Re: [PATCH v2 11/12] mm: make userland page table freeing RCU-safe

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Sep 08, 2026 at 01:32:20PM +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 all page tables are

You missed s/all/userspace/ from v1.

> 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.


> @@ -151,24 +148,15 @@
>   *  This might be useful if your architecture has size specific TLB
>   *  invalidation instructions.
>   *
> - *  MMU_GATHER_TABLE_FREE
> - *
> - *  This provides tlb_remove_table(), to be used instead of tlb_remove_page()
> - *  for page directores (__p*_free_tlb()).
> - *
> - *  Useful if your architecture has non-page page directories.
> + *  Page directories (__p*_free_tlb()) are always freed via tlb_remove_table(),
> + *  after an RCU grace period (see mm/mmu_gather.c).
>   *
> - *  When used, an architecture is expected to provide __tlb_remove_table() or
> - *  use the generic __tlb_remove_table(), which does the actual freeing of these
> - *  pages.
> + * This serialises against software page-table walkers, including architectures
> + * which do not use IPIs for remote TLB invalidates.

Indentation is broken.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov

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