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From:   Yu Zhao <yuzhao@google.com>
Date:   Sat, 10 Jul 2021 17:22:46 -0600
Message-ID: <CAOUHufZpU4uQOBb4p10uCXs-40MeETRUmGiqy96Eim1w3o_dgQ@mail.gmail.com>
Subject: Re: [PATCH 1/5] mm/vmscan: put the redirtied MADV_FREE pages back to
 anonymous LRU list
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     akpm@linux-foundation.org, hannes@cmpxchg.org, vbabka@suse.cz,
        mhocko@suse.com, axboe@kernel.dk, iamjoonsoo.kim@lge.com,
        alexs@kernel.org, apopple@nvidia.com, willy@infradead.org,
        minchan@kernel.org, david@redhat.com, shli@fb.com,
        hillf.zj@alibaba-inc.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
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On Sat, Jul 10, 2021 at 4:03 AM Miaohe Lin <linmiaohe@huawei.com> wrote:
>
> If the MADV_FREE pages are redirtied before they could be reclaimed, put
> the pages back to anonymous LRU list by setting SwapBacked flag and the
> pages will be reclaimed in normal swapout way. Otherwise MADV_FREE pages
> won't be reclaimed as expected.
>
> Fixes: 802a3a92ad7a ("mm: reclaim MADV_FREE pages")

This is not a bug -- the dirty check isn't needed but it was copied
from __remove_mapping().

The page has only one reference left, which is from the isolation.
After the caller puts the page back on lru and drops the reference,
the page will be freed anyway. It doesn't matter which lru it goes.

> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> ---
>  mm/vmscan.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index a7602f71ec04..6483fe0e2065 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1628,6 +1628,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
>                         if (!page_ref_freeze(page, 1))
>                                 goto keep_locked;
>                         if (PageDirty(page)) {
> +                               SetPageSwapBacked(page);
>                                 page_ref_unfreeze(page, 1);
>                                 goto keep_locked;
>                         }
> --
> 2.23.0
>
>