Re: [PATCH v4 11/12] mm: make userland page table freeing RCU-safe

[Lance Yang <lance.yang@linux.dev>]

On Tue, Sep 22, 2026 at 04:35:42PM +0100, Lorenzo Stoakes (ARM) wrote:
>Now every architecture has been converted to support
>CONFIG_MMU_GATHER_RCU_TABLE_FREE, this configuration option no longer makes
>any sense to keep around.
>
>Therefore remove it, and remove all the dead code that existed for
>!CONFIG_MMU_GATHER_RCU_TABLE_FREE architectures previously.
>
>Additionally, CONFIG_MMU_GATHER_TABLE_FREE is no longer necessary, as all
>architectures instead use CONFIG_HAVE_ARCH_TLB_REMOVE_TABLE when a custom
>__tlb_remove_table() is required, so remove this too.
>
>A number of architectures only enabled CONFIG_MMU_GATHER_RCU_TABLE_FREE if
>CONFIG_MMU was set, however the mmu_gather logic only actually does
>something meaningful if CONFIG_MMU is set (mmu_gather.c is only compiled in
>this case, for instance).
>
>As a result, there's no need to gate any of this logic on CONFIG_MMU
>explicitly.
>
>CONFIG_PT_RECLAIM however does have a strict dependency on CONFIG_MMU, so
>make this dependency explicit.
>
>Additionally, correct comments to remove references to non-RCU page table
>gathering and make it clear that this is not 'semi-RCU', nor has it been
>since commit 1fb3d8c20bfa ("mm/mmu_gather: replace IPI with
>synchronize_rcu() when batch allocation fails").
>
>With this change in place the kernel policy is now that userspace page
>tables are freed after an RCU grace period, and thus it is now safe to
>unconditionally perform page table walks under RCU, safe in the knowledge
>that page tables will not be freed underneath the walker.
>
>This is all that is guaranteed, however, so naturally it is still incumbent
>upon page table walkers to ensure that the page table entries are as
>expected.
>
>Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
>Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
>---

with David's feedback folded in, LGTM!

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