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Message-ID: <b069d5c5-7ea0-6960-74de-dcc6caf81a5e@collabora.com>
Date:   Tue, 6 Jun 2023 12:48:29 +0500
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Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        David Hildenbrand <david@redhat.com>,
        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>
Subject: Re: [PATCH v3 02/11] selftests/mm: fix unused variable warnings in
 hugetlb-madvise.c, migration.c
Content-Language: en-US
To:     John Hubbard <jhubbard@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20230606071637.267103-1-jhubbard@nvidia.com>
 <20230606071637.267103-3-jhubbard@nvidia.com>
From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
In-Reply-To: <20230606071637.267103-3-jhubbard@nvidia.com>
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Precedence: bulk

On 6/6/23 12:16 PM, John Hubbard wrote:
> Dummy variables are required in order to make these two (similar)
> routines work, so in both cases, declare the variables as volatile in
> order to avoid the clang compiler warning.
> 
> Furthermore, in order to ensure that each test actually does what is
> intended, add an asm volatile invocation (thanks to David Hildenbrand
> for the suggestion), with a clarifying comment so that it survives
> future maintenance.
> 
> Reviewed-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>

> ---
>  tools/testing/selftests/mm/hugetlb-madvise.c | 8 ++++++--
>  tools/testing/selftests/mm/migration.c       | 5 ++++-
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/hugetlb-madvise.c b/tools/testing/selftests/mm/hugetlb-madvise.c
> index 28426e30d9bc..d55322df4b73 100644
> --- a/tools/testing/selftests/mm/hugetlb-madvise.c
> +++ b/tools/testing/selftests/mm/hugetlb-madvise.c
> @@ -65,11 +65,15 @@ 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++)
> +	for (i = 0; i < nr_pages; i++) {
>  		dummy += *((unsigned long *)(addr + (i * huge_page_size)));
> +
> +		/* Prevent the compiler from optimizing out the entire loop: */
> +		asm volatile("" : "+r" (dummy));
> +	}
>  }
>  
>  int main(int argc, char **argv)
> diff --git a/tools/testing/selftests/mm/migration.c b/tools/testing/selftests/mm/migration.c
> index 1cec8425e3ca..379581567f27 100644
> --- a/tools/testing/selftests/mm/migration.c
> +++ b/tools/testing/selftests/mm/migration.c
> @@ -95,12 +95,15 @@ int migrate(uint64_t *ptr, int n1, int n2)
>  
>  void *access_mem(void *ptr)
>  {
> -	uint64_t y = 0;
> +	volatile uint64_t y = 0;
>  	volatile uint64_t *x = ptr;
>  
>  	while (1) {
>  		pthread_testcancel();
>  		y += *x;
> +
> +		/* Prevent the compiler from optimizing out the writes to y: */
> +		asm volatile("" : "+r" (y));
>  	}
>  
>  	return NULL;

-- 
BR,
Muhammad Usama Anjum