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 17:06:25 +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 4:54 PM, Wanpeng Li wrote:
>
>
> 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. :)

How about something like this:

+void put_hwpoison_page(struct page *page)
+{
+       struct page *head = compound_head(page);
+
+       if (PageHuge(head))
+               goto put_out;
+
+       if (PageTransHuge(head))
+               if (page != head)
+                       put_page(head);
+
+put_out:
+       put_page(page);
+       return;
+}
+

Any comments are welcome, I can update the patch by myself. :)

Regards,
Wanpeng Li

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

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