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 08:58:38 +0000
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Wanpeng Li <wanpeng.li@...mail.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 Mon, Aug 10, 2015 at 04:54:39PM +0800, 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. :)

Great, thank you :)

Naoya--
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