linux-kernel - Re: [PATCH 1/3] x86: fix access_ok() and valid_user_address() using wrong USER_PTR

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Message-ID: <20251106192645.4108a505@pumpkin>
Date: Thu, 6 Nov 2025 19:26:45 +0000
From: David Laight <david.laight.linux@...il.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Borislav Petkov <bp@...en8.de>, Linus Torvalds
 <torvalds@...ux-foundation.org>, "the arch/x86 maintainers"
 <x86@...nel.org>, brauner@...nel.org, viro@...iv.linux.org.uk,
 jack@...e.cz, linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 tglx@...utronix.de, pfalcato@...e.de
Subject: Re: [PATCH 1/3] x86: fix access_ok() and valid_user_address() using
 wrong USER_PTR_MAX in modules

On Thu, 6 Nov 2025 14:19:06 +0100
Mateusz Guzik <mjguzik@...il.com> wrote:

> On Thu, Nov 6, 2025 at 2:10 PM Borislav Petkov <bp@...en8.de> wrote:
> >
> > On Thu, Nov 06, 2025 at 01:06:06PM +0100, Mateusz Guzik wrote:  
> > > I don't know what are you trying to say here.
> > >
> > > Are you protesting the notion that reducing cache footprint of the
> > > memory allocator is a good idea, or perhaps are you claiming these
> > > vars are too problematic to warrant the effort, or something else?  
> >
> > I'm saying all work which does not change the code in a trivial way should
> > have numbers to back it up. As in: "this change X shows this perf improvement
> > Y with the benchmark Z."
> >
> > Because code uglification better have a fair justification.
> >
> > Not just random "oh yeah, it would be better to have this." If the changes are
> > trivial, sure. But the runtime const thing was added for a very narrow case,
> > AFAIR, and it wasn't supposed to have a widespread use. And it ain't that
> > trivial, codewise.
> >
> > IOW, no non-trivial changes which become a burden to maintainers without
> > a really good reason for them. This has been the guiding principle for
> > non-trivial perf optimizations in Linux. AFAIR at least.
> >
> > But hey, what do I know...  
> 
> Then, as I pointed out, you should be protesting the patching of
> USER_PTR_MAX as it came with no benchmarks and also resulted in a
> regression.
> 

IIRC it was a definite performance improvement for a specific workload
(compiling kernels) on a system where the relatively small d-cache
caused significant overhead reading the value from memory.

Look at the patch author for more info.

	David