+ mm-change-the-contract-for-free_pgtables-update-docs.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm: change the contract for free_pgtables(), update docs
has been added to the -mm mm-unstable branch.  Its filename is
     mm-change-the-contract-for-free_pgtables-update-docs.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-change-the-contract-for-free_pgtables-update-docs.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Subject: mm: change the contract for free_pgtables(), update docs
Date: Tue, 22 Sep 2026 16:35:43 +0100

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.

Link: https://lore.kernel.org/20260922-rcu-pagetable-freeing-v4-12-fe1ad1f1e303@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Guo Ren <guoren@kernel.org>
Cc: Brian Cain <bcain@kernel.org>
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Cc: Dinh Nguyen <dinguyen@kernel.org>
Cc: Simon Schuster <schuster.simon@siemens-energy.com>
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---

 Documentation/mm/process_addrs.rst |    6 ++++++
 mm/pgtable-generic.c               |   15 +++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

--- a/Documentation/mm/process_addrs.rst~mm-change-the-contract-for-free_pgtables-update-docs
+++ a/Documentation/mm/process_addrs.rst
@@ -537,6 +537,12 @@ We establish basic locking rules when in
 * 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).
 * 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
--- a/mm/pgtable-generic.c~mm-change-the-contract-for-free_pgtables-update-docs
+++ a/mm/pgtable-generic.c
@@ -397,10 +397,17 @@ pte_t *pte_offset_map_rw_nolock(struct m
  * 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)
_

Patches currently in -mm which might be from ljs@kernel.org are

mm-vma-predicate-setting-mmap_prepare-vma-fields-on-new-vma-alloc.patch
mm-mremap-fix-locked_vm-leak-from-mremap_dontunmap-self-merge.patch
mm-mremap-fix-locked_vm-leak-by-splitting-vma-for-mremap_dontunmap.patch
mm-vmpressure-remove-window-size-todo.patch
tools-testing-selftests-mm-add-missing-gitignore-entries.patch
mm-move-drivers-char-memc-to-mm-char-memc.patch
mm-implement-file_is_dev_zero-to-uniquely-identify-dev-zero.patch
mm-vma-only-permit-map_private-dev-zero-to-be-mapped-anonymous.patch
mm-vma-make-map_private-mapped-dev-zero-mappings-truly-anonymous.patch
tools-testing-vma-add-test-to-assert-map_private-dev-zero-is-anon.patch
tools-testing-selftests-mm-add-map_private-dev-zero-merge-tests.patch
mm-madvise-swap-in-cowd-map_private-file-mappings-on-madv_willneed.patch
mm-khugepaged-deposit-a-newly-allocated-page-table-on-collapse.patch
mm-khugepaged-deposit-a-newly-allocated-page-table-on-collapse-fix.patch
mm-enable-mmu_gather_rcu_table_free-for-most-2-level-architectures.patch
mm-enable-mmu_gather_rcu_table_free-for-mmu-riscv.patch
mm-enable-mmu_gather_rcu_table_free-for-mmu-arm.patch
mm-enable-mmu_gather_rcu_table_free-for-arc-microblaze-xtensa.patch
mm-enable-mmu_gather_rcu_table_free-for-sparc64.patch
mm-enable-mmu_gather_rcu_table_free-for-m68k-coldfire.patch
mm-enable-mmu_gather_rcu_table_free-for-sh-x2.patch
mm-enable-mmu_gather_rcu_table_free-for-m68k-motorola.patch
mm-enable-mmu_gather_rcu_table_free-for-sparc32.patch
mm-make-userland-page-table-freeing-rcu-safe.patch
mm-change-the-contract-for-free_pgtables-update-docs.patch
mm-vma-fix-mmap_prepare-file-handling-remove-file_doesnt_need_get.patch
mm-vma-introduce-and-use-vma_can_merge.patch
mm-consistently-validate-vma-state-after-mmap-hooks.patch
mm-vma-ensure-mmap_prepare-doesnt-set-actions-on-a-mergeable-vma.patch
mm-make-map_kernel_pages_-internal-and-unexported.patch
mm-vma-tidy-up-map-kernel-pages-enum-values.patch
mm-add-mmap-action-for-discontiguous-kernel-page-mapping.patch
docs-filesystems-update-mmap_prepare-docs-for-discontig-kernel-pgs.patch
drivers-usb-mon-update-to-use-mmap_prepare-map-kernel-pages.patch
infiniband-update-hfi1-to-use-remap_vmalloc_range.patch
selinux-reject-writable-opens-of-policy-file-drop-mmap-shared-write-check.patch
alsa-pcm-use-vm_insert_page-to-map-pcm-status-page.patch
bpf-arena-mark-arena_map_mmap-mappings-vm_mixedmap.patch
mm-vma-add-vma_is_kernel_owned-predicates.patch
mm-vma-only-allow-mmap-to-clear-vma_maywrite_bit-if-kernel-owned.patch
mm-vma-add-and-use-vma__is_fixed_mapping.patch
scsi-sg-convert-mmap-hook-to-mmap_prepare-and-rework.patch
fbdev-defio-assert-fbinfo_virtfb-drop-vm_io-add-vm_mixedmap.patch
hsi-cmt_speech-convert-mmap-hook-to-mmap_prepare-refactor.patch
mm-gup-error-out-early-on-vma_mayread_bit-vmas.patch
uprobes-remove-vm_io-set-vm_mixedmap-for-mapped-kernel-pages.patch
mm-mlock-clear-vma_locked_mask-over-mmap-callback.patch
mm-mlock-eliminate-weird-vma_io_bit-abuse-and-simplify.patch
mm-vma-enforce-that-only-kernel-owned-mappings-may-set-vma_io_bit.patch
mm-remove-vma_io_bit-check-in-vma_is_kernel_owned.patch
mm-remove-hugetlb_inlineh.patch
mm-rename-is_vm_hugetlb_page-to-vma_is_hugetlb.patch
mm-drop-some-redundant-checks-around-hugetlb-vmas.patch
mm-madvise-update-is_valid_guard_vma-to-use-vma_can_merge.patch
mm-vma-introduce-vma_is_persistent.patch
mm-uffd-use-predicates-for-userfaultfd-checks.patch
mm-madvise-use-predicates-for-madvise-madv_dofork.patch
mm-eliminate-vma_special_flags-usage-when-hugetlb-explicitly-tested.patch
mm-eliminate-vma_special_flags-check-in-lru_gen_look_around.patch
mm-avoid-use-of-vma_special_flags-in-migrate_vma_setup.patch
mm-eliminate-vm_special-vma_special_flags.patch
fuse-dax-do-not-set-vm_mixedmap.patch
mm-huge_memory-remove-vma_is_special_huge.patch
mm-vma-introduce-and-use-vma_can_gup.patch
mm-vma-const-ify-vma_assert_stabilised-and-associated-functions.patch
mm-implement-and-use-vma_has_anon_rmap-silence-kcsan.patch
mm-update-comments-to-refer-to-anon-rmap-rather-than-anon_vma.patch