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Date:   Sat, 20 Aug 2022 14:17:02 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Cc:     sj@kernel.org, muchun.song@linux.dev, mike.kravetz@oracle.com,
        damon@lists.linux.dev, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/2] mm/damon: validate if the pmd entry is present
 before accessing
Message-Id: <20220820141702.e731f2aade5da8cbe0c78cba@linux-foundation.org>
In-Reply-To: <58b1d1f5fbda7db49ca886d9ef6783e3dcbbbc98.1660805030.git.baolin.wang@linux.alibaba.com>
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On Thu, 18 Aug 2022 15:37:43 +0800 Baolin Wang <baolin.wang@linux.alibaba.com> wrote:

> 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.
> 
> 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.
> 
> Thus adding a pmd entry present validation to fix above issues.
> 

Do we have a Fixes: for this?

What are the user-visible runtime effects of the bug?  "make the access
statistics incorrect" is rather vague.

Do we feel that a cc:stable is warranted?