Subject: Re: [HMM v13 03/18] mm/ZONE_DEVICE/free_hot_cold_page: catch
 ZONE_DEVICE pages
To: Jerome Glisse <jglisse@redhat.com>
References: <1479493107-982-1-git-send-email-jglisse@redhat.com>
 <1479493107-982-4-git-send-email-jglisse@redhat.com>
 <5832ADD2.5000507@linux.vnet.ibm.com> <20161121125029.GG2392@redhat.com>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, 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: Tue, 22 Nov 2016 10:00:54 +0530
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On 11/21/2016 06:20 PM, Jerome Glisse wrote:
> On Mon, Nov 21, 2016 at 01:48:26PM +0530, Anshuman Khandual wrote:
>> On 11/18/2016 11:48 PM, J?r?me 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: J?r?me 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 ?
>>
> 
> 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.

Sure at the least move them up in the series as ZONE_DEVICE preparatory
fixes before expanding ZONE_DEVICE framework to accommodate the new
un-addressable memory representation.