linux-kernel - Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

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Date:   Wed, 2 Aug 2023 14:43:47 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Ryan Roberts <ryan.roberts@....com>,
        "Yin, Fengwei" <fengwei.yin@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        akpm@...ux-foundation.org, willy@...radead.org,
        vishal.moola@...il.com, wangkefeng.wang@...wei.com,
        minchan@...nel.org, yuzhao@...gle.com, shy828301@...il.com
Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

On 02.08.23 14:40, Ryan Roberts wrote:
> On 02/08/2023 13:35, Yin, Fengwei wrote:
>>
>>
>> On 8/2/2023 6:27 PM, Ryan Roberts wrote:
>>> On 28/07/2023 17:13, Yin Fengwei wrote:
>>>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
>>>> folio_mapcount() is used to check whether the folio is shared. But it's
>>>> not correct as folio_mapcount() returns total mapcount of large folio.
>>>>
>>>> Use folio_estimated_sharers() here as the estimated number is enough.
>>>>
>>>> Yin Fengwei (2):
>>>>    madvise: don't use mapcount() against large folio for sharing check
>>>>    madvise: don't use mapcount() against large folio for sharing check
>>>>
>>>>   mm/huge_memory.c | 2 +-
>>>>   mm/madvise.c     | 6 +++---
>>>>   2 files changed, 4 insertions(+), 4 deletions(-)
>>>>
>>>
>>> As a set of fixes, I agree this is definitely an improvement, so:
>>>
>>> Reviewed-By: Ryan Roberts
>> Thanks.
>>
>>>
>>>
>>> But I have a couple of comments around further improvements;
>>>
>>> Once we have the scheme that David is working on to be able to provide precise
>>> exclusive vs shared info, we will probably want to move to that. Although that
>>> scheme will need access to the mm_struct of a process known to be mapping the
>>> folio. We have that info, but its not passed to folio_estimated_sharers() so we
>>> can't just reimplement folio_estimated_sharers() - we will need to rework these
>>> call sites again.
>> Yes. This could be extra work. Maybe should delay till David's work is done.
> 
> What you have is definitely an improvement over what was there before. And is
> probably the best we can do without David's scheme. So I wouldn't delay this.
> Just pointing out that we will be able to make it even better later on (if
> David's stuff goes in).

Agreed, we just should be careful and clearly spell out the implications 
and that this is eventually also not what we 100% want.

That MADV_PAGEOUT now fails on a PTE-mapped THP -- as can be seen when 
executing the cow selftest where MADV_PAGEOUT will essentially fail -- 
is certainly undesired and should be fixed IMHO.

-- 
Cheers,

David / dhildenb