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

Vlastimil Babka
2014-06-12T11:56:32Z

On 06/12/2014 02:21 AM, David Rientjes wrote:
> On Wed, 11 Jun 2014, Vlastimil Babka wrote:
>
>>> I hate to belabor this point, but I think gcc does treat it differently.
>>> If you look at the assembly comparing your patch to if you do
>>>
>>> unsigned long freepage_order = ACCESS_ONCE(page_private(page));
>>>
>>> instead, then if you enable annotation you'll see that gcc treats the
>>> store as page_x->D.y.private in your patch vs. MEM[(volatile long unsigned
>>> int *)page_x + 48B] with the above.
>>
>> Hm sure you compiled a version that used page_order_unsafe() and not
>> page_order()? Because I do see:
>>
>> MEM[(volatile long unsigned int *)valid_page_114 + 48B];
>>
>> That's gcc 4.8.1, but our gcc guy said he tried 4.5+ and all was like this.
>> And that it would be a gcc bug if not.
>> He also did a test where page_order was called twice in one function and
>> page_order_unsafe twice in another function. page_order() was reduced to a
>> single access in the assembly, page_order_unsafe were two accesses.
>>
>
> Ok, and I won't continue to push the point.

I'd rather know I'm correct and not just persistent enough :) If you
confirm that your compiler behaves differently, then maybe making
page_order_unsafe a #define instead of inline function would prevent
this issue?

> I think the lockless
> suitable_migration_target() call that looks at page_order() is fine in the
> free scanner since we use it as a racy check, but it might benefit from
> either a comment describing the behavior or a sanity check for
> page_order(page) <= MAX_ORDER as you've done before.

OK, I'll add that.