Re: [PATCH v4 03/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU riscv

On Wed, Sep 23, 2026 at 02:15:51PM +0800, Lance Yang wrote:

On Tue, Sep 22, 2026 at 04:35:34PM +0100, Lorenzo Stoakes (ARM) wrote:

quoted

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.

Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---

Yeah, for RISC-V, page-table freeing already goes through
tlb_remove_ptdesc() :) Tested on QEMU, no issues seen :)

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

Thanks! :)

A cheeky A-b tag possible too? :P


--
Cheers, Lorenzo