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Message-ID: <f199d120-2347-4bc0-8940-155c3c465de9@redhat.com>
Date: Thu, 6 Jun 2024 17:57:21 +0200
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Subject: Re: [PATCH] selftests/mm: use volatile keyword to not optimize mmap
 read variable
From: David Hildenbrand <david@redhat.com>
To: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>,
 Shuah Khan <shuah@kernel.org>, Andrew Morton <akpm@linux-foundation.org>
Cc: willy@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
 gost.dev@samsung.com, mcgrof@kernel.org, linux-kselftest@vger.kernel.org,
 Zi Yan <zi.yan@sent.com>, Pankaj Raghav <p.raghav@samsung.com>
References: <20240606135835.600022-1-kernel@pankajraghav.com>
 <5b4e7ef2-3ced-4d4a-989c-e99b06598d32@redhat.com>
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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