linux-kernel - Re: [HMM v13 03/18] mm/ZONE_DEVICE/free_hot_cold_page: catch ZONE

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Date:   Mon, 21 Nov 2016 07:50:29 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, John Hubbard <jhubbard@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Subject: Re: [HMM v13 03/18] mm/ZONE_DEVICE/free_hot_cold_page: catch
 ZONE_DEVICE pages

On Mon, Nov 21, 2016 at 01:48:26PM +0530, Anshuman Khandual wrote:
> On 11/18/2016 11:48 PM, Jérôme Glisse wrote:
> > Catch page from ZONE_DEVICE in free_hot_cold_page(). This should never
> > happen as ZONE_DEVICE page must always have an elevated refcount.
> > 
> > This is to catch refcounting issues in a sane way for ZONE_DEVICE pages.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@...hat.com>
> > Cc: Dan Williams <dan.j.williams@...el.com>
> > Cc: Ross Zwisler <ross.zwisler@...ux.intel.com>
> > ---
> >  mm/page_alloc.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 0fbfead..09b2630 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -2435,6 +2435,16 @@ void free_hot_cold_page(struct page *page, bool cold)
> >  	unsigned long pfn = page_to_pfn(page);
> >  	int migratetype;
> >  
> > +	/*
> > +	 * This should never happen ! Page from ZONE_DEVICE always must have an
> > +	 * active refcount. Complain about it and try to restore the refcount.
> > +	 */
> > +	if (is_zone_device_page(page)) {
> > +		VM_BUG_ON_PAGE(is_zone_device_page(page), page);
> > +		page_ref_inc(page);
> > +		return;
> > +	}
> 
> This fixes an issue in the existing ZONE_DEVICE code, should not this
> patch be sent separately not in this series ?
> 

Well this is more like a safetynet feature, i can send it separately from the
series. It is not an issue per say as a trap to catch bugs. I had refcounting
bugs while working on this patchset and having this safetynet was helpful to
quickly pin-point issues.

Cheers,
Jérôme