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

From: Vlastimil Babka
Date: Tue Aug 16 2016 - 02:36:29 EST


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.

Thanks.