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

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

[Lance Yang]

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.
synchronize_rcu() waits for all such sections to complete, providing the
same guarantee as IPI but without disrupting all CPUs.

Since batch allocation already failed, we are in a way slow path, so
replacing the IPI with synchronize_rcu() is fine.

We are in process context (unmap_region, exit_mmap) with only mmap_lock
held, a sleeping lock. synchronize_rcu() will catch any invalid context
via might_sleep().

[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>
---
 mm/mmu_gather.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c
index fe5b6a031717..df670c219260 100644
--- a/mm/mmu_gather.c
+++ b/mm/mmu_gather.c
@@ -339,7 +339,8 @@ static inline void __tlb_remove_table_one(void *table)
 #else
 static inline void __tlb_remove_table_one(void *table)
 {
-	tlb_remove_table_sync_one();
+	if (IS_ENABLED(CONFIG_MMU_GATHER_RCU_TABLE_FREE))
+		synchronize_rcu();
 	__tlb_remove_table(table);
 }
 #endif /* CONFIG_PT_RECLAIM */
-- 
2.49.0