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

On Tue, Sep 01, 2026 at 02:57:13PM +0100, Kiryl Shutsemau wrote:
> 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?

Hmm yeah, ah page tables, what a rabbit hole of horror :)

I guess this is effectively a reflection of the 'write lock on write, RCU load
on read' pattern in RCU itself generally.

So, maybe something like:

 * Note that free_pgtables(), used after unampping detached vmas, or when
 * exiting the whoel mm, does not take a page table lock before freeing a page
 * table.
 *
 * As page table freeing itself is RCU-safe, page table readers can safely run
 * concurrently with page table teardown. However, writers cannot, as without
 * a lock nothing prevents concurrent teardown.
 */

Instead? What do you think?

>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov

--
Cheers, Lorenzo