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

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

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

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.

The default implementation returns false to maintain current behavior.

Suggested-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Lance Yang <lance.yang@linux.dev>
---
 include/asm-generic/tlb.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/asm-generic/tlb.h b/include/asm-generic/tlb.h
index 324a21f53b64..3f0add95604f 100644
--- a/include/asm-generic/tlb.h
+++ b/include/asm-generic/tlb.h
@@ -248,6 +248,21 @@ static inline void tlb_remove_table(struct mmu_gather *tlb, void *table)
 #define tlb_needs_table_invalidate() (true)
 #endif
 
+/*
+ * 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
+
 void tlb_remove_table_sync_one(void);
 
 #else
@@ -829,12 +844,17 @@ static inline void tlb_flush_unshared_tables(struct mmu_gather *tlb)
 	 * We only perform this when we are the last sharer of a page table,
 	 * as the IPI will reach all CPUs: any GUP-fast.
 	 *
+	 * However, if the TLB flush already synchronized with other CPUs
+	 * (indicated by tlb_table_flush_implies_ipi_broadcast()), we can skip
+	 * the additional IPI.
+	 *
 	 * Note that on configs where tlb_remove_table_sync_one() is a NOP,
 	 * the expectation is that the tlb_flush_mmu_tlbonly() would have issued
 	 * required IPIs already for us.
 	 */
 	if (tlb->fully_unshared_tables) {
-		tlb_remove_table_sync_one();
+		if (!tlb_table_flush_implies_ipi_broadcast())
+			tlb_remove_table_sync_one();
 		tlb->fully_unshared_tables = false;
 	}
 }
-- 
2.49.0