On Tue, Sep 01, 2026 at 12:01:32PM +0100, Lorenzo Stoakes (ARM) wrote: > 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? -- Kiryl Shutsemau / Kirill A. Shutemov
