[PATCH v5] mm/page_alloc: boost watermarks on atomic allocation failure

[Qiliang Yuan <realwujing@gmail.com>]

Atomic allocations (GFP_ATOMIC) are prone to failure under heavy memory
pressure as they cannot enter direct reclaim. This patch introduces a
'Soft Boost' mechanism to mitigate this.

When a GFP_ATOMIC request fails or enters the slowpath, the preferred
zone's watermark_boost is increased. This triggers kswapd to proactively
reclaim memory, creating a safety buffer for future atomic bursts.

To prevent excessive reclaim during packet storms, a 1-second debounce
timer (last_boost_jiffies) is added to each zone to rate-limit boosts.

This approach reuses existing watermark_boost infrastructure, ensuring
minimal overhead and asynchronous background reclaim via kswapd.

Allocation failure logs:
[38535644.718700] node 0: slabs: 1031, objs: 43328, free: 0
[38535644.725059] node 1: slabs: 339, objs: 17616, free: 317
[38535645.428345] SLUB: Unable to allocate memory on node -1, gfp=0x480020(GFP_ATOMIC)
[38535645.436888] cache: skbuff_head_cache, object size: 232, buffer size: 256, default order: 2, min order: 0
[38535645.447664] node 0: slabs: 940, objs: 40864, free: 144
[38535645.454026] node 1: slabs: 322, objs: 19168, free: 383
[38535645.556122] SLUB: Unable to allocate memory on node -1, gfp=0x480020(GFP_ATOMIC)
[38535645.564576] cache: skbuff_head_cache, object size: 232, buffer size: 256, default order: 2, min order: 0
[38535649.655523] warn_alloc: 59 callbacks suppressed
[38535649.655527] swapper/100: page allocation failure: order:0, mode:0x480020(GFP_ATOMIC), nodemask=(null)
[38535649.671692] swapper/100 cpuset=/ mems_allowed=0-1

Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
---
v5:
  - Replaced custom watermark_scale_boost and manual recomputations with 
    native boost_watermark reuse.
  - Simplified logic to use existing 'boost' architecture for better 
    community acceptability.
v4:
  - Introduced watermark_scale_boost and gradual decay via balance_pgdat.
  - Added proactive soft-boosting when entering slowpath.
v3:
  - Moved debounce timer to per-zone to avoid cross-node interference.
  - Optimized candidate zone selection to reduce global reclaim pressure.
v2:
  - Added basic debounce logic and scaled boosting strength based on zone size.
v1:
  - Initial proposal: Basic watermark boost on atomic allocation failure.
---
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        | 29 ++++++++++++++++++++++++++++-
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 75ef7c9f9307..8e37e4e6765b 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -882,6 +882,7 @@ struct zone {
 	/* zone watermarks, access with *_wmark_pages(zone) macros */
 	unsigned long _watermark[NR_WMARK];
 	unsigned long watermark_boost;
+	unsigned long last_boost_jiffies;
 
 	unsigned long nr_reserved_highatomic;
 	unsigned long nr_free_highatomic;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index c380f063e8b7..1faace9e2dc5 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2189,12 +2189,31 @@ static inline bool boost_watermark(struct zone *zone)
 
 	max_boost = max(pageblock_nr_pages, max_boost);
 
-	zone->watermark_boost = min(zone->watermark_boost + pageblock_nr_pages,
+	zone->watermark_boost = min(zone->watermark_boost +
+		max(pageblock_nr_pages, zone_managed_pages(zone) >> 10),
 		max_boost);
 
 	return true;
 }
 
+static void boost_zones_for_atomic(struct alloc_context *ac, gfp_t gfp_mask)
+{
+	struct zoneref *z;
+	struct zone *zone;
+	unsigned long now = jiffies;
+
+	for_each_zone_zonelist(zone, z, ac->zonelist, ac->highest_zoneidx) {
+		/* 1 second debounce to avoid spamming boosts in a burst */
+		if (time_after(now, zone->last_boost_jiffies + HZ)) {
+			zone->last_boost_jiffies = now;
+			if (boost_watermark(zone))
+				wakeup_kswapd(zone, gfp_mask, 0, ac->highest_zoneidx);
+			/* Only boost the preferred zone to be precise */
+			break;
+		}
+	}
+}
+
 /*
  * When we are falling back to another migratetype during allocation, should we
  * try to claim an entire block to satisfy further allocations, instead of
@@ -4742,6 +4761,10 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (page)
 		goto got_pg;
 
+	/* Proactively boost for atomic requests entering slowpath */
+	if ((gfp_mask & GFP_ATOMIC) && order == 0)
+		boost_zones_for_atomic(ac, gfp_mask);
+
 	/*
 	 * For costly allocations, try direct compaction first, as it's likely
 	 * that we have enough base pages and don't need to reclaim. For non-
@@ -4947,6 +4970,10 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 		goto retry;
 	}
 fail:
+	/* Boost watermarks on atomic allocation failure to trigger kswapd */
+	if (unlikely(page == NULL && (gfp_mask & GFP_ATOMIC) && order == 0))
+		boost_zones_for_atomic(ac, gfp_mask);
+
 	warn_alloc(gfp_mask, ac->nodemask,
 			"page allocation failure: order:%u", order);
 got_pg:
-- 
2.51.0