[PATCH v4 04/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU arm

[Lance Yang]

On Tue, Sep 22, 2026 at 04:35:35PM +0100, Lorenzo Stoakes (ARM) wrote:
>Commit a0ad5496b2b3 ("arm: mm: enable HAVE_RCU_TABLE_FREE logic") enabled
>CONFIG_MMU_GATHER_RCU_TABLE_FREE (then named HAVE_RCU_TABLE_FREE) for SMP
>arm architectures with LPAE enabled.
>
>Regardless of whether CONFIG_ARM_LPAE is enabled or not, the same page
>table freeing functions __pte_free_tlb() and __pmd_free_tlb() are used.
>
>Non-LPAE PMD page tables are folded into the PGD and freed by
>pgd_free() (PGD freeing is not part of mmu_gather page table freeing in any
>case), so this is a noop in this case.
>
>Since commit 358d1c39c82a ("arm: convert various functions to use ptdescs")
>both LPAE and non-LPAE PTE page table freeing uses tlb_remove_ptdesc().
>
>Thus all page table freeing is performed under RCU with
>CONFIG_MMU_GATHER_RCU_TABLE_FREE enabled for LPAE and non-LPAE and thus it
>need not be gated on LPAE.
>
>A UP arm system can set CONFIG_PREEMPT_RCU, so a future pure RCU page
>table walker requires MMU_GATHER_RCU_TABLE_FREE to be enabled on UP as
>well, even if concurrent GUP fast is not possible there.
>
>Therefore, it is both safe and desirable to set
>CONFIG_MMU_GATHER_RCU_TABLE_FREE for all MMU arm architectures (nommu does
>not perform mmu_gather operations).
>
>This forms part of an overall effort to switch every architecture to this
>mode.
>
>Acked-by: Kiryl Shutsemau (Meta) <kas at kernel.org>
>Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
>---

Tested under heavy page-table teardown load on qemu-system-arm,
with and without ARM_LPAE. No issues seen :D

Tested-by: Lance Yang <lance.yang at linux.dev>

Also, feel free to add:

Acked-by: Lance Yang <lance.yang at linux.dev>


> arch/arm/Kconfig | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
>index 408aa58a2a5b..72b9afc6ae10 100644
>--- a/arch/arm/Kconfig
>+++ b/arch/arm/Kconfig
>@@ -134,7 +134,7 @@ config ARM
> 	select HAVE_PERF_REGS
> 	select HAVE_PERF_USER_STACK_DUMP
> 	select HAVE_POSIX_CPU_TIMERS_TASK_WORK
>-	select MMU_GATHER_RCU_TABLE_FREE if SMP && ARM_LPAE
>+	select MMU_GATHER_RCU_TABLE_FREE if MMU
> 	select HAVE_REGS_AND_STACK_ACCESS_API
> 	select HAVE_RSEQ
> 	select HAVE_RUST if CPU_LITTLE_ENDIAN && CPU_32v7 && !KASAN
>
>-- 
>2.55.0
>
>