Re: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner

[Vlastimil Babka <vbabka@suse.cz>]

On 06/10/2014 12:25 AM, David Rientjes wrote:
> On Mon, 9 Jun 2014, Vlastimil Babka wrote:
>
>>>>> Sorry, I meant ACCESS_ONCE(page_private(page)) in the migration scanner
>>>>
>>>> Hm but that's breaking the abstraction of page_order(). I don't know if
>>>> it's
>>>> worse to create a new variant of page_order() or to do this. BTW, seems
>>>> like
>>>> next_active_pageblock() in memory-hotplug.c should use this variant too.
>>>>
>>>
>>> The compiler seems free to disregard the access of a volatile object above
>>> because the return value of the inline function is unsigned long.  What's
>>> the difference between unsigned long order = page_order_unsafe(page) and
>>> unsigned long order = (unsigned long)ACCESS_ONCE(page_private(page)) and
>>
>> I think there's none functionally, but one is abstraction layer violation and
>> the other imply the context of usage as you say (but is that so uncommon?).
>>
>>> the compiler being able to reaccess page_private() because the result is
>>> no longer volatile qualified?
>>
>> You think it will reaccess? That would defeat all current ACCESS_ONCE usages,
>> no?
>>
>
> I think the compiler is allowed to turn this into
>
> 	if (ACCESS_ONCE(page_private(page)) > 0 &&
> 	    ACCESS_ONCE(page_private(page)) < MAX_ORDER)
> 		low_pfn += (1UL << ACCESS_ONCE(page_private(page))) - 1;
>
> since the inline function has a return value of unsigned long but gcc may
> not do this.  I think
>
> 	/*
> 	 * Big fat comment describing why we're using ACCESS_ONCE(), that
> 	 * we're ok to race, and that this is meaningful only because of
> 	 * the previous PageBuddy() check.
> 	 */
> 	unsigned long pageblock_order = ACCESS_ONCE(page_private(page));
>
> is better.

I've talked about it with a gcc guy and (although he didn't actually see 
the code so it might be due to me not explaining it perfectly), the 
compiler will inline page_order_unsafe() so that there's effectively.

unsigned long freepage_order = ACCESS_ONCE(page_private(page));

and now it cannot just replace all freepage_order occurences with new 
page_private() accesses. So thanks to the inlining, the volatile 
qualification propagates to where it matters. It makes sense to me, but 
if it's according to standard or gcc specific, I don't know.


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