Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210427133214.2270207-1-linmiaohe@huawei.com> <20210427133214.2270207-6-linmiaohe@huawei.com>
In-Reply-To: <20210427133214.2270207-6-linmiaohe@huawei.com>
From:   Yang Shi <shy828301@gmail.com>
Date:   Tue, 27 Apr 2021 14:22:48 -0700
Message-ID: <CAHbLzkrYqKw+iF_+sDccyBEAsRySTPKyhd01p9f6KV0FLLAbnQ@mail.gmail.com>
Subject: Re: [PATCH 5/5] mm/huge_memory.c: don't discard hugepage if other
 processes are mapping it
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, Zi Yan <ziy@nvidia.com>,
        william.kucharski@oracle.com, Matthew Wilcox <willy@infradead.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        aneesh.kumar@linux.ibm.com, Ralph Campbell <rcampbell@nvidia.com>,
        Song Liu <songliubraving@fb.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@surriel.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Apr 27, 2021 at 6:32 AM Miaohe Lin <linmiaohe@huawei.com> 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@gmail.com>

>
> Fixes: b8d3c4c3009d ("mm/huge_memory.c: don't split THP page when MADV_FREE syscall is called")
> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  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
>
>