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Message-ID: <0764cf72-26fe-627b-f56c-b0d2629bc9e3@redhat.com>
Date:   Mon, 5 Jun 2023 13:35:57 +0200
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Subject: Re: [PATCH v2 02/11] selftests/mm: fix unused variable warnings in
 hugetlb-madvise.c, migration.c
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To:     John Hubbard <jhubbard@nvidia.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: <20230603021558.95299-1-jhubbard@nvidia.com>
 <20230603021558.95299-3-jhubbard@nvidia.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230603021558.95299-3-jhubbard@nvidia.com>
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On 03.06.23 04:15, 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.
> 
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.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;

With the asm, I think the "volatile" might be completely unnecessary. 
But it doesn't hurt.

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb