Re: [PATCH v6 06/16] mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail

Lokesh Gidra
2024-12-17T11:31:34Z

On Mon, Dec 16, 2024 at 11:24 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> With upcoming replacement of vm_lock with vm_refcnt, we need to handle a
> possibility of vma_start_read_locked/vma_start_read_locked_nested failing
> due to refcount overflow. Prepare for such possibility by changing these
> APIs and adjusting their users.
>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Cc: Lokesh Gidra <lokeshgidra@google.com>
> ---
> include/linux/mm.h | 6 ++++--
> mm/userfaultfd.c | 17 ++++++++++++-----
> 2 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 689f5a1e2181..0ecd321c50b7 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -747,10 +747,11 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
> * not be used in such cases because it might fail due to mm_lock_seq overflow.
> * This functionality is used to obtain vma read lock and drop the mmap read lock.
> */
> -static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> +static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass)
> {
> mmap_assert_locked(vma->vm_mm);
> down_read_nested(&vma->vm_lock.lock, subclass);
> + return true;
> }
>
> /*
> @@ -759,10 +760,11 @@ static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int
> * not be used in such cases because it might fail due to mm_lock_seq overflow.
> * This functionality is used to obtain vma read lock and drop the mmap read lock.
> */
> -static inline void vma_start_read_locked(struct vm_area_struct *vma)
> +static inline bool vma_start_read_locked(struct vm_area_struct *vma)
> {
> mmap_assert_locked(vma->vm_mm);
> down_read(&vma->vm_lock.lock);
> + return true;
> }
>
> static inline void vma_end_read(struct vm_area_struct *vma)
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index bc9a66ec6a6e..79e8ae676f75 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -85,7 +85,8 @@ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm,
> mmap_read_lock(mm);
> vma = find_vma_and_prepare_anon(mm, address);
> if (!IS_ERR(vma))
> - vma_start_read_locked(vma);
> + if (!vma_start_read_locked(vma))
> + vma = ERR_PTR(-EAGAIN);
>
> mmap_read_unlock(mm);
> return vma;
> @@ -1483,10 +1484,16 @@ static int uffd_move_lock(struct mm_struct *mm,
> mmap_read_lock(mm);
> err = find_vmas_mm_locked(mm, dst_start, src_start, dst_vmap, src_vmap);
> if (!err) {
> - vma_start_read_locked(*dst_vmap);
> - if (*dst_vmap != *src_vmap)
> - vma_start_read_locked_nested(*src_vmap,
> - SINGLE_DEPTH_NESTING);
> + if (!vma_start_read_locked(*dst_vmap)) {

I think you mistakenly reversed the condition. This block should be
executed if we manage to lock dst_vma successfully.
> + if (*dst_vmap != *src_vmap) {
> + if (!vma_start_read_locked_nested(*src_vmap,
> + SINGLE_DEPTH_NESTING)) {
> + vma_end_read(*dst_vmap);
> + err = -EAGAIN;
> + }
> + }
> + } else
> + err = -EAGAIN;
> }
> mmap_read_unlock(mm);
> return err;
> --
> 2.47.1.613.gc27f4b7a9f-goog
>