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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Aug 1, 2023 at 3:07 PM Suren Baghdasaryan <surenb@google.com> wrote:
>
> 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

v3 is posted at
https://lore.kernel.org/all/20230803172652.2849981-1-surenb@google.com/

>
> 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
>