Re: [PATCH v4 04/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU arm
From: Lance Yang <lance.yang@linux.dev>
Date: 2026-09-23 08:05:48
Also in:
linux-alpha, linux-arch, linux-doc, linux-m68k, linux-mips, linux-mm, linux-riscv, linux-s390, linux-sh, linux-um, lkml, loongarch, sparclinux
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@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@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@linux.dev> Also, feel free to add: Acked-by: Lance Yang <lance.yang@linux.dev>
quoted hunk ↗ jump to hunk
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 ARMselect 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