+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
