linux-kernel - Re: [PATCH 8/8] mm: memory-failure: fix race window when trying to get hugetlb folio

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Date:   Mon, 10 Jul 2023 17:39:25 +0900
From:   Naoya Horiguchi <naoya.horiguchi@...ux.dev>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     akpm@...ux-foundation.org, naoya.horiguchi@....com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] mm: memory-failure: fix race window when trying to
 get hugetlb folio

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@...wei.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.

Thanks,
Naoya Horiguchi