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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 1 Jul 2019 18:00:31 -0700
Message-ID: <CALvZod5Fb+2mR_KjKq06AHeRYyykZatA4woNt_K5QZNETvw4nw@mail.gmail.com>
Subject: Re: [PATCH v2] mm/z3fold.c: Lock z3fold page before __SetPageMovable()
To:     Henry Burns <henryburns@google.com>
Cc:     Vitaly Wool <vitalywool@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vitaly Vul <vitaly.vul@sony.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Xidong Wang <wangxidong_97@163.com>,
        Jonathan Adams <jwadams@google.com>,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Jul 1, 2019 at 5:51 PM Henry Burns <henryburns@google.com> wrote:
>
> __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't
> lock the page. Following zsmalloc.c's example we call trylock_page() and
> unlock_page(). Also makes z3fold_page_migrate() assert that newpage is
> passed in locked, as documentation.
>
> Signed-off-by: Henry Burns <henryburns@google.com>
> Suggested-by: Vitaly Wool <vitalywool@gmail.com>
> ---
>  Changelog since v1:
>  - Added an if statement around WARN_ON(trylock_page(page)) to avoid
>    unlocking a page locked by a someone else.
>
>  mm/z3fold.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index e174d1549734..6341435b9610 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -918,7 +918,10 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp,
>                 set_bit(PAGE_HEADLESS, &page->private);
>                 goto headless;
>         }
> -       __SetPageMovable(page, pool->inode->i_mapping);
> +       if (!WARN_ON(!trylock_page(page))) {
> +               __SetPageMovable(page, pool->inode->i_mapping);
> +               unlock_page(page);
> +       }

Can you please comment why lock_page() is not used here?

>         z3fold_page_lock(zhdr);
>
>  found:
> @@ -1325,6 +1328,7 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
>
>         VM_BUG_ON_PAGE(!PageMovable(page), page);
>         VM_BUG_ON_PAGE(!PageIsolated(page), page);
> +       VM_BUG_ON_PAGE(!PageLocked(newpage), newpage);
>
>         zhdr = page_address(page);
>         pool = zhdr_to_pool(zhdr);
> --
> 2.22.0.410.gd8fdbe21b5-goog
>