Re: [PATCH v4 12/12] mm: change the contract for free_pgtables(), update docs

[Lorenzo Stoakes (ARM)]

On Wed, Sep 23, 2026 at 11:08:11AM +0200, David Hildenbrand (Arm) wrote:
> On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote:
> > Now that page tables are freed after an RCU grace period, it is safe for
> > read-only page table walkers to walk page table ranges that are being
> > concurrently torn down, provided the mm is kept alive via mmgrab().
> >
> > It is however unsafe for writers to do so, as they must obtain an
> > appropriate lock to do so safely.
> >
> > Update the pte_offset_map_lock()'s comment block to reflect this.
> >
> > Similarly update the process addresses documentation.
> >
> > Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> > Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> > ---
> >  Documentation/mm/process_addrs.rst |  6 ++++++
> >  mm/pgtable-generic.c               | 15 +++++++++++----
> >  2 files changed, 17 insertions(+), 4 deletions(-)
> >
> > diff --git a/Documentation/mm/process_addrs.rst b/Documentation/mm/process_addrs.rst
> > index a7296f251799..b1f4f44d75eb 100644
> > --- a/Documentation/mm/process_addrs.rst
> > +++ b/Documentation/mm/process_addrs.rst
> > @@ -537,6 +537,12 @@ We establish basic locking rules when interacting with page tables:
> >  * When changing a page table entry the page table lock for that page table
> >    **must** be held, except if you can safely assume nobody can access the page
> >    tables concurrently (such as on invocation of :c:func:`!free_pgtables`).
> > +* Page tables may be *walked* under RCU alone, as page tables are freed only
> > +  after an RCU grace period has elapsed. However, any entry found must be
> > +  revalidated after the page table lock is taken (such as the
> > +  :c:func:`!pmd_same` recheck performed by :c:func:`!pte_offset_map_lock`)
> > +  before it is acted upon. Changing an entry requires the page table
> > +  lock and one of the locks that excludes teardown (mmap or VMA lock).
>
> While we can walk the page tables, I assume there are some limits to what we can
> actually do with leaf entries. E.g., Doing a careless pte->folio lookup might be
> dangerous.

Yes.

>
> I wonder if we want to hint at that here: that walking under RCU (traversing
> page tables) is something different than actually operating on the leaf entries.

Yeah I can add a new entry in the list perhaps like:

* When traversing page tables under RCU alone it is important to take care when
  operating upon leaf entries - if the value is operated upon (for instance
  getting the folio associated with a PTE) an appropriate lock must be taken to
  prevent concurrent modification.

?

>
> Apart from that LGTM.
>
> --
> Cheers,
>
> David

--
Cheers, Lorenzo