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

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Message-ID: <d56776f2-d7bb-4db9-9e32-7cbe7fb9c752@lucifer.local>
Date: Tue, 6 Jan 2026 14:06:54 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.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 6/8] mm/mmap_lock: add vma_is_attached() helper

On Tue, Dec 30, 2025 at 11:50:34AM -0800, Suren Baghdasaryan wrote:
> On Wed, Dec 17, 2025 at 4:27 AM Lorenzo Stoakes
> <lorenzo.stoakes@...cle.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

You're welcome! :)

> 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.

Right, yeah makes sense separately I think as this change was to allow us
to use this for an assert :)

>
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>
> Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>

Thanks!

>
> > ---
> >  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