Re: [PATCH 12/12] mm: change the contract for free_pgtables(), update docs

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Sep 01, 2026 at 12:01:32PM +0100, Lorenzo Stoakes (ARM) wrote:
> diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> index b91b1a98029c..ff8ff3706485 100644
> --- a/mm/pgtable-generic.c
> +++ b/mm/pgtable-generic.c
> @@ -386,9 +386,21 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
>   * be read-only/read-write protected.
>   *
>   * Note that free_pgtables(), used after unmapping detached vmas, or when
> - * exiting the whole mm, does not take page table lock before freeing a page
> - * table, and may not use RCU at all: "outsiders" like khugepaged should avoid
> - * pte_offset_map() and co once the vma is detached from mm or mm_users is zero.
> + * exiting the whole mm, does not take the page table lock before freeing a
> + * table.
> + *
> + * However, the PMD entry is cleared first, and the table freed only after
> + * an RCU grace period, so a walker that mapped the table under
> + * rcu_read_lock() stays safe, and the pmd_same() recheck in
> + * pte_offset_map_lock() detects the teardown.
> + *
> + * Therefore it is safe for "outsiders" like khugepaged to use
> + * pte_offset_map() and co. for VMAs that might be undergoing page table
> + * teardown.

Is it strictly true?

free_pte_range() clears the pmd without taking the PTL, so the
pmd_same() recheck in pte_offset_map_lock() can pass and the pmd gets
cleared right after.

Readers are fine -- __pte_offset_map() holds rcu_read_lock() until
pte_unmap(), so the table cannot go away.

khugepaged is an odd example here. collapse_pte_mapped_thp() and
retract_page_tables() don't rely on this recheck -- they take pmd_lock()
plus the ptl and do their own pmd_same() under both.

Can we say walks are safe and leave the write rule where it is?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov