Re: [PATCH 2/2] mm/hwpoison: fix refcount of THP head page in no-injection case

[Wanpeng Li]

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@xxxxxxxxxxx>
> > > ---
> > >   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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