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

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Date:   Sat, 20 Aug 2022 22:46:19 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     sj@...nel.org, muchun.song@...ux.dev, 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 Sun, 21 Aug 2022 13:22:42 +0800 Baolin Wang <baolin.wang@...ux.alibaba.com> wrote:

> 
> 
> On 8/21/2022 5:17 AM, Andrew Morton wrote:
> > On Thu, 18 Aug 2022 15:37:43 +0800 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.
> >>
> > 
> > Do we have a Fixes: for this?
> 
> OK, should be:
> Fixes: 3f49584b262c ("mm/damon: implement primitives for the virtual 
> memory address spaces")
> 
> > What are the user-visible runtime effects of the bug?  "make the access
> > statistics incorrect" is rather vague.
> 
> "access statistics incorrect" means that the DAMON may make incorrect 
> decision according to the incorrect statistics, for example, DAMON may 
> can not reclaim cold page in time due to this cold page was regarded as 
> accessed mistakenly if DAMOS_PAGEOUT operation is specified.
> 
> > Do we feel that a cc:stable is warranted?
> 
> Though this is not a regular case, I think this patch is suitable to be 
> backported to cover this unusual case. So please help to add a stable 
> tag when you apply this patch, or please let me know if you want a new 
> version with adding Fixes and stable tags. Thanks.

Thanks, I took care of all that.