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

[Kiryl Shutsemau <kas@kernel.org>]

On Tue, Sep 01, 2026 at 03:31:26PM +0100, Lorenzo Stoakes (ARM) wrote:
> 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?

s/whoel/whole/

Otherwise, LGTM.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov