Re: [PATCH RESEND v3 09/10] mm/vma: update vma_assert_locked() to use lockdep

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jan 22, 2026 at 5:02 AM Lorenzo Stoakes
<lorenzo.stoakes@oracle.com> wrote:
>
> We can use lockdep to avoid unnecessary work here, otherwise update the
> code to logically evaluate all pertinent cases and share code with
> vma_assert_write_locked().
>
> Make it clear here that we treat the VMA being detached at this point as a
> bug, this was only implicit before.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

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

> ---
>  include/linux/mmap_lock.h | 42 ++++++++++++++++++++++++++++++++++++---
>  1 file changed, 39 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index b00d34b5ad10..92ea07f0da4e 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -319,6 +319,10 @@ int vma_start_write_killable(struct vm_area_struct *vma)
>         return __vma_start_write(vma, mm_lock_seq, TASK_KILLABLE);
>  }
>
> +/**
> + * vma_assert_write_locked() - assert that @vma holds a VMA write lock.
> + * @vma: The VMA to assert.
> + */
>  static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>  {
>         unsigned int mm_lock_seq;
> @@ -326,16 +330,48 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma)
>         VM_BUG_ON_VMA(!__is_vma_write_locked(vma, &mm_lock_seq), vma);
>  }
>
> +/**
> + * vma_assert_locked() - assert that @vma holds either a VMA read or a VMA write
> + * lock and is not detached.
> + * @vma: The VMA to assert.
> + */
>  static inline void vma_assert_locked(struct vm_area_struct *vma)
>  {
> -       unsigned int mm_lock_seq;
> +       unsigned int refs;
>
>         /*
>          * See the comment describing the vm_area_struct->vm_refcnt field for
>          * details of possible refcnt values.
>          */
> -       VM_BUG_ON_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
> -                     !__is_vma_write_locked(vma, &mm_lock_seq), vma);
> +
> +       /*
> +        * If read-locked or currently excluding readers, then the VMA is
> +        * locked.
> +        */
> +#ifdef CONFIG_LOCKDEP
> +       if (lock_is_held(&vma->vmlock_dep_map))
> +               return;
> +#endif
> +
> +       refs = refcount_read(&vma->vm_refcnt);
> +
> +       /*
> +        * In this case we're either read-locked, write-locked with temporary
> +        * readers, or in the midst of excluding readers, all of which means
> +        * we're locked.
> +        */
> +       if (refs > 1)
> +               return;
> +
> +       /* It is a bug for the VMA to be detached here. */
> +       VM_BUG_ON_VMA(!refs, vma);
> +
> +       /*
> +        * OK, the VMA has a reference count of 1 which means it is either
> +        * unlocked and attached or write-locked, so assert that it is
> +        * write-locked.
> +        */
> +       vma_assert_write_locked(vma);
>  }
>
>  static inline bool vma_is_attached(struct vm_area_struct *vma)
> --
> 2.52.0