linux-kernel - Re: [PATCH 2/2] mm/hwpoison: fix refcount of THP head page in no-injection case

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Date:	Mon, 10 Aug 2015 08:35:30 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] mm/hwpoison: fix refcount of THP head page in
 no-injection case

On Mon, Aug 10, 2015 at 02:32:31PM +0800, Wanpeng Li wrote:
> Hwpoison injection takes a refcount of target page and another refcount
> of head page of THP if the target page is the tail page of a THP. However,
> current code doesn't release the refcount of head page if the THP is not 
> supported to be injected wrt hwpoison filter. 
> 
> Fix it by reducing the refcount of head page if the target page is the tail 
> page of a THP and it is not supported to be injected.
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
> ---
>  mm/hwpoison-inject.c |    2 ++
>  1 files changed, 2 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/hwpoison-inject.c b/mm/hwpoison-inject.c
> index 5015679..c343a45 100644
> --- a/mm/hwpoison-inject.c
> +++ b/mm/hwpoison-inject.c
> @@ -56,6 +56,8 @@ inject:
>  	return memory_failure(pfn, 18, MF_COUNT_INCREASED);
>  put_out:
>  	put_page(p);
> +	if (p != hpage)
> +		put_page(hpage);

Yes, we need this when we inject to a thp tail page and "goto put_out" is
called. But it seems that this code can be called also when injecting error
to a hugetlb tail page and hwpoison_filter() returns non-zero, which is not
expected. Unfortunately simply doing like below

+	if (!PageHuge(p) && p != hpage)
+		put_page(hpage);

doesn't work, because exisiting put_page(p) can release refcount of hugetlb
tail page, while get_hwpoison_page() takes refcount of hugetlb head page.

So I feel that we need put_hwpoison_page() to properly release the refcount
taken by memory error handlers.
I'll post some patch(es) to address this problem this week.

Thanks,
Naoya Horiguchi--
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