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

["David Hildenbrand (Arm)" <david@kernel.org>]

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.

> 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>

-- 
Cheers,

David