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

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Date:	Thu, 18 Aug 2016 14:20:10 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Hocko <mhocko@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	David Rientjes <rientjes@...gle.com>,
	Rik van Riel <riel@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 10/11] mm, compaction: require only min watermarks for
 non-costly orders

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?

----8<----
>From 5d060f4222a637e1005ff32ae0fd4330625b6675 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <vbabka@...e.cz>
Date: Thu, 18 Aug 2016 14:18:08 +0200
Subject: [PATCH] mm, compaction: require only min watermarks for non-costly
 orders-fix

Clarify why __isolate_free_page() does a order-0 watermark check with
apparent (1UL << order) gap, per Joonsoo.

Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
---
 mm/page_alloc.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index a5c0f914ec00..216715504fb4 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2491,7 +2491,12 @@ int __isolate_free_page(struct page *page, unsigned int order)
 	mt = get_pageblock_migratetype(page);
 
 	if (!is_migrate_isolate(mt)) {
-		/* Obey watermarks as if the page was being allocated */
+		/*
+		 * Obey watermarks as if the page was being allocated. We can
+		 * emulate a high-order watermark check with a raised order-0
+		 * watermark, because we already know our high-order page
+		 * exists.
+		 */
 		watermark = min_wmark_pages(zone) + (1UL << order);
 		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
 			return 0;
-- 
2.9.2