Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/28 13:52, Joonsoo Kim wrote:

> On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
>> On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
>>> On 2016/9/26 17:43, Michal Hocko wrote:
>>>
>>>> On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
>>>>> On 2016/9/26 16:58, Michal Hocko wrote:
>>>>>
>>>>>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>>>>>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>>>>>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>>>>>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>>>>>>> quietly fix a bug. Please see the following.
>>>>>>>
>>>>>>> Before this patch, the high-order check is this:
>>>>>>> __zone_watermark_ok()
>>>>>>> 	...
>>>>>>> 	for (o = 0; o < order; o++) {
>>>>>>> 		/* At the next order, this order's pages become unavailable */
>>>>>>> 		free_pages -= z->free_area[o].nr_free << o;
>>>>>>>
>>>>>>> 		/* Require fewer higher order pages to be free */
>>>>>>> 		min >>= 1;
>>>>>>>
>>>>>>> 		if (free_pages <= min)
>>>>>>> 			return false;
>>>>>>> 	}
>>>>>>> 	...
>>>>>>>
>>>>>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>>>>>>
>>>>>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>>>>>>> and there are a lot of high-order cma pages, but little high-order unmovable
>>>>>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>>>>>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
>>>>>>
>>>>>> AFAIR CMA wmark check was always tricky and the above commit has made
>>>>>> the situation at least a bit more clear. Anyway IIRC 
>>>>>>
>>>>>> #ifdef CONFIG_CMA
>>>>>> 	/* If allocation can't use CMA areas don't use free CMA pages */
>>>>>> 	if (!(alloc_flags & ALLOC_CMA))
>>>>>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>>>>>> #endif
>>>>>>
>>>>>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
>>>>>> 		return false;
>>>>>>
>>>>>> should reduce the prioblem because a lot of CMA pages should just get us
>>>>>> below the wmark + reserve boundary.
>>>>>
>>>>> Hi Michal,
>>>>>
>>>>> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
>>>>> are also enough, but they are fragment, then it will triger the problem.
>>>>> If we alloc a high-order unmovable page, water mark check return *true*, but we
>>>>> will alloc *failed*, right?
>>>>
>>>> As Vlastimil has written. There were known issues with the wmark checks
>>>> and high order requests.
>>>
>>> Shall we backport to stable?
>>
>> I dunno, it was a part of a larger series with high atomic reserves and
>> changes which sound a bit intrusive for the stable kernel. Considering
>> that CMA was known to be problematic and there are still some issues
>> left I do not think this is worth the trouble/risk.
> 
> CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
> removed due to Mel's high atomic reserve series.
> 
> That series is rather large and has some problems so I think that it
> is not suitable for stable tree.
> 
> Thanks.
> 

OK, I know, thank you very much.

Thanks,
Xishi Qiu

> .
>