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

On Wed, Sep 23, 2026 at 11:08:11AM +0200, David Hildenbrand (Arm) wrote:

On 9/22/26 17:35, Lorenzo Stoakes (ARM) wrote:

quoted

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@kernel.org>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 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