From: Linus Torvalds <torvalds@linux-foundation.org>
To: Dan Williams <dan.j.williams@intel.com>
Cc: "Luck, Tony" <tony.luck@intel.com>,
Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
Peter Zijlstra <peterz@infradead.org>,
Borislav Petkov <bp@alien8.de>, stable <stable@vger.kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>,
Paul Mackerras <paulus@samba.org>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Erwin Tsaur <erwin.tsaur@intel.com>,
Michael Ellerman <mpe@ellerman.id.au>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-nvdimm <linux-nvdimm@lists.01.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()
Date: Thu, 30 Apr 2020 17:24:49 -0700 [thread overview]
Message-ID: <CAHk-=wh1SPyuGkTkQESsacwKTpjWd=_-KwoCK5o=SuC3yMdf7A@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiMs=A90np0Hv5WjHY8HXQWpgtuq-xrrJvyk7_pNB4meg@mail.gmail.com>
On Thu, Apr 30, 2020 at 5:10 PM Linus Torvalds
<torvalds@linux-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
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next prev parent reply other threads:[~2020-05-01 0:31 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-30 8:24 [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 1/2] copy_safe: Rename memcpy_mcsafe() to copy_safe() Dan Williams
2020-04-30 8:25 ` [PATCH v2 2/2] x86/copy_safe: Introduce copy_safe_fast() Dan Williams
2020-04-30 14:02 ` [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe() Linus Torvalds
2020-04-30 16:51 ` Andy Lutomirski
2020-04-30 17:17 ` Linus Torvalds
2020-04-30 18:42 ` Andy Lutomirski
2020-04-30 19:22 ` Luck, Tony
2020-04-30 19:50 ` Linus Torvalds
2020-04-30 20:25 ` Luck, Tony
2020-04-30 23:52 ` Dan Williams
2020-05-01 0:10 ` Linus Torvalds
2020-05-01 0:23 ` Andy Lutomirski
2020-05-01 0:39 ` Linus Torvalds
2020-05-01 1:10 ` Andy Lutomirski
2020-05-01 14:09 ` Luck, Tony
2020-05-03 0:29 ` Andy Lutomirski
2020-05-04 20:05 ` Luck, Tony
2020-05-04 20:26 ` Andy Lutomirski
2020-05-04 21:30 ` Dan Williams
2020-05-01 0:24 ` Linus Torvalds [this message]
2020-05-01 1:20 ` Andy Lutomirski
2020-05-01 1:21 ` Dan Williams
2020-05-01 18:28 ` Linus Torvalds
2020-05-01 20:17 ` Dave Hansen
2020-05-03 12:57 ` David Laight
2020-05-04 18:33 ` Dan Williams
2020-05-11 15:24 ` Vivek Goyal
2020-04-30 19:51 ` Dan Williams
2020-04-30 20:07 ` Andy Lutomirski
2020-05-01 7:46 ` David Laight
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