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

[Lorenzo Stoakes (ARM)]

On Thu, Sep 24, 2026 at 11:26:25AM +0800, Lance Yang wrote:
>
> On Tue, Sep 22, 2026 at 04:35:43PM +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.
> >
> >Acked-by: Kiryl Shutsemau (Meta) <kas@xxxxxxxxxx>
> >Signed-off-by: Lorenzo Stoakes (ARM) <ljs@xxxxxxxxxx>
> >---
> > 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).
>
> What about rmap walkers? try_to_unmap() clears PTEs under the rmap lock
> and PTL, without an mmap or VMA lock.

That's an abomination but yep will update.

>
> Cheers, Lance
>
> > * Reads from and writes to page table entries must be *appropriately*
> >   atomic. See the section on atomicity below for details.
> > * Populating previously empty entries requires that the mmap or VMA locks are
> >diff --git a/mm/pgtable-generic.c b/mm/pgtable-generic.c
> >index b91b1a98029c..a127e3e8f9b9 100644
> >--- a/mm/pgtable-generic.c
> >+++ b/mm/pgtable-generic.c
> >@@ -385,10 +385,17 @@ pte_t *pte_offset_map_rw_nolock(struct mm_struct *mm, pmd_t *pmd,
> >  * Note: "RO" / "RW" expresses the intended semantics, not that the *kmap* will
> >  * 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.
> >+ * Note that free_pgtables(), used after unmapping detached vmas or when exiting
> >+ * the whole 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 being held, nothing prevents
> >+ * concurrent teardown.
> >+ *
> >+ * Also note that the PGD itself is freed at mmdrop() time, not under RCU - so
> >+ * the walker must keep the mm alive either by pinning the mm or the VMA.
> >  */
> > pte_t *pte_offset_map_lock(struct mm_struct *mm, pmd_t *pmd,
> > 			   unsigned long addr, spinlock_t **ptlp)
> >
> >--
> >2.55.0
> >
> >

--
Cheers, Lorenzo