FAILED: patch "[PATCH] mm/page_alloc: prevent pcp corruption with SMP=n" failed to apply to 6.1-stable tree

PATCH New [email protected] 2 participants 2 messages Jan 20 – Jan 21

Subsystems (3)

Stable branch Memory management Page allocator

lore

Share & tools

lore

[email protected]

author maintainer

2026-01-20 13:08 UTC #0 lore raw

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x 038a102535eb49e10e93eafac54352fcc5d78847
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026012044-late-implode-126e@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 038a102535eb49e10e93eafac54352fcc5d78847 Mon Sep 17 00:00:00 2001
From: Vlastimil Babka <[email protected]>
Date: Mon, 5 Jan 2026 16:08:56 +0100
Subject: [PATCH] mm/page_alloc: prevent pcp corruption with SMP=n

The kernel test robot has reported:

 BUG: spinlock trylock failure on UP on CPU#0, kcompactd0/28
  lock: 0xffff888807e35ef0, .magic: dead4ead, .owner: kcompactd0/28, .owner_cpu: 0
 CPU: 0 UID: 0 PID: 28 Comm: kcompactd0 Not tainted 6.18.0-rc5-00127-ga06157804399 #1 PREEMPT  8cc09ef94dcec767faa911515ce9e609c45db470
 Call Trace:
  <IRQ>
  __dump_stack (lib/dump_stack.c:95)
  dump_stack_lvl (lib/dump_stack.c:123)
  dump_stack (lib/dump_stack.c:130)
  spin_dump (kernel/locking/spinlock_debug.c:71)
  do_raw_spin_trylock (kernel/locking/spinlock_debug.c:?)
  _raw_spin_trylock (include/linux/spinlock_api_smp.h:89 kernel/locking/spinlock.c:138)
  __free_frozen_pages (mm/page_alloc.c:2973)
  ___free_pages (mm/page_alloc.c:5295)
  __free_pages (mm/page_alloc.c:5334)
  tlb_remove_table_rcu (include/linux/mm.h:? include/linux/mm.h:3122 include/asm-generic/tlb.h:220 mm/mmu_gather.c:227 mm/mmu_gather.c:290)
  ? __cfi_tlb_remove_table_rcu (mm/mmu_gather.c:289)
  ? rcu_core (kernel/rcu/tree.c:?)
  rcu_core (include/linux/rcupdate.h:341 kernel/rcu/tree.c:2607 kernel/rcu/tree.c:2861)
  rcu_core_si (kernel/rcu/tree.c:2879)
  handle_softirqs (arch/x86/include/asm/jump_label.h:36 include/trace/events/irq.h:142 kernel/softirq.c:623)
  __irq_exit_rcu (arch/x86/include/asm/jump_label.h:36 kernel/softirq.c:725)
  irq_exit_rcu (kernel/softirq.c:741)
  sysvec_apic_timer_interrupt (arch/x86/kernel/apic/apic.c:1052)
  </IRQ>
  <TASK>
 RIP: 0010:_raw_spin_unlock_irqrestore (arch/x86/include/asm/preempt.h:95 include/linux/spinlock_api_smp.h:152 kernel/locking/spinlock.c:194)
  free_pcppages_bulk (mm/page_alloc.c:1494)
  drain_pages_zone (include/linux/spinlock.h:391 mm/page_alloc.c:2632)
  __drain_all_pages (mm/page_alloc.c:2731)
  drain_all_pages (mm/page_alloc.c:2747)
  kcompactd (mm/compaction.c:3115)
  kthread (kernel/kthread.c:465)
  ? __cfi_kcompactd (mm/compaction.c:3166)
  ? __cfi_kthread (kernel/kthread.c:412)
  ret_from_fork (arch/x86/kernel/process.c:164)
  ? __cfi_kthread (kernel/kthread.c:412)
  ret_from_fork_asm (arch/x86/entry/entry_64.S:255)
  </TASK>

Matthew has analyzed the report and identified that in drain_page_zone() we are in a section protected by spin_lock(&pcp->lock) and then get an interrupt that attempts spin_trylock() on the same lock. The code is designed to work this way without disabling IRQs and occasionally fail the trylock with a fallback. However, the SMP=n spinlock implementation assumes spin_trylock() will always succeed, and thus it's normally a no-op. Here the enabled lock debugging catches the problem, but otherwise it could cause a corruption of the pcp structure.

The problem has been introduced by commit 574907741599 ("mm/page_alloc:
leave IRQs enabled for per-cpu page allocations"). The pcp locking scheme recognizes the need for disabling IRQs to prevent nesting spin_trylock() sections on SMP=n, but the need to prevent the nesting in spin_lock() has not been recognized. Fix it by introducing local wrappers that change the spin_lock() to spin_lock_iqsave() with SMP=n and use them in all places that do spin_lock(&pcp->lock).

[[email protected]: add pcp_ prefix to the spin_lock_irqsave wrappers, per Steven]

Link https://lkml.kernel.org/r/[email protected] open here

Fixes 574907741599 ("mm/page_alloc: leave IRQs enabled for per-cpu page allocations")

Signed-off-by Vlastimil Babka <[email protected]>

Reported-by kernel test robot <[email protected]>

Closes https://lore.kernel.org/oe-lkp/[email protected] open here

Analyzed-by Matthew Wilcox <[email protected]>

Link https://lore.kernel.org/all/[email protected]/ open here

Acked-by Mel Gorman <[email protected]>

Cc Brendan Jackman <[email protected]>

Cc Johannes Weiner <[email protected]>

Cc Michal Hocko <[email protected]>

Cc Sebastian Andrzej Siewior <[email protected]>

Cc Steven Rostedt <[email protected]>

Cc Suren Baghdasaryan <[email protected]>

Cc Zi Yan <[email protected]>

Cc <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

mm/page_alloc.c +39 -8

index 3dce96f3be49..f65c4edf199d 100644
@@ -167,6 +167,33 @@ static inline void __pcp_trylock_noop(unsigned long *flags) { }
 	pcp_trylock_finish(UP_flags);					\
 })
 
+/*
+ * With the UP spinlock implementation, when we spin_lock(&pcp->lock) (for i.e.
+ * a potentially remote cpu drain) and get interrupted by an operation that
+ * attempts pcp_spin_trylock(), we can't rely on the trylock failure due to UP
+ * spinlock assumptions making the trylock a no-op. So we have to turn that
+ * spin_lock() to a spin_lock_irqsave(). This works because on UP there are no
+ * remote cpu's so we can only be locking the only existing local one.
+ */
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT)
+static inline void __flags_noop(unsigned long *flags) { }
+#define pcp_spin_lock_maybe_irqsave(ptr, flags)		\
+({							\
+	 __flags_noop(&(flags));			\
+	 spin_lock(&(ptr)->lock);			\
+})
+#define pcp_spin_unlock_maybe_irqrestore(ptr, flags)	\
+({							\
+	 spin_unlock(&(ptr)->lock);			\
+	 __flags_noop(&(flags));			\
+})
+#else
+#define pcp_spin_lock_maybe_irqsave(ptr, flags)		\
+		spin_lock_irqsave(&(ptr)->lock, flags)
+#define pcp_spin_unlock_maybe_irqrestore(ptr, flags)	\
+		spin_unlock_irqrestore(&(ptr)->lock, flags)
+#endif
+
 #ifdef CONFIG_USE_PERCPU_NUMA_NODE_ID
 DEFINE_PER_CPU(int, numa_node);
 EXPORT_PER_CPU_SYMBOL(numa_node);

@@ -2556,6 +2583,7 @@ static int rmqueue_bulk(struct zone *zone, unsigned int order,
 bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 {
 	int high_min, to_drain, to_drain_batched, batch;
+	unsigned long UP_flags;
 	bool todo = false;
 
 	high_min = READ_ONCE(pcp->high_min);

@@ -2575,9 +2603,9 @@ bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
 	to_drain = pcp->count - pcp->high;
 	while (to_drain > 0) {
 		to_drain_batched = min(to_drain, batch);
-		spin_lock(&pcp->lock);
+		pcp_spin_lock_maybe_irqsave(pcp, UP_flags);
 		free_pcppages_bulk(zone, to_drain_batched, pcp, 0);
-		spin_unlock(&pcp->lock);
+		pcp_spin_unlock_maybe_irqrestore(pcp, UP_flags);
 		todo = true;
 
 		to_drain -= to_drain_batched;

@@ -2594,14 +2622,15 @@ bool decay_pcp_high(struct zone *zone, struct per_cpu_pages *pcp)
  */
 void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
 {
+	unsigned long UP_flags;
 	int to_drain, batch;
 
 	batch = READ_ONCE(pcp->batch);
 	to_drain = min(pcp->count, batch);
 	if (to_drain > 0) {
-		spin_lock(&pcp->lock);
+		pcp_spin_lock_maybe_irqsave(pcp, UP_flags);
 		free_pcppages_bulk(zone, to_drain, pcp, 0);
-		spin_unlock(&pcp->lock);
+		pcp_spin_unlock_maybe_irqrestore(pcp, UP_flags);
 	}
 }
 #endif

@@ -2612,10 +2641,11 @@ void drain_zone_pages(struct zone *zone, struct per_cpu_pages *pcp)
 static void drain_pages_zone(unsigned int cpu, struct zone *zone)
 {
 	struct per_cpu_pages *pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
+	unsigned long UP_flags;
 	int count;
 
 	do {
-		spin_lock(&pcp->lock);
+		pcp_spin_lock_maybe_irqsave(pcp, UP_flags);
 		count = pcp->count;
 		if (count) {
 			int to_drain = min(count,

@@ -2624,7 +2654,7 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone)
 			free_pcppages_bulk(zone, to_drain, pcp, 0);
 			count -= to_drain;
 		}
-		spin_unlock(&pcp->lock);
+		pcp_spin_unlock_maybe_irqrestore(pcp, UP_flags);
 	} while (count);
 }

@@ -6109,6 +6139,7 @@ static void zone_pcp_update_cacheinfo(struct zone *zone, unsigned int cpu)
 {
 	struct per_cpu_pages *pcp;
 	struct cpu_cacheinfo *cci;
+	unsigned long UP_flags;
 
 	pcp = per_cpu_ptr(zone->per_cpu_pageset, cpu);
 	cci = get_cpu_cacheinfo(cpu);

@@ -6119,12 +6150,12 @@ static void zone_pcp_update_cacheinfo(struct zone *zone, unsigned int cpu)
 	 * This can reduce zone lock contention without hurting
 	 * cache-hot pages sharing.
 	 */
-	spin_lock(&pcp->lock);
+	pcp_spin_lock_maybe_irqsave(pcp, UP_flags);
 	if ((cci->per_cpu_data_slice_size >> PAGE_SHIFT) > 3 * pcp->batch)
 		pcp->flags |= PCPF_FREE_HIGH_BATCH;
 	else
 		pcp->flags &= ~PCPF_FREE_HIGH_BATCH;
-	spin_unlock(&pcp->lock);
+	pcp_spin_unlock_maybe_irqrestore(pcp, UP_flags);
 }
 
 void setup_pcp_cacheinfo(unsigned int cpu)

Sasha Levin

maintainer

2026-01-21 12:15 UTC #1 lore raw

PATCH 1/1 mm/page_alloc: prevent pcp corruption with SMP=n

From: Vlastimil Babka <[email protected]>

[ Upstream commit 038a102535eb49e10e93eafac54352fcc5d78847 ]