[PATCH v2 16/18] mm, compaction: require only min watermarks for non-costly orders

[Vlastimil Babka]

The __compaction_suitable() function checks the low watermark plus a
compact_gap() gap to decide if there's enough free memory to perform
compaction. Then __isolate_free_page uses low watermark check to decide if
particular free page can be isolated. In the latter case, using low watermark
is needlessly pessimistic, as the free page isolations are only temporary. For
__compaction_suitable() the higher watermark makes sense for high-order
allocations where more freepages increase the chance of success, and we can
typically fail with some order-0 fallback when the system is struggling to
reach that watermark. But for low-order allocation, forming the page should not
be that hard. So using low watermark here might just prevent compaction from
even trying, and eventually lead to OOM killer even if we are above min
watermarks.

So after this patch, we use min watermark for non-costly orders in
__compaction_suitable(), and for all orders in __isolate_free_page().

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/compaction.c | 6 +++++-
 mm/page_alloc.c | 2 +-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/mm/compaction.c b/mm/compaction.c
index 4ffa0870192b..d854519a5302 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -1345,10 +1345,14 @@ static enum compact_result __compaction_suitable(struct zone *zone, int order,
 	 * isolation. We however do use the direct compactor's classzone_idx to
 	 * skip over zones where lowmem reserves would prevent allocation even
 	 * if compaction succeeds.
+	 * For costly orders, we require low watermark instead of min for
+	 * compaction to proceed to increase its chances.
 	 * ALLOC_CMA is used, as pages in CMA pageblocks are considered
 	 * suitable migration targets
 	 */
-	watermark = low_wmark_pages(zone) + compact_gap(order);
+	watermark = (order > PAGE_ALLOC_COSTLY_ORDER) ?
+				low_wmark_pages(zone) : min_wmark_pages(zone);
+	watermark += compact_gap(order);
 	if (!__zone_watermark_ok(zone, 0, watermark, classzone_idx,
 						ALLOC_CMA, wmark_target))
 		return COMPACT_SKIPPED;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 09dc9db8a7e9..5b4c9e567fc1 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2489,7 +2489,7 @@ 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);
 		if (!zone_watermark_ok(zone, 0, watermark, 0, ALLOC_CMA))
 			return 0;
 
-- 
2.8.3