linux-kernel - Re: [PATCH v2 6/8] mm/mmap_lock: add vma_is

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Message-ID: <aps74d2c2jkyrwstifkaf2etqjdvzskm7sn3a3kvy6mdp4egfn@q54mbprw4o23>
Date: Tue, 6 Jan 2026 13:56:07 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeel.butt@...ux.dev>,
        David Hildenbrand <david@...nel.org>, Rik van Riel <riel@...riel.com>,
        Harry Yoo <harry.yoo@...cle.com>, Jann Horn <jannh@...gle.com>,
        Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Pedro Falcato <pfalcato@...e.de>, Chris Li <chriscli@...gle.com>,
        Barry Song <v-songbaohua@...o.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 6/8] mm/mmap_lock: add vma_is_attached() helper

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [260106 10:04]:
> 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.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.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
>