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

[David Hildenbrand <david@redhat.com>]

On 06.06.24 15:58, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> 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@samsung.com>
> ---
>   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));


-- 
Cheers,

David / dhildenb