Re: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns

From: Linus Torvalds <torvalds@linux-foundation.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH] lib/string.c: Disable tree-loop-distribute-patterns
Date: Tue, 18 Aug 2020 20:32:58 -0700	[thread overview]
Message-ID: <CAHk-=wiJLvqS1_O+yAQSZr-Lj49HdJyLpt3J_nW=otHLfEN4RA@mail.gmail.com> (raw)
In-Reply-To: <20200819030442.GA3396810@rani.riverdale.lan>

On Tue, Aug 18, 2020 at 8:04 PM Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> On Tue, Aug 18, 2020 at 05:44:03PM -0700, Linus Torvalds wrote:
> > Using -fno-tree-loop-distribute-patterns seems to really be a bit too
> > incestuous with internal compiler knowledge.
>
> Fair enough -- you ok with just the -ffreestanding? That's what protects
> the memset in arch/x86/boot/compressed/string.c.

Yeah, I think -ffreestanding makes sense. It may not be optimal, but
it doesn't smell wrong to me.

> > Looking at the implementation of "strscpy()" in the same file, and
> > then comparing that to the ludicrously simplisting "memcpy()", I
> > really get the feeling that that memcpy() is not worth having.
>
> I don't think anything actually uses the generic memcpy, and I think
> only c6x uses the generic memset.

I do think maybe we should just remove the generic memcpy and memset
and say "hey people, you really do need to implement your own".

Even if you don't have this "recognize and recurse" issue, you end up
having other issues like just tracing etc. Yeah, we've hopefully
turned everything like that off, but considering that no major
architecture uses this, I wonder how many small details we've missed
with ftrace recursion etc?

> Might be worth optimizing strnlen etc with the word-at-a-time thing though.

Yeah, possibly. Except the kernel almost never uses strnlen for
anything bigger. At least I haven't seen it very much in the profiles.

The "strncpy_from_user()" stuff shows up like a sore thumb on some
loads (lots and lots of strings from user space for pathnames and
execve), but the kernel itself tends to seldom deal a lot with any
longer strings.  Stuff like device names etc, I  guess, but any time I
see string handling in profiles, it tends to be in user space (GNU
make spends all of its time in string handling, it sometimes seems).

Of course, that may be just me looking at very particular profiles, so
maybe I've just not seen the loads where the kernel strnlen matters.

memcpy and memset? Those matter. A lot.

            Linus