On Wed, Sep 09, 2026 at 10:24:21AM +0100, Kiryl Shutsemau wrote: > On Tue, Sep 08, 2026 at 01:32:21PM +0100, 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. > > > > 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..1e65b139f355 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 always requires the page table > > + lock. > > This says the PTL plus a recheck is enough to act on the entry. It is > not: free_pte_range() clears the PMD without the PTL, so pmd_same() can > pass and the table is torn down right after. That is the case we settled > on for the pte_offset_map_lock() comment, and the two now disagree. > > Something like "Changing an entry requires the page table lock and one > of the locks that excludes teardown (mmap or VMA lock)" would match the > comment.
Ack will change! -- Cheers, Lorenzo
