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: > > 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
