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From: Zi Yan <ziy@nvidia.com>
To: "Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Shuah Khan <shuah@kernel.org>,
 linux-mm@kvack.org, gost.dev@samsung.com,
 David Hildenbrand <david@redhat.com>, willy@infradead.org, mcgrof@kernel.org,
 linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
 Pankaj Raghav <p.raghav@samsung.com>
Subject: Re: [PATCH v3] selftests/mm: use asm volatile to not optimize mmap
 read variable
Date: Thu, 06 Jun 2024 13:52:36 -0700
Message-ID: <5B059AE5-AA46-465D-A91E-9142A4FFA0F2@nvidia.com>
In-Reply-To: <20240606203619.677276-1-kernel@pankajraghav.com>
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On 6 Jun 2024, at 13:36, 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.
>
> Use asm volatile() trick to force the compiler not to optimize out
> the loop where we read from the mmaped addr. This is similar to what is
> being done in other tests (cow.c, etc)
>
> As the variable is now used in the asm statement, remove the unused
> attribute.
>
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
> Changes since v2:
> - Use the asm volatile trick to force the compiler to not optimize the
>   read into dummy variable. (David)
>
>  tools/testing/selftests/mm/split_huge_page_test.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
LGTM. Reviewed-by: Zi Yan <ziy@nvidia.com>

Best Regards,
Yan, Zi

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