linux-kernel - Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split while folio_estimated

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Date: Mon, 26 Feb 2024 13:46:20 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Barry Song <21cnbao@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
 Yin Fengwei <fengwei.yin@...el.com>, Yu Zhao <yuzhao@...gle.com>,
 David Hildenbrand <david@...hat.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>,
 Matthew Wilcox <willy@...radead.org>, Minchan Kim <minchan@...nel.org>,
 Vishal Moola <vishal.moola@...il.com>, Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH] madvise:madvise_cold_or_pageout_pte_range(): allow split
 while folio_estimated_sharers = 0

On 21/02/2024 08:50, Barry Song wrote:
> From: Barry Song <v-songbaohua@...o.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@...el.com>
> Cc: Yu Zhao <yuzhao@...gle.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Kefeng Wang <wangkefeng.wang@...wei.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Minchan Kim <minchan@...nel.org>
> Cc: Vishal Moola (Oracle) <vishal.moola@...il.com>
> Cc: Yang Shi <shy828301@...il.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.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

It shouldn't cause a problem for the pmd case, and there are definitely other
cases where it will help. e.g. madvise_free_pte_range().

Regardless:

Reviewed-by: Ryan Roberts <ryan.roberts@....com>