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

[David Hildenbrand <david@redhat.com>]

On 06.06.24 17:56, David Hildenbrand wrote:
> 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));

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

-- 
Cheers,

David / dhildenb