mm/mmu_gather: replace IPI with synchronize_rcu() when batch allocation fails

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 slow path where sleeping is
acceptable - we are in process context (unmap_region, exit_mmap) with only
mmap_lock held.

tlb_remove_table_sync_one() is retained for other callers (e.g.,
khugepaged after pmdp_collapse_flush(), tlb_finish_mmu() when
tlb->fully_unshared_tables) that are not slow paths.  Converting those may
require different approaches such as targeted IPIs.

Link: https://lore.kernel.org/linux-mm/1b27a3fa-359a-43d0-bdeb-c31341749367@kernel.org/ [1]
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/
Link: https://lkml.kernel.org/r/20260224142101.20500-1-lance.yang@linux.dev
Signed-off-by: Lance Yang <lance.yang@linux.dev>
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Suggested-by: Dave Hansen <dave.hansen@intel.com>
Suggested-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 24 insertions, 1 deletions

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..3985d856de7f 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -296,6 +296,25 @@ 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.
+ */
+void tlb_remove_table_sync_rcu(void)
+{
+	synchronize_rcu();
+}
+
 #else /* !CONFIG_MMU_GATHER_RCU_TABLE_FREE */
 
 static void tlb_remove_table_free(struct mmu_table_batch *batch)
@@ -339,7 +358,7 @@ 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 */