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

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

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:
> >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