[PATCH v2 1/3] mm/tlb: allow architectures to skip redundant TLB sync IPIs

[Lance Yang]

From: Lance Yang <lance.yang@xxxxxxxxx>

When unsharing hugetlb PMD page tables, we currently send two IPIs: one
for TLB invalidation, and another to synchronize with concurrent GUP-fast
walkers.

However, if the TLB flush already reaches all CPUs, the second IPI is
redundant. GUP-fast runs with IRQs disabled, so when the TLB flush IPI
completes, any concurrent GUP-fast must have finished.

Add tlb_table_flush_implies_ipi_broadcast() to let architectures indicate
their TLB flush provides full synchronization, enabling the redundant IPI
to be skipped.

Suggested-by: David Hildenbrand (Red Hat) <david@xxxxxxxxxx>
Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
---
 include/asm-generic/tlb.h | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 4d679d2a206b..e8d99b5e831f 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -261,6 +261,20 @@ static inline void tlb_remove_table_sync_one(void) { }
 
 #endif /* CONFIG_MMU_GATHER_RCU_TABLE_FREE */
 
+/*
+ * Architectures can override if their TLB flush already broadcasts IPIs to all
+ * CPUs when freeing or unsharing page tables.
+ *
+ * Return true only when the flush guarantees:
+ * - IPIs reach all CPUs with potentially stale paging-structure cache entries
+ * - Synchronization with IRQ-disabled code like GUP-fast
+ */
+#ifndef tlb_table_flush_implies_ipi_broadcast
+static inline bool tlb_table_flush_implies_ipi_broadcast(void)
+{
+	return false;
+}
+#endif
 
 #ifndef CONFIG_MMU_GATHER_NO_GATHER
 /*
-- 
2.49.0