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From:   Yang Shi <shy828301@gmail.com>
Date:   Wed, 19 Aug 2020 16:04:47 -0700
Message-ID: <CAHbLzkrLK3gkeA2_B7jyf99jmA0yvV0fQE53sqOLHyocaCGDSw@mail.gmail.com>
Subject: Re: [PATCH v2 2/3] mm: remove superfluous __ClearPageActive()
To:     Yu Zhao <yuzhao@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        Huang Ying <ying.huang@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Michal Hocko <mhocko@suse.com>,
        Yang Shi <yang.shi@linux.alibaba.com>, Qian Cai <cai@lca.pw>,
        Mel Gorman <mgorman@suse.de>,
        Nicholas Piggin <npiggin@gmail.com>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        Hugh Dickins <hughd@google.com>, Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>
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On Tue, Aug 18, 2020 at 11:47 AM Yu Zhao <yuzhao@google.com> wrote:
>
> To activate a page, mark_page_accessed() always holds a reference
> on it. It either gets a new reference when adding a page to
> lru_pvecs.activate_page or reuses an existing one it previously
> got when it added a page to lru_pvecs.lru_add. So it doesn't call
> SetPageActive() on a page that doesn't have any reference left.
> Therefore, the race is impossible these days (I didn't brother to
> dig into its history).
>
> For other paths, namely reclaim and migration, a reference count is
> always held while calling SetPageActive() on a page.
>
> SetPageSlabPfmemalloc() also uses SetPageActive(), but it's irrelevant
> to LRU pages.

Seems fine to me. Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Signed-off-by: Yu Zhao <yuzhao@google.com>
> ---
>  mm/memremap.c | 2 --
>  mm/swap.c     | 2 --
>  2 files changed, 4 deletions(-)
>
> diff --git a/mm/memremap.c b/mm/memremap.c
> index 03e38b7a38f1..3a06eb91cb59 100644
> --- a/mm/memremap.c
> +++ b/mm/memremap.c
> @@ -451,8 +451,6 @@ void free_devmap_managed_page(struct page *page)
>                 return;
>         }
>
> -       /* Clear Active bit in case of parallel mark_page_accessed */
> -       __ClearPageActive(page);
>         __ClearPageWaiters(page);
>
>         mem_cgroup_uncharge(page);
> diff --git a/mm/swap.c b/mm/swap.c
> index 25c4043491b3..999a84dbe12c 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -900,8 +900,6 @@ void release_pages(struct page **pages, int nr)
>                         del_page_from_lru_list(page, lruvec, page_off_lru(page));
>                 }
>
> -               /* Clear Active bit in case of parallel mark_page_accessed */
> -               __ClearPageActive(page);
>                 __ClearPageWaiters(page);
>
>                 list_add(&page->lru, &pages_to_free);
> --
> 2.28.0.220.ged08abb693-goog
>
>