[PATCH] mm/compaction: cap compact_gap() at COMPACT_CLUSTER_MAX

["JP Kobryn (Meta)" <jp.kobryn@linux.dev>]

compact_gap() returns 2 << order, which is used as watermark headroom in
__compaction_suitable() and as a reclaim target in kswapd. The computed
value scales exponentially by order. For order-9 THP allocations this
evaluates to 1024 pages, but the compaction free scanner's working set is
bounded by COMPACT_CLUSTER_MAX (32 pages). The scanner stops isolating free
pages once it matches the migration batch. The current gap over-reserves by
32x.

On fragmented production hosts, kswapd will try and reclaim up to the gap,
but it only reaches that threshold 18% of the time, causing reclaim to
continue a majority of the time. The over-sized gap also causes 46% of
order-9 compaction suitability checks to fail unnecessarily - the zone has
sufficient free pages for the scanner to operate, but not enough to clear
the inflated threshold.

Cap compact_gap() at COMPACT_CLUSTER_MAX to align the watermark headroom
with the scanner's actual capacity. Orders 0-4 are unaffected since their
gap is <= 32.

A/B test on ~100 instagram production hosts (64GB, 60s measurement):

Unpatched (43 hosts)
pgscan_kswapd (mean/host): ~1.6M
reclaim efficiency (steal/scan): 83.8%
compaction success (success/stall): 2.1%
THP success (alloc/alloc+fallback): 4.9%
forced lru_add_drain (mean/host): ~107K

Patched (59 hosts)
pgscan_kswapd (mean/host): ~449K
reclaim efficiency (steal/scan): 91.0%
compaction success (success/stall): 28.3%
THP success (alloc/alloc+fallback): 17.2%
forced lru_add_drain (mean/host): ~64K

Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
---
 include/linux/compaction.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index 173d9c07a8952..09aea63b8a89d 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -2,6 +2,8 @@
 #ifndef _LINUX_COMPACTION_H
 #define _LINUX_COMPACTION_H
 
+#include <linux/swap.h>
+
 /*
  * Determines how hard direct compaction should try to succeed.
  * Lower value means higher priority, analogically to reclaim priority.
@@ -73,11 +75,9 @@ static inline unsigned long compact_gap(unsigned int order)
 	 * effectively limited by COMPACT_CLUSTER_MAX, as that's the maximum
 	 * that the migrate scanner can have isolated on migrate list, and free
 	 * scanner is only invoked when the number of isolated free pages is
-	 * lower than that. But it's not worth to complicate the formula here
-	 * as a bigger gap for higher orders than strictly necessary can also
-	 * improve chances of compaction success.
+	 * lower than that.
 	 */
-	return 2UL << order;
+	return min(2UL << order, COMPACT_CLUSTER_MAX);
 }
 
 static inline int current_is_kcompactd(void)
-- 
2.53.0-Meta