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From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
To: Andrew Morton <akpm@linux-foundation.org>,
	 David Hildenbrand <david@kernel.org>, Zi Yan <ziy@nvidia.com>,
	 Baolin Wang <baolin.wang@linux.alibaba.com>,
	 "Liam R. Howlett" <liam@infradead.org>,
	Nico Pache <nico.pache@linux.dev>,
	 Ryan Roberts <ryan.roberts@arm.com>, Dev Jain <dev.jain@arm.com>,
	 Barry Song <baohua@kernel.org>,
	Lance Yang <lance.yang@linux.dev>,
	 Usama Arif <usama.arif@linux.dev>,
	Kiryl Shutsemau <kas@kernel.org>,  Guo Ren <guoren@kernel.org>,
	Brian Cain <bcain@kernel.org>,
	 Geert Uytterhoeven <geert@linux-m68k.org>,
	 Dinh Nguyen <dinguyen@kernel.org>,
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	 Jonas Bonn <jonas@southpole.se>,
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	 Stafford Horne <shorne@gmail.com>,
	 Yoshinori Sato <ysato@users.sourceforge.jp>,
	Rich Felker <dalias@libc.org>,
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Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	 linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org,
	 linux-m68k@lists.linux-m68k.org, linux-openrisc@vger.kernel.org,
	 linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org,
	 linux-arm-kernel@lists.infradead.org,
	linux-snps-arc@lists.infradead.org,  linux-arch@vger.kernel.org,
	sparclinux@vger.kernel.org,  linux-alpha@vger.kernel.org,
	loongarch@lists.linux.dev,  linux-mips@vger.kernel.org,
	linux-parisc@vger.kernel.org,  linuxppc-dev@lists.ozlabs.org,
	linux-s390@vger.kernel.org,  linux-um@lists.infradead.org,
	Hugh Dickins <hughd@google.com>,  Qi Zheng <qi.zheng@linux.dev>,
	"Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Subject: [PATCH v2 04/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU arm
Date: Tue, 08 Sep 2026 13:32:13 +0100	[thread overview]
Message-ID: <20260908-rcu-pagetable-freeing-v2-4-1f60b64e878e@kernel.org> (raw)
In-Reply-To: <20260908-rcu-pagetable-freeing-v2-0-1f60b64e878e@kernel.org>

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@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


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  parent reply	other threads:[~2026-09-08 12:34 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-09-08 12:32 [PATCH v2 00/12] mm: make userland page table freeing RCU-safe Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 02/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for most 2-level architectures Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 03/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU riscv Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` Lorenzo Stoakes (ARM) [this message]
2026-09-08 12:32 ` [PATCH v2 05/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for arc, microblaze, xtensa Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 06/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for sparc64 Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 07/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for m68k-coldfire Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 08/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for sh-X2 Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 09/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for m68k-motorola Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 10/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for sparc32 Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 11/12] mm: make userland page table freeing RCU-safe Lorenzo Stoakes (ARM)
2026-09-09  9:15   ` Kiryl Shutsemau
2026-09-09 16:44     ` Lorenzo Stoakes (ARM)
2026-09-08 12:32 ` [PATCH v2 12/12] mm: change the contract for free_pgtables(), update docs Lorenzo Stoakes (ARM)
2026-09-09  9:24   ` Kiryl Shutsemau
2026-09-09 16:42     ` Lorenzo Stoakes (ARM)
2026-09-08 21:15 ` [PATCH v2 00/12] mm: make userland page table freeing RCU-safe Andrew Morton
2026-09-09 11:24   ` Lorenzo Stoakes (ARM)
2026-09-09  9:26 ` Kiryl Shutsemau
2026-09-09 11:07   ` Lorenzo Stoakes (ARM)

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