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

[Lorenzo Stoakes (ARM)]

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.

Signed-off-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
---
 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