-=wh1SPyuGkTkQESsacwKTpjWd=_-KwoCK5o=SuC3yMdf7A@mail.gmail.com>
Date:   Thu, 30 Apr 2020 17:24:49 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

On Thu, Apr 30, 2020 at 5:10 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> It's a horrible word, btw. The word doesn't actually mean what Andy
> means it to mean. "fallible" means "can make mistakes", not "can
> fault".

Btw, on naming: the name should be about _why_ it can fault, not about
whether it faults.

Which hasn't been explained to me.

I know why user accesses can fault. I still don't know why these new
accesses can fault. I know of the old name (mcs), but the newly
suggested name (safe) is the _opposite_ of an explanation of why it
faults.

Naming - like comments - shouldn't be about what some implementation
is, but about the concept.

Again, let me use that "copy_to_user()" as an example of this. Yes, it
can fault. Notice how the name doesn't say "copy_to_faulting()". That
would be WRONG. It can fault _because_ it is user memory, so
"copy_to_user()" not only describes what it does, but it also
implicitly describes that it can fault.

THAT is the kind of explanation I'm looking for. The "memcpy_mcsafe()"
at least had _some_ of that in it. It was wrong for all the _other_
reasons (not having a direction, and the hardware just being complete
and utter garbage), but at least there was a reason in the name.

I am not interested in adding new infrastructure that cannot even be
explained. Why would writes ever fault, considering they are posted
(and again, "user space" is not a valid reason, we have that case
already and have had it since day #1 even if the original naming was
the same kind of bad implementation-specific name that "mcsafe" was).

If the ONLY reason for the fault is a machine check, then the name
should say so, and "copy_mc_to_user()" would be a perfectly fine name
(along with copy_to_mc(), copy_from_mc(), and copy_in_mc()).

It wasn't clear how "copy_to_mc()" could ever fault. Poisoning
after-the-fact? Why would that be preferable to just mapping a dummy
page? But even if it cannot fault, I can see that you might want to do
it as a special kind of copy to avoid any read-mask-write artifacts
(which can definitely happen on other architectures, and I could see a
manual memcpy() implementation doing even on x86)

                  Linus