patches/mm-page_alloc-simplify-and-cleanup-pcp-locking.patch

From: Vlastimil Babka <vbabka@suse.cz>
Subject: mm/page_alloc: simplify and cleanup pcp locking
Date: Wed, 15 Oct 2025 19:50:38 +0200

The pcp locking relies on pcp_spin_trylock() which has to be used together
with pcp_trylock_prepare()/pcp_trylock_finish() to work properly on !SMP
!RT configs.  This is tedious and error-prone.

We can remove pcp_spin_lock() and underlying pcpu_spin_lock() because we
don't use it.  Afterwards pcp_spin_unlock() is only used together with
pcp_spin_trylock().  Therefore we can add the UP_flags parameter to them
both and handle pcp_trylock_prepare()/finish() within.

Additionally for the configs where pcp_trylock_prepare()/finish() are
no-op (SMP || RT) make them pass &UP_flags to a no-op inline function. 
This ensures typechecking and makes the local variable "used" so we can
remove the __maybe_unused attributes.

In my compile testing, bloat-o-meter reported no change on SMP config, so
the compiler is capable of optimizing away the no-ops same as before, and
we have simplified the code using pcp_spin_trylock().

Link: https://lkml.kernel.org/r/20251015-b4-pcp-lock-cleanup-v2-1-740d999595d5@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/page_alloc.c |   99 ++++++++++++++++++----------------------------
 1 file changed, 40 insertions(+), 59 deletions(-)

--- a/mm/page_alloc.c~mm-page_alloc-simplify-and-cleanup-pcp-locking
+++ a/mm/page_alloc.c
@@ -99,9 +99,12 @@ static DEFINE_MUTEX(pcp_batch_high_lock)
 /*
  * On SMP, spin_trylock is sufficient protection.
  * On PREEMPT_RT, spin_trylock is equivalent on both SMP and UP.
+ * Pass flags to a no-op inline function to typecheck and silence the unused
+ * variable warning.
  */
-#define pcp_trylock_prepare(flags)	do { } while (0)
-#define pcp_trylock_finish(flag)	do { } while (0)
+static inline void __pcp_trylock_noop(unsigned long *flags) { }
+#define pcp_trylock_prepare(flags)	__pcp_trylock_noop(&(flags))
+#define pcp_trylock_finish(flags)	__pcp_trylock_noop(&(flags))
 #else
 
 /* UP spin_trylock always succeeds so disable IRQs to prevent re-entrancy. */
@@ -129,15 +132,6 @@ static DEFINE_MUTEX(pcp_batch_high_lock)
  * Generic helper to lookup and a per-cpu variable with an embedded spinlock.
  * Return value should be used with equivalent unlock helper.
  */
-#define pcpu_spin_lock(type, member, ptr)				\
-({									\
-	type *_ret;							\
-	pcpu_task_pin();						\
-	_ret = this_cpu_ptr(ptr);					\
-	spin_lock(&_ret->member);					\
-	_ret;								\
-})
-
 #define pcpu_spin_trylock(type, member, ptr)				\
 ({									\
 	type *_ret;							\
@@ -157,14 +151,21 @@ static DEFINE_MUTEX(pcp_batch_high_lock)
 })
 
 /* struct per_cpu_pages specific helpers. */
-#define pcp_spin_lock(ptr)						\
-	pcpu_spin_lock(struct per_cpu_pages, lock, ptr)
-
-#define pcp_spin_trylock(ptr)						\
-	pcpu_spin_trylock(struct per_cpu_pages, lock, ptr)
+#define pcp_spin_trylock(ptr, UP_flags)					\
+({									\
+	struct per_cpu_pages *__ret;					\
+	pcp_trylock_prepare(UP_flags);					\
+	__ret = pcpu_spin_trylock(struct per_cpu_pages, lock, ptr);	\
+	if (!__ret)							\
+		pcp_trylock_finish(UP_flags);				\
+	__ret;								\
+})
 
-#define pcp_spin_unlock(ptr)						\
-	pcpu_spin_unlock(lock, ptr)
+#define pcp_spin_unlock(ptr, UP_flags)					\
+({									\
+	pcpu_spin_unlock(lock, ptr);					\
+	pcp_trylock_finish(UP_flags);					\
+})
 
 #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
 DEFINE_PER_CPU(int, numa_node);
@@ -2887,13 +2888,10 @@ static bool free_frozen_page_commit(stru
 		if (to_free == 0 || pcp->count == 0)
 			break;
 
-		pcp_spin_unlock(pcp);
-		pcp_trylock_finish(*UP_flags);
+		pcp_spin_unlock(pcp, *UP_flags);
 
-		pcp_trylock_prepare(*UP_flags);
-		pcp = pcp_spin_trylock(zone->per_cpu_pageset);
+		pcp = pcp_spin_trylock(zone->per_cpu_pageset, *UP_flags);
 		if (!pcp) {
-			pcp_trylock_finish(*UP_flags);
 			ret = false;
 			break;
 		}
@@ -2904,8 +2902,7 @@ static bool free_frozen_page_commit(stru
 		 * returned in an unlocked state.
 		 */
 		if (smp_processor_id() != cpu) {
-			pcp_spin_unlock(pcp);
-			pcp_trylock_finish(*UP_flags);
+			pcp_spin_unlock(pcp, *UP_flags);
 			ret = false;
 			break;
 		}
@@ -2937,7 +2934,7 @@ static bool free_frozen_page_commit(stru
 static void __free_frozen_pages(struct page *page, unsigned int order,
 				fpi_t fpi_flags)
 {
-	unsigned long __maybe_unused UP_flags;
+	unsigned long UP_flags;
 	struct per_cpu_pages *pcp;
 	struct zone *zone;
 	unsigned long pfn = page_to_pfn(page);
@@ -2973,17 +2970,15 @@ static void __free_frozen_pages(struct p
 		add_page_to_zone_llist(zone, page, order);
 		return;
 	}
-	pcp_trylock_prepare(UP_flags);
-	pcp = pcp_spin_trylock(zone->per_cpu_pageset);
+	pcp = pcp_spin_trylock(zone->per_cpu_pageset, UP_flags);
 	if (pcp) {
 		if (!free_frozen_page_commit(zone, pcp, page, migratetype,
 						order, fpi_flags, &UP_flags))
 			return;
-		pcp_spin_unlock(pcp);
+		pcp_spin_unlock(pcp, UP_flags);
 	} else {
 		free_one_page(zone, page, pfn, order, fpi_flags);
 	}
-	pcp_trylock_finish(UP_flags);
 }
 
 void free_frozen_pages(struct page *page, unsigned int order)
@@ -2996,7 +2991,7 @@ void free_frozen_pages(struct page *page
  */
 void free_unref_folios(struct folio_batch *folios)
 {
-	unsigned long __maybe_unused UP_flags;
+	unsigned long UP_flags;
 	struct per_cpu_pages *pcp = NULL;
 	struct zone *locked_zone = NULL;
 	int i, j;
@@ -3039,8 +3034,7 @@ void free_unref_folios(struct folio_batc
 		if (zone != locked_zone ||
 		    is_migrate_isolate(migratetype)) {
 			if (pcp) {
-				pcp_spin_unlock(pcp);
-				pcp_trylock_finish(UP_flags);
+				pcp_spin_unlock(pcp, UP_flags);
 				locked_zone = NULL;
 				pcp = NULL;
 			}
@@ -3059,10 +3053,8 @@ void free_unref_folios(struct folio_batc
 			 * trylock is necessary as folios may be getting freed
 			 * from IRQ or SoftIRQ context after an IO completion.
 			 */
-			pcp_trylock_prepare(UP_flags);
-			pcp = pcp_spin_trylock(zone->per_cpu_pageset);
+			pcp = pcp_spin_trylock(zone->per_cpu_pageset, UP_flags);
 			if (unlikely(!pcp)) {
-				pcp_trylock_finish(UP_flags);
 				free_one_page(zone, &folio->page, pfn,
 					      order, FPI_NONE);
 				continue;
@@ -3085,10 +3077,8 @@ void free_unref_folios(struct folio_batc
 		}
 	}
 
-	if (pcp) {
-		pcp_spin_unlock(pcp);
-		pcp_trylock_finish(UP_flags);
-	}
+	if (pcp)
+		pcp_spin_unlock(pcp, UP_flags);
 	folio_batch_reinit(folios);
 }
 
@@ -3339,15 +3329,12 @@ static struct page *rmqueue_pcplist(stru
 	struct per_cpu_pages *pcp;
 	struct list_head *list;
 	struct page *page;
-	unsigned long __maybe_unused UP_flags;
+	unsigned long UP_flags;
 
 	/* spin_trylock may fail due to a parallel drain or IRQ reentrancy. */
-	pcp_trylock_prepare(UP_flags);
-	pcp = pcp_spin_trylock(zone->per_cpu_pageset);
-	if (!pcp) {
-		pcp_trylock_finish(UP_flags);
+	pcp = pcp_spin_trylock(zone->per_cpu_pageset, UP_flags);
+	if (!pcp)
 		return NULL;
-	}
 
 	/*
 	 * On allocation, reduce the number of pages that are batch freed.
@@ -3357,8 +3344,7 @@ static struct page *rmqueue_pcplist(stru
 	pcp->free_count >>= 1;
 	list = &pcp->lists[order_to_pindex(migratetype, order)];
 	page = __rmqueue_pcplist(zone, order, migratetype, alloc_flags, pcp, list);
-	pcp_spin_unlock(pcp);
-	pcp_trylock_finish(UP_flags);
+	pcp_spin_unlock(pcp, UP_flags);
 	if (page) {
 		__count_zid_vm_events(PGALLOC, page_zonenum(page), 1 << order);
 		zone_statistics(preferred_zone, zone, 1);
@@ -5045,7 +5031,7 @@ unsigned long alloc_pages_bulk_noprof(gf
 			struct page **page_array)
 {
 	struct page *page;
-	unsigned long __maybe_unused UP_flags;
+	unsigned long UP_flags;
 	struct zone *zone;
 	struct zoneref *z;
 	struct per_cpu_pages *pcp;
@@ -5139,10 +5125,9 @@ retry_this_zone:
 		goto failed;
 
 	/* spin_trylock may fail due to a parallel drain or IRQ reentrancy. */
-	pcp_trylock_prepare(UP_flags);
-	pcp = pcp_spin_trylock(zone->per_cpu_pageset);
+	pcp = pcp_spin_trylock(zone->per_cpu_pageset, UP_flags);
 	if (!pcp)
-		goto failed_irq;
+		goto failed;
 
 	/* Attempt the batch allocation */
 	pcp_list = &pcp->lists[order_to_pindex(ac.migratetype, 0)];
@@ -5159,8 +5144,8 @@ retry_this_zone:
 		if (unlikely(!page)) {
 			/* Try and allocate at least one page */
 			if (!nr_account) {
-				pcp_spin_unlock(pcp);
-				goto failed_irq;
+				pcp_spin_unlock(pcp, UP_flags);
+				goto failed;
 			}
 			break;
 		}
@@ -5171,8 +5156,7 @@ retry_this_zone:
 		page_array[nr_populated++] = page;
 	}
 
-	pcp_spin_unlock(pcp);
-	pcp_trylock_finish(UP_flags);
+	pcp_spin_unlock(pcp, UP_flags);
 
 	__count_zid_vm_events(PGALLOC, zone_idx(zone), nr_account);
 	zone_statistics(zonelist_zone(ac.preferred_zoneref), zone, nr_account);
@@ -5180,9 +5164,6 @@ retry_this_zone:
 out:
 	return nr_populated;
 
-failed_irq:
-	pcp_trylock_finish(UP_flags);
-
 failed:
 	page = __alloc_pages_noprof(gfp, 0, preferred_nid, nodemask);
 	if (page)
_