Re: [PATCH 6/8] mm/mmap_lock: add vma_is_attached() helper

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Dec 17, 2025 at 4:27 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> This makes it easy to explicitly check for VMA detachment, which is useful
> for things like asserts.
>
> Note that we intentionally do not allow this function to be available
> should CONFIG_PER_VMA_LOCK be set - this is because vma_assert_attached()
> and vma_assert_detached() are no-ops if !CONFIG_PER_VMA_LOCK, so there is
> no correct state for vma_is_attached() to be in if this configuration
> option is not specified.
>
> Therefore users elsewhere must invoke this function only after checking for
> CONFIG_PER_VMA_LOCK.
>
> We rework the assert functions to utilise this.

Thank you! This nicely documents vm_refcnt attached state. Another
step in this direction is adding:

static inline bool vma_is_read_locked(struct vm_area_struct *vma)
{
        return refcount_read(&vma->vm_refcnt) > 1;
}

and changing vma_assert_locked() to use it.
But I can do that in a separate patch, so LGTM.

>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

> ---
>  include/linux/mmap_lock.h | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index d53f72dba7fe..b50416fbba20 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -251,6 +251,11 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
>                       !__is_vma_write_locked(vma, &mm_lock_seq), vma);
>  }
>
> +static inline bool vma_is_attached(struct vm_area_struct *vma)
> +{
> +       return refcount_read(&vma->vm_refcnt);
> +}
> +
>  /*
>   * WARNING: to avoid racing with vma_mark_attached()/vma_mark_detached(), these
>   * assertions should be made either under mmap_write_lock or when the object
> @@ -258,12 +263,12 @@ static inline void vma_assert_locked(struct vm_area_struct *vma)
>   */
>  static inline void vma_assert_attached(struct vm_area_struct *vma)
>  {
> -       WARN_ON_ONCE(!refcount_read(&vma->vm_refcnt));
> +       WARN_ON_ONCE(!vma_is_attached(vma));
>  }
>
>  static inline void vma_assert_detached(struct vm_area_struct *vma)
>  {
> -       WARN_ON_ONCE(refcount_read(&vma->vm_refcnt));
> +       WARN_ON_ONCE(vma_is_attached(vma));
>  }
>
>  static inline void vma_mark_attached(struct vm_area_struct *vma)
> --
> 2.52.0