linux-kernel - Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone

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Date:   Thu, 24 Oct 2019 12:26:29 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     Hugh Dickins <hughd@...gle.com>, kirill.shutemov@...ux.intel.com,
        aarcange@...hat.com, Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: thp: clear PageDoubleMap flag when the last PMD map gone

On Thu, Oct 24, 2019 at 10:28 AM Yang Shi <yang.shi@...ux.alibaba.com> wrote:
>
> File THP sets PageDoubleMap flag when the first it gets PTE mapped, but
> the flag is never cleared until the THP is freed.  This result in
> unbalanced state although it is not a big deal.
>
> Clear the flag when the last compound_mapcount is gone.  It should be
> cleared when all the PTE maps are gone (become PMD mapped only) as well,
> but this needs check all subpage's _mapcount every time any subpage's
> rmap is removed, the overhead may be not worth.  The anonymous THP also
> just clears PageDoubleMap flag when the last PMD map is gone.
>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> Cc: Andrea Arcangeli <aarcange@...hat.com>
> Signed-off-by: Yang Shi <yang.shi@...ux.alibaba.com>

Looks good to me. Thanks!

Acked-by: Song Liu <songliubraving@...com>

> ---
> Hugh thought it is unnecessary to fix it completely due to the overhead
> (https://lkml.org/lkml/2019/10/22/1011), but it sounds simple to achieve
> the similar balance as anonymous THP.
>
>  mm/rmap.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 0c7b2a9..d17cbf3 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1236,6 +1236,9 @@ static void page_remove_file_rmap(struct page *page, bool compound)
>                         __dec_node_page_state(page, NR_SHMEM_PMDMAPPED);
>                 else
>                         __dec_node_page_state(page, NR_FILE_PMDMAPPED);
> +
> +               /* The last PMD map is gone */
> +               ClearPageDoubleMap(compound_head(page));
>         } else {
>                 if (!atomic_add_negative(-1, &page->_mapcount))
>                         goto out;
> --
> 1.8.3.1
>
>