Re: Suspicious error for CMA stress test
From: Xishi Qiu
Date: Tue Mar 08 2016 - 05:46:32 EST
On 2016/3/8 15:48, Joonsoo Kim wrote:
> 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.
Isolation will appear when do cma alloc, so there are two following threads.
C(free)C(used) -> start_isolate_page_range -> I(free)C(used) -> I(free)I(someone free it) -> undo_isolate_page_range -> C(free)C(free)
so free cma is 2M -> 0M -> 0M -> 4M, the increased 2M was freed by someone.
C(used)C(free) -> start_isolate_page_range -> C(used)I(free) -> C(someone free it)C(free) -> undo_isolate_page_range -> C(free)C(free)
so free cma is 2M -> 0M -> 4M -> 4M, the increased 2M was freed by someone.
so these two cases are no problem, right?
> 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.