Suspicious error for CMA stress test

[Joonsoo Kim]

On Mon, Mar 07, 2016 at 01:59:12PM +0100, Vlastimil Babka wrote:
> On 03/07/2016 05:34 AM, Joonsoo Kim wrote:
> >On Fri, Mar 04, 2016 at 03:35:26PM +0800, Hanjun Guo wrote:
> >>>Sad to hear that.
> >>>
> >>>Could you tell me your system's MAX_ORDER and pageblock_order?
> >>>
> >>
> >>MAX_ORDER is 11, pageblock_order is 9, thanks for your help!
> 
> I thought that CMA regions/operations (and isolation IIRC?) were
> supposed to be MAX_ORDER aligned exactly to prevent needing these
> extra checks for buddy merging. So what's wrong?

CMA isolates MAX_ORDER aligned blocks, but, during the process,
partialy isolated block exists. If MAX_ORDER is 11 and
pageblock_order is 9, two pageblocks make up MAX_ORDER
aligned block and I can think following scenario because pageblock
(un)isolation would be done one by one.

(each character means one pageblock. 'C', 'I' means MIGRATE_CMA,
MIGRATE_ISOLATE, respectively.

CC -> IC -> II (Isolation)
II -> CI -> CC (Un-isolation)

If some pages are freed at this intermediate state such as IC or CI,
that page could be merged to the other page that is resident on
different type of pageblock and it will cause wrong freepage count.

If we don't release zone lock during whole isolation process, there
would be no problem and CMA can use that implementation. But,
isolation is used by another feature and I guess it cannot use that
kind of implementation.

Thanks.