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Subject: Re: [PATCH 8/8] mm: memory-failure: fix race window when trying to
 get hugetlb folio
To:     Naoya Horiguchi <naoya.horiguchi@linux.dev>
CC:     <akpm@linux-foundation.org>, <naoya.horiguchi@nec.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
References: <20230708085744.3599311-1-linmiaohe@huawei.com>
 <20230708085744.3599311-9-linmiaohe@huawei.com>
 <20230710075812.GH1686200@ik1-406-35019.vs.sakura.ne.jp>
 <74acb109-76cc-3990-c770-8330e343ef54@huawei.com>
 <20230710083925.GA1754785@ik1-406-35019.vs.sakura.ne.jp>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <147fb145-1289-4526-862e-0cb63c9711e0@huawei.com>
Date:   Mon, 10 Jul 2023 16:59:10 +0800
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On 2023/7/10 16:39, Naoya Horiguchi wrote:
> On Mon, Jul 10, 2023 at 04:32:27PM +0800, Miaohe Lin wrote:
>> On 2023/7/10 15:58, Naoya Horiguchi wrote:
>>> On Sat, Jul 08, 2023 at 04:57:44PM +0800, Miaohe Lin wrote:
>>>> page_folio() is fetched before calling get_hwpoison_hugetlb_folio()
>>>> without hugetlb_lock being held. So hugetlb page could be demoted
>>>> before get_hwpoison_hugetlb_folio() holding hugetlb_lock but after
>>>> page_folio() is fetched. So get_hwpoison_hugetlb_folio() will hold
>>>> unexpected extra refcnt of hugetlb folio while leaving demoted page
>>>> un-refcnted.
>>>
>>> Very nice, thank you for finding the issue.
>>>
>>>>
>>>> Fixes: 25182f05ffed ("mm,hwpoison: fix race with hugetlb page allocation")
>>>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>>>> ---
>>>>  mm/memory-failure.c | 18 ++++++++++++++----
>>>>  1 file changed, 14 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
>>>> index 76d88d27cdbe..066bf57f2d22 100644
>>>> --- a/mm/memory-failure.c
>>>> +++ b/mm/memory-failure.c
>>>> @@ -1388,8 +1388,14 @@ static int __get_hwpoison_page(struct page *page, unsigned long flags)
>>>>  	bool hugetlb = false;
>>>>  
>>>>  	ret = get_hwpoison_hugetlb_folio(folio, &hugetlb, false);
>>>> -	if (hugetlb)
>>>> -		return ret;
>>>> +	if (hugetlb) {
>>>> +		if (folio == page_folio(page))
>>>> +			return ret;
>>>
>>> Some short comment about the race against demotion here is helpful.
>>
>> Does the below comment makes sense to you?
>>
>> "
>>   Make sure hugetlb demotion did not happen from under us.
>> "
> 
> Yes, this sounds fine.

Will do it in v2. Thanks.

> 
> Thanks,
> Naoya Horiguchi
> 
> .
>