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

[Xishi Qiu <qiuxishi@huawei.com>]

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?

Thanks,
Xishi Qiu