Re: [PATCH 5/5] mm/huge_memory.c: don't discard hugepage if other processes are mapping it

[Yang Shi]

On Tue, Apr 27, 2021 at 6:32 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>
> If other processes are mapping any other subpages of the hugepage, i.e. in
> pte-mapped thp case, page_mapcount() will return 1 incorrectly. Then we
> would discard the page while other processes are still mapping it. Fix it
> by using total_mapcount() which can tell whether other processes are still
> mapping it.

Seems correct to me. It is possible that the THP is PTE-mapped by the
other processes.

Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

>
> Fixes: b8d3c4c3009d ("mm/huge_memory.c: don't split THP page when MADV_FREE syscall is called")
> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index f652be6ecca3..d14fecb8cd00 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1604,7 +1604,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>          * If other processes are mapping this page, we couldn't discard
>          * the page unless they all do MADV_FREE so let's skip the page.
>          */
> -       if (page_mapcount(page) != 1)
> +       if (total_mapcount(page) != 1)
>                 goto out;
>
>         if (!trylock_page(page))
> --
> 2.23.0
>
>