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

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

On Wed, Sep 09, 2026 at 10:15:13AM +0100, Kiryl Shutsemau wrote:
> 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.

Hmm I thought I got this... will fix on respin!

>
> > 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.

Ugh I thought I fixed this before I respun too, doh!

Will fix.

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

--
Cheers, Lorenzo