New subject: [PATCH v2 1/6] mm: enable page walking API to lock vmas during the walk

1 Aug 2023


      During recent vma locking patch reviews Linus and Jann Horn noted a number
of issues with vma locking and suggested improvements:
1. walk_page_range() does not have ability to write-lock a vma during the
walk when it's done under mmap_write_lock. For example s390_reset_cmma().
2. Vma locking is hidden inside vm_flags modifiers and is hard to follow.
Suggestion is to change vm_flags_reset{_once} to assert that vma is
write-locked and require an explicit locking.
3. Same issue with vma_prepare() hiding vma locking.
4. In userfaultfd vm_flags are modified after vma->vm_userfaultfd_ctx and
page faults can operate on a context while it's changed.
5. do_brk_flags() and __install_special_mapping() not locking a newly
created vma before adding it into the mm. While not strictly a problem,
this is fragile if vma is modified after insertion, as in the
mmap_region() case which was recently fixed. Suggestion is to always lock
a new vma before inserting it and making it visible to page faults.
6. vma_assert_write_locked() for CONFIG_PER_VMA_LOCK=n would benefit from
being mmap_assert_write_locked() instead of no-op and then any place which
operates on a vma and calls mmap_assert_write_locked() can be converted
into vma_assert_write_locked().
I CC'ed stable only on the first patch because others are cleanups and the
bug in userfaultfd does not affect stable (lock_vma_under_rcu prevents
uffds from being handled under vma lock protection). However I would be
happy if the whole series is merged into stable 6.4 since it makes vma
locking more maintainable.
The patches apply cleanly over Linus' ToT and will conflict when applied
over mm-unstable due to missing [1]. The conflict can be easily resolved
by ignoring conflicting deletions but probably simpler to take [1] into
mm-unstable and avoid later conflict.
[1] commit 6c21e066f925 ("mm/mempolicy: Take VMA lock before replacing policy")
Changes since v1:
- replace walk_page_range() parameter with mm_walk_ops.walk_lock,
per Linus
- introduced page_walk_lock enum to allow different locking modes
during a walk, per Linus
- added Liam's Reviewed-by
Suren Baghdasaryan (6):
  mm: enable page walking API to lock vmas during the walk
  mm: for !CONFIG_PER_VMA_LOCK equate write lock assertion for vma and
    mmap
  mm: replace mmap with vma write lock assertions when operating on a
    vma
  mm: lock vma explicitly before doing vm_flags_reset and
    vm_flags_reset_once
  mm: always lock new vma before inserting into vma tree
  mm: move vma locking out of vma_prepare
arch/powerpc/kvm/book3s_hv_uvmem.c      |  1 +
 arch/powerpc/mm/book3s64/subpage_prot.c |  1 +
 arch/riscv/mm/pageattr.c                |  1 +
 arch/s390/mm/gmap.c                     |  5 ++++
 drivers/infiniband/hw/hfi1/file_ops.c   |  1 +
 fs/proc/task_mmu.c                      |  5 ++++
 fs/userfaultfd.c                        |  6 +++++
 include/linux/mm.h                      | 13 ++++++---
 include/linux/pagewalk.h                | 11 ++++++++
 mm/damon/vaddr.c                        |  2 ++
 mm/hmm.c                                |  1 +
 mm/hugetlb.c                            |  2 +-
 mm/khugepaged.c                         |  5 ++--
 mm/ksm.c                                | 25 ++++++++++-------
 mm/madvise.c                            |  8 +++---
 mm/memcontrol.c                         |  2 ++
 mm/memory-failure.c                     |  1 +
 mm/memory.c                             |  2 +-
 mm/mempolicy.c                          | 22 +++++++++------
 mm/migrate_device.c                     |  1 +
 mm/mincore.c                            |  1 +
 mm/mlock.c                              |  4 ++-
 mm/mmap.c                               | 29 +++++++++++++-------
 mm/mprotect.c                           |  2 ++
 mm/pagewalk.c                           | 36 ++++++++++++++++++++++---
 mm/vmscan.c                             |  1 +
 26 files changed, 146 insertions(+), 42 deletions(-)
-- 
2.41.0.585.gd2178a4bd4-goog

[PATCH v2 0/6] make vma locking more obvious