Re: [PATCH v2 01/12] mm/huge_memory: zap deposited page tables after an RCU grace period

[sashiko-bot@kernel.org]

> When an anonymous mapping is collapsed for THP, a PTE page table is
> 'deposited' with the installed PMD entry.
> 
> This is done in order that a split can be performed without needing to
> allocate additional memory.
> 
> The freeing occurs in zap_deposited_table() and is done directly without
> any delay via pte_free().
> 
> This is currently not a problem as existing page table walks are protected
> by the mmap or anon rmap lock.
> 
> However this becomes problematic in a future where RCU-only page table
> walkers exist, as there is nothing to prevent a page table walker that
> started the walk prior to collapse having its PTE table freed underneath
> [ ... ]
> __collapse_huge_page_isolate() determines whether it is safe to proceed by
> checking folio_ref_count() against folio_expected_ref_count(), so the two
> must be mutually excluded.
> 
> 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/20260908-rcu-pagetable-freeing-v2-0-1f60b64e878e@kernel.org?part=1