Subject: Re: [HMM v13 03/18] mm/ZONE_DEVICE/free_hot_cold_page: catch
 ZONE_DEVICE pages
To: =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <1479493107-982-1-git-send-email-jglisse@redhat.com>
 <1479493107-982-4-git-send-email-jglisse@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Date: Mon, 21 Nov 2016 13:48:26 +0530
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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@redhat.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: Ross Zwisler <ross.zwisler@linux.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 ?