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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>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <74acb109-76cc-3990-c770-8330e343ef54@huawei.com>
Date:   Mon, 10 Jul 2023 16:32:27 +0800
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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.
"

> 
> Anyway, the patch looks good to me.
> 
> Acked-by: Naoya Horiguchi <naoya.horiguchi@nec.com>

Many thanks for your review and comment, Naoya.