From mboxrd@z Thu Jan  1 00:00:00 1970
From: Avi Kivity <avi@qumranet.com>
Subject: [ofa-general] Re: [patch 3/6] mmu_notifier: invalidate_page
	callbacks
Date: Sat, 16 Feb 2008 21:54:41 +0200
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Cc: steiner@sgi.com, Andrea Arcangeli <andrea@qumranet.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>, linux-mm@kvack.org,
	Izik Eidus <izike@qumranet.com>, Kanoj Sarcar <kanojsarcar@yahoo.com>,
	Roland Dreier <rdreier@cisco.com>, linux-kernel@vger.kernel.org,
	kvm-devel@lists.sourceforge.net, daniel.blueman@quadrics.com,
	Robin Holt <holt@sgi.com>, general@lists.openfabrics.org,
	Andrew Morton <akpm@linux-foundation.org>
To: Christoph Lameter <clameter@sgi.com>
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Christoph Lameter wrote:
> On Fri, 15 Feb 2008, Andrew Morton wrote:
>
>   
>>> @@ -287,7 +288,8 @@ static int page_referenced_one(struct pa
>>>  	if (vma->vm_flags & VM_LOCKED) {
>>>  		referenced++;
>>>  		*mapcount = 1;	/* break early from loop */
>>> -	} else if (ptep_clear_flush_young(vma, address, pte))
>>> +	} else if (ptep_clear_flush_young(vma, address, pte) |
>>> +		   mmu_notifier_age_page(mm, address))
>>>  		referenced++;
>>>       
>> The "|" is obviously deliberate.  But no explanation is provided telling us
>> why we still call the callback if ptep_clear_flush_young() said the page
>> was recently referenced.  People who read your code will want to understand
>> this.
>>     
>
> Andrea?
>
>   

I'm not Andrea, but the way I read it, ptep_clear_flush_young() and 
->age_page() each have two effects: check whether the page has been 
referenced and clear the referenced bit.  || would retain the semantics 
of the check but lose the clearing.  | does the right thing.

-- 
Any sufficiently difficult bug is indistinguishable from a feature.