[PATCH v2 1/1] mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails

[Lance Yang <lance.yang@linux.dev>]

From: Lance Yang <lance.yang@linux.dev>

When freeing page tables, we try to batch them. If batch allocation fails
(GFP_NOWAIT), __tlb_remove_table_one() immediately frees the one without
batching.

On !CONFIG_PT_RECLAIM, the fallback sends an IPI to all CPUs via
tlb_remove_table_sync_one(). It disrupts all CPUs even when only a single
process is unmapping memory. IPI broadcast was reported to hurt RT
workloads[1].

tlb_remove_table_sync_one() synchronizes with lockless page-table walkers
(e.g. GUP-fast) that rely on IRQ disabling. These walkers use
local_irq_disable(), which is also an RCU read-side critical section.

This patch introduces tlb_remove_table_sync_rcu() which uses RCU grace
period (synchronize_rcu()) instead of IPI broadcast. This provides the
same guarantee as IPI but without disrupting all CPUs. Since batch
allocation already failed, we are in a way slow path where sleeping is
acceptable - we are in process context (unmap_region, exit_mmap) with only
mmap_lock held. might_sleep() will catch any invalid context.

[1] https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/

Link: https://lore.kernel.org/linux-mm/20260202150957.GD1282955@noisy.programming.kicks-ass.net/
Link: https://lore.kernel.org/linux-mm/dfdfeac9-5cd5-46fc-a5c1-9ccf9bd3502a@intel.com/
Link: https://lore.kernel.org/linux-mm/bc489455-bb18-44dc-8518-ae75abda6bec@kernel.org/
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
v1 -> v2:
- Wrap synchronize_rcu() in tlb_remove_table_sync_rcu() with proper
  kerneldoc (per David)
- Add might_sleep() to make sleeping constraint explicit (per Dave)
- Clarify this is for synchronization, not memory freeing (per Dave)
- https://lore.kernel.org/linux-mm/20260223033604.10198-1-lance.yang@linux.dev/

 include/asm-generic/tlb.h |  4 ++++
 mm/mmu_gather.c           | 22 +++++++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 4aeac0c3d3f0..bdcc2778ac64 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -251,6 +251,8 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
 
 void tlb_remove_table_sync_one(void);
 
+void tlb_remove_table_sync_rcu(void);
+
 #else
 
 #ifdef tlb_needs_table_invalidate
@@ -259,6 +261,8 @@ void tlb_remove_table_sync_one(void);
 
 static inline void tlb_remove_table_sync_one(void) { }
 
+static inline void tlb_remove_table_sync_rcu(void) { }
+
 #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
 
 
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index fe5b6a031717..2c6fa8db55df 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -296,6 +296,26 @@ static void tlb_remove_table_free(struct mmu_table_batch *batch)
 	call_rcu(&batch->rcu, tlb_remove_table_rcu);
 }
 
+/**
+ * tlb_remove_table_sync_rcu() - synchronize with software page-table walkers
+ *
+ * Like tlb_remove_table_sync_one() but uses RCU grace period instead of IPI
+ * broadcast. Use in slow paths where sleeping is acceptable.
+ *
+ * Software/Lockless page-table walkers use local_irq_disable(), which is also
+ * an RCU read-side critical section. synchronize_rcu() waits for all such
+ * sections, providing the same guarantee as tlb_remove_table_sync_one() but
+ * without disrupting all CPUs with IPIs.
+ *
+ * Do not use for freeing memory. Use RCU callbacks instead to avoid latency
+ * spikes. Cannot be called from any atomic context.
+ */
+void tlb_remove_table_sync_rcu(void)
+{
+	might_sleep();
+	synchronize_rcu();
+}
+
 #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */
 
 static void tlb_remove_table_free(struct mmu_table_batch *batch)
@@ -339,7 +359,7 @@ static inline void __tlb_remove_table_one(void *table)
 #else
 static inline void __tlb_remove_table_one(void *table)
 {
-	tlb_remove_table_sync_one();
+	tlb_remove_table_sync_rcu();
 	__tlb_remove_table(table);
 }
 #endif /* CONFIG_PT_RECLAIM */
-- 
2.49.0