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

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Message-ID: <3541d2de-5cf8-2f84-8153-277e2bfc0101@arm.com>
Date:   Wed, 2 Aug 2023 13:40:18 +0100
From:   Ryan Roberts <ryan.roberts@....com>
To:     "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, david@...hat.com,
        shy828301@...il.com
Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

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).

> 
>>
>> Given the aspiration for most of the memory to be large folios going forwards,
>> wouldn't it be better to avoid splitting the large folio where the large folio
>> is mapped entirely within the range of the madvise operation? Sorry if this has
>> already been discussed and decided against - I didn't follow the RFC too
>> closely. Or perhaps you plan to do this as a follow up?
> Yes. We are on same page. RFC patchset did that. But there are some other opens
> on the RFC. So I tried to submit this part of change which is bug fix. The other
> thing left in RFC is optimization (avoid split large folio if we can).
> 
> 
> Regards
> Yin, Fengwei
> 
>>
>> Thanks,
>> Ryan
>>