Re: [patch 3/6] mmu_notifier: invalidate_page callbacks

[Avi Kivity <avi@qumranet.com>]

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.