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[03/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU riscv

Message ID 20260901-rcu-pagetable-freeing-v1-3-5456a81c8212@kernel.org
State New
Headers show
Series mm: make userland page table freeing RCU-safe | expand

Commit Message

Lorenzo Stoakes (ARM) Sept. 1, 2026, 11:01 a.m. UTC 
Currently riscv gates MMU_GATHER_RCU_TABLE_FREE on CONFIG_SMP and
CONFIG_MMU.

Commit 69be3fb111e7 ("riscv: enable MMU_GATHER_RCU_TABLE_FREE for SMP &&
MMU") enabled CONFIG_MMU_GATHER_RCU_TABLE_FREE for CONFIG_SMP, CONFIG_MMU
riscv builds.

This is expressly for the safety of GUP-fast walkers (CONFIG_HAVE_GUP_FAST
is enabled if CONFIG_MMU is enabled).

Naturally a single core system does not encounter issues with software page
table walkers being correctly synchronised across cores, as there is only a
single core.

However, CONFIG_PREEMPT_RCU is still available on a riscv UP system, so for
a future RCU-only page table walker, this guarantee is required to prevent
concurrent page table teardown.

All page table freeing is already done via tlb_remove_ptdesc() so the
conditions of CONFIG_MMU_GATHER_RCU_TABLE_FREE are already met.

This forms part of an overall effort to switch every architecture to this
mode.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 arch/riscv/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 505eed4af932..3529ed1861ce 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -208,7 +208,7 @@  config RISCV
 	select IRQ_FORCED_THREADING
 	select KASAN_VMALLOC if KASAN
 	select LOCK_MM_AND_FIND_VMA
-	select MMU_GATHER_RCU_TABLE_FREE if SMP && MMU
+	select MMU_GATHER_RCU_TABLE_FREE if MMU
 	select MODULES_USE_ELF_RELA if MODULES
 	select OF
 	select OF_EARLY_FLATTREE