linux-kernel - Re: [PATCH] selftests/mm: fix FORCE_READ to read input value correctly.

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Message-ID: <vahlhhwb3vh33c7nh67usxfraeotqq5pw5cwfak5ke3ysegv5l@bgtlqmv2wqyf>
Date: Wed, 6 Aug 2025 11:34:57 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Zi Yan <ziy@...dia.com>, wang lian <lianux.mm@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, David Hildenbrand <david@...hat.com>, 
	Wei Yang <richard.weiyang@...il.com>, Christian Brauner <brauner@...nel.org>, 
	Jann Horn <jannh@...gle.com>, Kairui Song <ryncsn@...il.com>, 
	Liam Howlett <liam.howlett@...cle.com>, Mark Brown <broonie@...nel.org>, SeongJae Park <sj@...nel.org>, 
	Shuah Khan <shuah@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org, 
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] selftests/mm: fix FORCE_READ to read input value
 correctly.

On Tue, Aug 05, 2025 at 08:21:23PM +0100, Lorenzo Stoakes wrote:
> +cc Pedro
> 
> On Tue, Aug 05, 2025 at 03:09:54PM -0400, Zi Yan wrote:
> > On 5 Aug 2025, at 15:00, Lorenzo Stoakes wrote:
> >
> > > On Tue, Aug 05, 2025 at 01:51:40PM -0400, Zi Yan wrote:
> > >> diff --git a/tools/testing/selftests/mm/vm_util.h b/tools/testing/selftests/mm/vm_util.h
> > >> index c20298ae98ea..b55d1809debc 100644
> > >> --- a/tools/testing/selftests/mm/vm_util.h
> > >> +++ b/tools/testing/selftests/mm/vm_util.h
> > >> @@ -23,7 +23,7 @@
> > >>   * anything with it in order to trigger a read page fault. We therefore must use
> > >>   * volatile to stop the compiler from optimising this away.
> > >>   */
> > >> -#define FORCE_READ(x) (*(volatile typeof(x) *)x)
> > >> +#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x))
> > >
> > > NIT: but wonder if const is necessary, and also (as discussed off-list
> >
> > I just used READ_ONCE() code, but it is not necessary.
> 
> It's not end of the world though.
> 
> >
> > > again :) will this work with a (void) prefixed, just to a. make it clear
> > > we're reading but discarding and b. to avoid any possible compiler warning
> > > on this?
> >
> > Adding (void) makes no difference, at least from godbolt.
>

I disagree with adding (void), because volatile being properly propagated into
the type should hide any Wunused-value warnings (because volatile reads can have
side effects, so discarding a read is most definitely valid).

And as I was seeing in https://godbolt.org/z/jnWsET1vx yesterday, GCC (and clang)
can silently drop the volatile qualifier For Some Reason.

-- 
Pedro