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Date:   Fri, 2 Jun 2023 11:38:53 -0700
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Subject: Re: [PATCH 02/12] selftests/mm: fix unused variable warning in
 hugetlb-madvise.c
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To:     David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     Peter Xu <peterx@redhat.com>, Shuah Khan <shuah@kernel.org>,
        "Nathan Chancellor" <nathan@kernel.org>, <linux-mm@kvack.org>,
        <linux-kselftest@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <20230602013358.900637-1-jhubbard@nvidia.com>
 <20230602013358.900637-3-jhubbard@nvidia.com>
 <dd78871f-c95c-6691-9ceb-532271b1f453@redhat.com>
From:   John Hubbard <jhubbard@nvidia.com>
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On 6/2/23 03:01, David Hildenbrand wrote:
> On 02.06.23 03:33, John Hubbard wrote:
>> The dummy variable is required in order to make this work, so declare it
>> as volatile in order to avoid the clang compiler warning.
>>
>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>> ---
>>   tools/testing/selftests/mm/hugetlb-madvise.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
>> index 28426e30d9bc..3296ccaf7525 100644
>> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
>> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
>> @@ -65,7 +65,7 @@ void write_fault_pages(void *addr, unsigned long nr_pages)
>>   void read_fault_pages(void *addr, unsigned long nr_pages)
>>   {
>> -    unsigned long dummy = 0;
>> +    volatile unsigned long dummy = 0;
>>       unsigned long i;
>>       for (i = 0; i < nr_pages; i++)
> 
> The compiler can still decide to optimize it all out, because it's not a global variable.
> 
> 
> Placing a
> 
> asm volatile("" : "+r" (dummy));
> 
> after the write tells the compiler that the value will be read and cannot be optimized out (we use that trick in the cow selftest and I've been using it in QEMU for the same purpose as well).
> 

Done. I will send a v2 with this, thanks!


thanks,
-- 
John Hubbard
NVIDIA