On 08/16/2016 08:16 AM, Joonsoo Kim wrote:

On Wed, Aug 10, 2016 at 11:12:25AM +0200, Vlastimil Babka wrote:

quoted

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.

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