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

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

On Wed, Sep 23, 2026 at 11:04:17AM +0200, David Hildenbrand (Arm) wrote:
> On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote:
>
> The patch subject confused me at first: that's not what the patch is doing, no?
> Previous patches did that, and now you're just removing the leftovers?
>
> Maybe I'm missing something that this patch does along the lines of teaching the
> code to be RCU-safe.

It is the final piece that makes that the case, so I think it's OK to allow
oneself a nice subject line? No? :)

If people are confused they can read the commit message which goes into some
detail.

>
> > 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>
>
>
> [...]
>
> > --- a/mm/Kconfig
> > +++ b/mm/Kconfig
> > @@ -1465,7 +1465,7 @@ config HAVE_ARCH_TLB_REMOVE_TABLE
> >
> >  config PT_RECLAIM
> >  	def_bool y
> > -	depends on MMU_GATHER_RCU_TABLE_FREE && !HAVE_ARCH_TLB_REMOVE_TABLE
> > +	depends on MMU && !HAVE_ARCH_TLB_REMOVE_TABLE
> >  	help
> >  	  Try to reclaim empty user page table pages in paths other than munmap
> >  	  and exit_mmap path.
>
> $ git grep PT_RECLAIM
> mm/Kconfig:config PT_RECLAIM
> mm/memory.c:    if (!IS_ENABLED(CONFIG_PT_RECLAIM))
> mm/mmu_gather.c:#ifdef CONFIG_PT_RECLAIM
> mm/mmu_gather.c:#endif /* CONFIG_PT_RECLAIM */
>
> whereby all files are MMU-only.
>
> Makes sense.
>
>
> Very nice
>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Thanks.

>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo