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

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:

quoted

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?

--
  Kiryl Shutsemau / Kirill A. Shutemov

--
Cheers, Lorenzo