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Date: Mon, 26 Feb 2024 14:50:26 +0100
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Subject: Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split
 while folio_estimated_sharers = 0
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To: Ryan Roberts <ryan.roberts@arm.com>, Barry Song <21cnbao@gmail.com>,
 akpm@linux-foundation.org, linux-mm@kvack.org
Cc: linux-kernel@vger.kernel.org, Barry Song <v-songbaohua@oppo.com>,
 Yin Fengwei <fengwei.yin@intel.com>, Yu Zhao <yuzhao@google.com>,
 Kefeng Wang <wangkefeng.wang@huawei.com>,
 Matthew Wilcox <willy@infradead.org>, Minchan Kim <minchan@kernel.org>,
 Vishal Moola <vishal.moola@gmail.com>, Yang Shi <shy828301@gmail.com>
References: <20240221085036.105621-1-21cnbao@gmail.com>
 <71fa4302-2df6-4e55-a5a8-7609476c41d4@arm.com>
From: David Hildenbrand <david@redhat.com>
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In-Reply-To: <71fa4302-2df6-4e55-a5a8-7609476c41d4@arm.com>
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On 26.02.24 14:46, Ryan Roberts wrote:
> On 21/02/2024 08:50, Barry Song wrote:
>> From: Barry Song <v-songbaohua@oppo.com>
>>
>> The purpose is stopping splitting large folios whose mapcount are 2 or
>> above. Folios whose estimated_shares = 0 should be still perfect and
>> even better candidates than estimated_shares = 1.
>>
>> Consider a pte-mapped large folio with 16 subpages, if we unmap 1-15,
>> the current code will split folios and reclaim them while madvise goes
>> on this folio; but if we unmap subpage 0, we will keep this folio and
>> break. This is weird.
>>
>> For pmd-mapped large folios, we can still use "= 1" as the condition
>> as anyway we have the entire map for it. So this patch doesn't change
>> the condition for pmd-mapped large folios.
>> This also explains why we had been using "= 1" for both pmd-mapped and
>> pte-mapped large folios before commit 07e8c82b5eff ("madvise: convert
>> madvise_cold_or_pageout_pte_range() to use folios"), because in the
>> past, we used the mapcount of the specific subpage, since the subpage
>> had pte present, its mapcount wouldn't be 0.
>>
>> The problem can be quite easily reproduced by writing a small program,
>> unmapping the first subpage of a pte-mapped large folio vs. unmapping
>> anyone other than the first subpage.
>>
>> Fixes: 2f406263e3e9 ("madvise:madvise_cold_or_pageout_pte_range(): don't use mapcount() against large folio for sharing check")
>> Cc: Yin Fengwei <fengwei.yin@intel.com>
>> Cc: Yu Zhao <yuzhao@google.com>
>> Cc: Ryan Roberts <ryan.roberts@arm.com>
>> Cc: David Hildenbrand <david@redhat.com>
>> Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
>> Cc: Matthew Wilcox <willy@infradead.org>
>> Cc: Minchan Kim <minchan@kernel.org>
>> Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
>> Cc: Yang Shi <shy828301@gmail.com>
>> Signed-off-by: Barry Song <v-songbaohua@oppo.com>
>> ---
>>   mm/madvise.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/madvise.c b/mm/madvise.c
>> index cfa5e7288261..abde3edb04f0 100644
>> --- a/mm/madvise.c
>> +++ b/mm/madvise.c
>> @@ -453,7 +453,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>>   		if (folio_test_large(folio)) {
>>   			int err;
>>   
>> -			if (folio_estimated_sharers(folio) != 1)
>> +			if (folio_estimated_sharers(folio) > 1)
>>   				break;
>>   			if (pageout_anon_only_filter && !folio_test_anon(folio))
>>   				break;
> 
> I wonder if we should change all the instances:
> 
> folio_estimated_sharers() != 1   ->   folio_estimated_sharers() > 1
> folio_estimated_sharers() == 1   ->   folio_estimated_sharers() <= 1

I'll send out something that wraps that in folio_mapped_shared() later 
today or tomorrow.

-- 
Cheers,

David / dhildenb