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Date: Wed, 21 Feb 2024 10:15:32 +0100
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Subject: Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split
 while folio_estimated_sharers = 0
To: 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>,
 Ryan Roberts <ryan.roberts@arm.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>
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From: David Hildenbrand <david@redhat.com>
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On 21.02.24 09:50, 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;

That's also what I do in

https://lkml.kernel.org/r/20231124132626.235350-4-david@redhat.com

I'll revive that soon.

-- 
Cheers,

David / dhildenb