[PATCH v10] 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.

Handle these failures by introducing a watermark boost mechanism for
atomic requests. Refactor boost_watermark() using an internal helper to
support both fragmentation and atomic paths. Apply zone-proportional
boosts (~0.1% of managed pages) for atomic allocations, while
decoupling it from watermark_boost_factor.

Implement boost_zones_for_atomic() to iterate through and boost all
eligible zones in the zonelist, respecting nodemasks. Use a per-zone
1-second debounce timer via last_boost_jiffies to prevent excessive
boosting. Protect modifications with zone->lock and verify with
lockdep. Integrate the mechanism into the page allocation slowpath
specifically for order-0 GFP_ATOMIC requests.

This approach reuses existing infrastructure and ensures emergency
reserves even if fragmentation boosting is disabled.

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

Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Qiliang Yuan <realwujing@gmail.com>
---
v10:
- Refactor watermark boosting into mechanism (__boost_watermark) and policy logic.
- Decouple Atomic boost from watermark_boost_factor to ensure emergency reserves.
- Simplify Atomic boost calculation to ~0.1% of managed pages with a 10% high-WM cap.
- Boost all eligible zones in the zonelist while respecting nodemasks.

v9:
- Use mult_frac() for boost calculation.
- Add !can_direct_reclaim check.
- Code cleanup: naming, scope, and line limits.
- Update tags: Add Vlastimil's Acked-by.
- Link: https://lore.kernel.org/r/20260213-wujing-mm-page_alloc-v8-v9-1-cd99f3a6cb70@gmail.com

v8:
- Use spin_lock_irqsave() to prevent inconsistent lock state.

v7:
- Use local variable for boost_amount.
- Add zone->lock protection.
- Add lockdep assertion.

v6:
- Use ATOMIC_BOOST_SCALE_SHIFT define.
- Add documentation for 0.1% rationale.

v5:
- Use native boost_watermark().

v4:
- Add watermark_scale_boost and gradual decay.

v3:
- Per-zone debounce timer.

v2:
- Debounce logic and zone-proportional boosting.

v1:
- Initial version.
---
 include/linux/mmzone.h |  1 +
 mm/page_alloc.c        | 77 ++++++++++++++++++++++++++++++++++++++++++++------
 2 files changed, 69 insertions(+), 9 deletions(-)

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..9219bfca806b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2158,12 +2158,15 @@ bool pageblock_unisolate_and_move_free_pages(struct zone *zone, struct page *pag
 
 #endif /* CONFIG_MEMORY_ISOLATION */
 
-static inline bool boost_watermark(struct zone *zone)
+/*
+ * Helper for boosting watermarks. Called with zone->lock held.
+ * Use max_boost to limit the boost to a percentage of the high watermark.
+ */
+static inline bool __boost_watermark(struct zone *zone, unsigned long amount,
+				     unsigned long max_boost)
 {
-	unsigned long max_boost;
+	lockdep_assert_held(&zone->lock);
 
-	if (!watermark_boost_factor)
-		return false;
 	/*
 	 * Don't bother in zones that are unlikely to produce results.
 	 * On small machines, including kdump capture kernels running
@@ -2173,9 +2176,6 @@ static inline bool boost_watermark(struct zone *zone)
 	if ((pageblock_nr_pages * 4) > zone_managed_pages(zone))
 		return false;
 
-	max_boost = mult_frac(zone->_watermark[WMARK_HIGH],
-			watermark_boost_factor, 10000);
-
 	/*
 	 * high watermark may be uninitialised if fragmentation occurs
 	 * very early in boot so do not boost. We do not fall
@@ -2189,12 +2189,67 @@ 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,
-		max_boost);
+	zone->watermark_boost = min(zone->watermark_boost + amount,
+				    max_boost);
 
 	return true;
 }
 
+/*
+ * Boost watermarks to increase reclaim pressure when fragmentation occurs
+ * and we fall back to other migratetypes.
+ */
+static inline bool boost_watermark(struct zone *zone)
+{
+	if (!watermark_boost_factor)
+		return false;
+
+	return __boost_watermark(zone, pageblock_nr_pages,
+			mult_frac(zone->_watermark[WMARK_HIGH],
+				  watermark_boost_factor, 10000));
+}
+
+/*
+ * Boost watermarks by ~0.1% of zone size on atomic allocation pressure.
+ * This provides zone-proportional safety buffers: ~1MB per 1GB of zone
+ * size. Max boost ceiling is fixed at ~10% of high watermark.
+ *
+ * This emergency reserve is independent of watermark_boost_factor.
+ */
+static inline bool boost_watermark_atomic(struct zone *zone)
+{
+	return __boost_watermark(zone,
+			max(pageblock_nr_pages, zone_managed_pages(zone) / 1000),
+			zone->_watermark[WMARK_HIGH] / 10);
+}
+
+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_nodemask(zone, z, ac->zonelist,
+					ac->highest_zoneidx, ac->nodemask) {
+		/* Rate-limit boosts to once per second per zone */
+		if (time_after(now, zone->last_boost_jiffies + HZ)) {
+			unsigned long flags;
+			bool should_wake;
+
+			zone->last_boost_jiffies = now;
+
+			/* Modify watermark under lock, wake kswapd outside */
+			spin_lock_irqsave(&zone->lock, flags);
+			should_wake = boost_watermark_atomic(zone);
+			spin_unlock_irqrestore(&zone->lock, flags);
+
+			if (should_wake)
+				wakeup_kswapd(zone, gfp_mask, 0,
+					      ac->highest_zoneidx);
+		}
+	}
+}
+
 /*
  * 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 +4797,10 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
 	if (page)
 		goto got_pg;
 
+	/* Boost watermarks for atomic requests entering slowpath */
+	if (((gfp_mask & GFP_ATOMIC) == GFP_ATOMIC) && order == 0 && !can_direct_reclaim)
+		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-

---
base-commit: b54345928fa1dbde534e32ecaa138678fd5d2135
change-id: 20260206-wujing-mm-page_alloc-v8-fb1979bac6fe

Best regards,
-- 
Qiliang Yuan <realwujing@gmail.com>