Re: [PATCH] selftests/mm: use volatile keyword to not optimize mmap read variable

[David Hildenbrand]

On 06.06.24 17:56, David Hildenbrand wrote:
> On 06.06.24 15:58, Pankaj Raghav (Samsung) wrote:
> > From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> >
> > create_pagecache_thp_and_fd() in split_huge_page_test.c used the
> > variable dummy to perform mmap read.
> >
> > However, this test was skipped even on XFS which has large folio
> > support. The issue was compiler (gcc 13.2.0) was optimizing out the
> > dummy variable, therefore, not creating huge page in the page cache.
> >
> > Add volatile keyword to force compiler not to optimize out the loop
> > where we read from the mmaped addr.
> >
> > Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> > ---
> >    tools/testing/selftests/mm/split_huge_page_test.c | 2 +-
> >    1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
> > index d3c7f5fb3e7b..c573a58f80ab 100644
> > --- a/tools/testing/selftests/mm/split_huge_page_test.c
> > +++ b/tools/testing/selftests/mm/split_huge_page_test.c
> > @@ -300,7 +300,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
> >    		char **addr)
> >    {
> >    	size_t i;
> > -	int __attribute__((unused)) dummy = 0;
> > +	volatile int __attribute__((unused)) dummy = 0;
> >    
> >    	srand(time(NULL));
> >    
> >
> > base-commit: d97496ca23a2d4ee80b7302849404859d9058bcd
>
> The rick we do in some other tests is:
>
> char *tmp;
>
> tmp = *whatever;
> asm volatile("" : "+r" (tmp));

char tmp; of course. See cow.c as an example.

--
Cheers,

David / dhildenb