On 06/16/2016 12:34 AM, David Rientjes wrote:
The goal of memory compaction is to defragment memory by moving migratable
pages to free pages at the end of the zone. No additional memory is being
allocated.
Ignore per-zone low watermarks in __isolate_free_page() because memory is
either fully migrated or isolated free pages are returned when migration
fails.
Michal Hocko suggested that too, but I didn't think it safe that
compaction should go below the min watermark, even temporarily. It means
the system is struggling with order-0 allocations, so making it worse
for the benefit of high-order allocations doesn't make sense. The
high-order allocation would likely fail anyway due to watermark checks,
even if the page of sufficient order was formed by compaction. So in my
series, I just changed the low watermark check to min [1].
This fixes an issue where the compaction freeing scanner can isolate
memory but the zone drops below its low watermark for that page order, so
the scanner must continue to scan all memory pointlessly.
Good point, looks like failing the watermark is the only reason when
__isolate_free_page() can fail. isolate_freepages_block() and its
callers should take this as an indication that compaction should return
with failure immediately.
[1] http://article.gmane.org/gmane.linux.kernel/2231369
Signed-off-by: David Rientjes <rientjes@xxxxxxxxxx>
---
mm/page_alloc.c | 14 ++------------
1 file changed, 2 insertions(+), 12 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2484,23 +2484,14 @@ EXPORT_SYMBOL_GPL(split_page);
int __isolate_free_page(struct page *page, unsigned int order)
{
- unsigned long watermark;
struct zone *zone;
- int mt;
+ const int mt = get_pageblock_migratetype(page);
BUG_ON(!PageBuddy(page));
-
zone = page_zone(page);
- mt = get_pageblock_migratetype(page);
-
- if (!is_migrate_isolate(mt)) {
- /* Obey watermarks as if the page was being allocated */
- watermark = low_wmark_pages(zone) + (1 << order);
- if (!zone_watermark_ok(zone, 0, watermark, 0, 0))
- return 0;
+ if (!is_migrate_isolate(mt))
__mod_zone_freepage_state(zone, -(1UL << order), mt);
- }
/* Remove page from free list */
list_del(&page->lru);
@@ -2520,7 +2511,6 @@ int __isolate_free_page(struct page *page, unsigned int order)
}
}
-
return 1UL << order;
}
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