Re: [PATCH v4 09/10] mm/vma: update vma_assert_locked() to use lockdep

[Vlastimil Babka <vbabka@suse.cz>]

On 1/23/26 21:12, Lorenzo Stoakes 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.
> 
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

Nit:

> ---
>  include/linux/mmap_lock.h | 41 +++++++++++++++++++++++++++++++++++++--
>  1 file changed, 39 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
> index 23bde4bd5a85..4a0aafc66c5d 100644
> --- a/include/linux/mmap_lock.h
> +++ b/include/linux/mmap_lock.h
> @@ -322,19 +322,56 @@ int vma_start_write_killable(struct vm_area_struct *vma)
>  	return __vma_start_write(vma, __vma_raw_mm_seqnum(vma), 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)
>  {
>  	VM_WARN_ON_ONCE_VMA(!__is_vma_write_locked(vma), 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 refcnt;
> +
> +	/*
> +	 * If read-locked or currently excluding readers, then the VMA is
> +	 * locked.
> +	 */
> +#ifdef CONFIG_LOCKDEP
> +	if (lock_is_held(&vma->vmlock_dep_map))
> +		return;

Wouldn't this work a tiny bit better?

	if (!lock_is_held(&vma->vmlock_dep_map))
		vma_assert_write_locked(vma);
	return;

> +#endif
> +
>  	/*
>  	 * See the comment describing the vm_area_struct->vm_refcnt field for
>  	 * details of possible refcnt values.
>  	 */
> -	VM_WARN_ON_ONCE_VMA(refcount_read(&vma->vm_refcnt) <= 1 &&
> -			    !__is_vma_write_locked(vma), vma);
> +	refcnt = 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 (refcnt > 1)
> +		return;
> +
> +	/* It is a bug for the VMA to be detached here. */
> +	VM_WARN_ON_ONCE_VMA(!refcnt, 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)