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

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Message-ID: <dc139b0b-e461-4edb-ba1d-fc824f875142@lucifer.local>
Date: Tue, 5 Aug 2025 20:21:23 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Zi Yan <ziy@...dia.com>
Cc: 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,
        Pedro Falcato <pfalcato@...e.de>
Subject: Re: [PATCH] selftests/mm: fix FORCE_READ to read input value
 correctly.

+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.

Yeah I won't pretend to understand _exactly_ what the compiler is doing here, if
this is working in practice across multiple compilers and read-faulting the page
that's good enough for me :)

>
> >
> > I know for some reason this form doesn't generate one currently (not sure
> > why), but we may hit that in future.
>
> Neither gcc nor clang complains without (void). My guess is that volatile
> is doing something there.

Indeed possibly, be interesting if you or Pedro who's also playing with this
could nail down exactly what's going on here.

>
> Best Regards,
> Yan, Zi

But from my point of view this patch is fine - ship it! :)

Cheers, Lorenzo