linux-kernel - Re: [RFC] mm/thp: Update mm's MM_ANONPAGES stat in set_huge_zero

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Date:   Thu, 13 May 2021 09:50:55 -0700
From:   Yang Shi <shy828301@...il.com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Linux MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Zi Yan <ziy@...dia.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] mm/thp: Update mm's MM_ANONPAGES stat in set_huge_zero_page()

On Thu, May 13, 2021 at 12:20 AM Anshuman Khandual
<anshuman.khandual@....com> wrote:
>
> Although the zero huge page is being shared across various processes, each
> mapping needs to update its mm's MM_ANONPAGES stat by HPAGE_PMD_NR in order
> to be consistent. This just updates the stats in set_huge_zero_page() after
> the mapping gets created.

I don't get why MM_ANONPAGES needs to be inc'ed when huge zero page is
installed. This may cause inconsistency between some counters, for
example, MM_ANONPAGES may be much bigger than anon LRU.

MM_ANONPAGES should not be inc'ed unless a new page is allocated and
installed, right?

>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: linux-mm@...ck.org
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> ---
> Should it update MM_SHMEM_PAGES instead ? Applies on latest mainline.
>
>  mm/huge_memory.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 63ed6b25deaa..262703304807 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -706,6 +706,7 @@ static void set_huge_zero_page(pgtable_t pgtable, struct mm_struct *mm,
>         if (pgtable)
>                 pgtable_trans_huge_deposit(mm, pmd, pgtable);
>         set_pmd_at(mm, haddr, pmd, entry);
> +       add_mm_counter(mm, MM_ANONPAGES, HPAGE_PMD_NR);
>         mm_inc_nr_ptes(mm);
>  }
>
> --
> 2.20.1
>
>