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

Vlastimil Babka
2014-06-10T07:26:26Z

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.