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) <[email protected]> > > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> > > --- > > 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
