Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for non-costly orders

[Vlastimil Babka]

On 08/16/2016 08:46 AM, Joonsoo Kim wrote:
> On Tue, Aug 16, 2016 at 08:36:12AM +0200, Vlastimil Babka wrote:
>> On 08/16/2016 08:16 AM, Joonsoo Kim wrote:
>>> On Wed, Aug 10, 2016 at 11:12:25AM +0200, Vlastimil Babka wrote:
>>>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>>>> index 621e4211ce16..a5c0f914ec00 100644
>>>> --- a/mm/page_alloc.c
>>>> +++ b/mm/page_alloc.c
>>>> @@ -2492,7 +2492,7 @@ int __isolate_free_page(struct page *page, unsigned int order)
>>>>
>>>> 	if (!is_migrate_isolate(mt)) {
>>>> 		/* Obey watermarks as if the page was being allocated */
>>>> -		watermark = low_wmark_pages(zone) + (1 << order);
>>>> +		watermark = min_wmark_pages(zone) + (1UL << order);
>>>
>>> This '1 << order' also needs some comment. Why can't we use
>>> compact_gap() in this case?
>>
>> This is just short-cutting the high-order watermark check to check
>> only order-0, because we already know the high-order page exists.
>> We can't use compact_gap() as that's too high to use for a single
>> allocation watermark, since we can be already holding some free
>> pages on the list. So it would defeat the gap purpose.
> 
> Oops. I missed that. Thanks for clarifying it.

So let's expand the comment?

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