Re: [PATCH 04/12] mm: enable MMU_GATHER_RCU_TABLE_FREE for MMU arm

[sashiko-bot@kernel.org]

> Commit a0ad5496b2b3 ("arm: mm: enable HAVE_RCU_TABLE_FREE logic") enabled
> CONFIG_MMU_GATHER_RCU_TABLE_FREE (then named HAVE_RCU_TABLE_FREE) for SMP
> arm architectures with LPAE enabled.
> 
> Regardless of whether CONFIG_ARM_LPAE is enabled or not, the same page
> table freeing functions __pte_free_tlb() and __pmd_free_tlb() are used.
> 
> Non-LPAE PMD page tables are folded into the PGD and freed by
> pgd_free() (PGD freeing is not part of mmu_gather page table freeing in any
> case), so this is a noop in this case.
> 
> Since commit 358d1c39c82a ("arm: convert various functions to use ptdescs")
> both LPAE and non-LPAE PTE page table freeing uses tlb_remove_ptdesc().
> 
> Thus all page table freeing is performed under RCU with
> CONFIG_MMU_GATHER_RCU_TABLE_FREE enabled for LPAE and non-LPAE and thus it
> need not be gated on LPAE.
> 
> A UP arm system can set CONFIG_PREEMPT_RCU, so a future pure RCU page
> table walker requires MMU_GATHER_RCU_TABLE_FREE to be enabled on UP as
> well, even if concurrent GUP fast is not possible there.
> 
> Therefore, it is both safe and desirable to set
> CONFIG_MMU_GATHER_RCU_TABLE_FREE for all MMU arm architectures (nommu does
> not perform mmu_gather operations).
> 
> This forms part of an overall effort to switch every architecture to this
> mode.
> 
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

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Sashiko AI review · https://sashiko.dev/#/patchset/20260901-rcu-pagetable-freeing-v1-0-5456a81c8212@kernel.org?part=4