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Date:   Wed, 19 Dec 2018 11:20:10 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Qian Cai <cai@lca.pw>
Cc:     akpm@linux-foundation.org, mingo@kernel.org, hpa@zytor.com,
        mgorman@techsingularity.net, tglx@linutronix.de,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: skip checking poison pattern for page_to_nid()
Message-ID: <20181219102010.GF5758@dhcp22.suse.cz>
References: <1545172285.18411.26.camel@lca.pw>
 <20181219015732.26179-1-cai@lca.pw>
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On Tue 18-12-18 20:57:32, Qian Cai wrote:
[...]
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 5411de93a363..f083f366ea90 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -985,9 +985,7 @@ extern int page_to_nid(const struct page *page);
>  #else
>  static inline int page_to_nid(const struct page *page)
>  {
> -	struct page *p = (struct page *)page;
> -
> -	return (PF_POISONED_CHECK(p)->flags >> NODES_PGSHIFT) & NODES_MASK;
> +	return (page->flags >> NODES_PGSHIFT) & NODES_MASK;
>  }
>  #endif

I didn't get to think about a proper fix but this is clearly worng. If
the page is still poisoned then flags are clearly bogus and the node you
get is a garbage as well. Have you actually tested this patch?
-- 
Michal Hocko
SUSE Labs