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From:   Yang Shi <shy828301@gmail.com>
Date:   Fri, 17 Dec 2021 11:06:31 -0800
Message-ID: <CAHbLzkp8B8zPSE-jdObqLJ=YxST8=J58FaiHxRq_ESgOG_LjCg@mail.gmail.com>
Subject: Re: [PATCH v1 02/11] mm: thp: consolidate mapcount logic on THP split
To:     David Hildenbrand <david@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
        Michal Hocko <mhocko@kernel.org>,
        Nadav Amit <namit@vmware.com>, Rik van Riel <riel@surriel.com>,
        Roman Gushchin <guro@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Peter Xu <peterx@redhat.com>,
        Donald Dutile <ddutile@redhat.com>,
        Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>, Jan Kara <jack@suse.cz>,
        Linux MM <linux-mm@kvack.org>, linux-kselftest@vger.kernel.org,
        linux-doc@vger.kernel.org
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On Fri, Dec 17, 2021 at 3:33 AM David Hildenbrand <david@redhat.com> wrote:
>
> Let's consolidate the mapcount logic to make it easier to understand and
> to prepare for further changes.
>
> Reviewed-by: Peter Xu <peterx@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Yang Shi <shy828301@gmail.com>

> ---
>  mm/huge_memory.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index e5483347291c..4751d03947da 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2101,21 +2101,25 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
>                 pte = pte_offset_map(&_pmd, addr);
>                 BUG_ON(!pte_none(*pte));
>                 set_pte_at(mm, addr, pte, entry);
> -               if (!pmd_migration)
> -                       atomic_inc(&page[i]._mapcount);
>                 pte_unmap(pte);
>         }
>
>         if (!pmd_migration) {
> +               /* Sub-page mapcount accounting for above small mappings. */
> +               int val = 1;
> +
>                 /*
>                  * Set PG_double_map before dropping compound_mapcount to avoid
>                  * false-negative page_mapped().
> +                *
> +                * The first to set PageDoubleMap() has to increment all
> +                * sub-page mapcounts by one.
>                  */
> -               if (compound_mapcount(page) > 1 &&
> -                   !TestSetPageDoubleMap(page)) {
> -                       for (i = 0; i < HPAGE_PMD_NR; i++)
> -                               atomic_inc(&page[i]._mapcount);
> -               }
> +               if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page))
> +                       val++;
> +
> +               for (i = 0; i < HPAGE_PMD_NR; i++)
> +                       atomic_add(val, &page[i]._mapcount);
>
>                 lock_page_memcg(page);
>                 if (atomic_add_negative(-1, compound_mapcount_ptr(page))) {
> --
> 2.31.1
>