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 16:54:39 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.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 8/10/15 4:35 PM, Naoya Horiguchi wrote:
> 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.

Good point. I think I will continue to do it and will post it out soon. :)

Regards,
Wanpeng Li

> I'll post some patch(es) to address this problem this week.
>
> Thanks,
> Naoya Horiguchi

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