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

[Xishi Qiu <qiuxishi@huawei.com>]

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?

Thanks,
Xishi Qiu

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