linux-kernel - Re: [PATCH v2 1/2] mm/damon: validate if the pmd entry is present before accessing

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Message-Id: <A18421D4-7BD6-41E6-A284-846C4EF53B2F@linux.dev>
Date:   Thu, 18 Aug 2022 16:27:41 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     sj@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>, damon@...ts.linux.dev,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/damon: validate if the pmd entry is present
 before accessing



> On Aug 18, 2022, at 15:37, Baolin Wang <baolin.wang@...ux.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.
> 
> Signed-off-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> Reviewed-by: SeongJae Park <sj@...nel.org>

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

Thanks.