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Subject: Re: [PATCH v2 1/2] mm/damon: validate if the pmd entry is present
 before accessing
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <58b1d1f5fbda7db49ca886d9ef6783e3dcbbbc98.1660805030.git.baolin.wang@linux.alibaba.com>
Date:   Thu, 18 Aug 2022 16:27:41 +0800
Cc:     sj@kernel.org, Andrew Morton <akpm@linux-foundation.org>,
        Mike Kravetz <mike.kravetz@oracle.com>, damon@lists.linux.dev,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
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Message-Id: <A18421D4-7BD6-41E6-A284-846C4EF53B2F@linux.dev>
References: <58b1d1f5fbda7db49ca886d9ef6783e3dcbbbc98.1660805030.git.baolin.wang@linux.alibaba.com>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Precedence: bulk


> On Aug 18, 2022, at 15:37, Baolin Wang <baolin.wang@linux.alibaba.com> =
wrote:
>=20
> The pmd_huge() is used to validate if the pmd entry is mapped by a =
huge
> page, also including the case of non-present (migration or hwpoisoned)
> pmd entry on arm64 or x86 architectures. That means the pmd_pfn() can
> not get the correct pfn number for the non-present pmd entry, which
> will cause damon_get_page() to get an incorrect page struct (also
> may be NULL by pfn_to_online_page()) to make the access statistics
> incorrect.
>=20
> Moreover it does not make sense that we still waste time to get the
> page of the non-present entry, just treat it as not-accessed and skip =
it,
> that keeps consistent with non-present pte level entry.
>=20
> Thus adding a pmd entry present validation to fix above issues.
>=20
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reviewed-by: SeongJae Park <sj@kernel.org>

Reviewed-by: Muchun Song <songmuchun@bytedance.com>

Thanks.