Re: [PATCH v2 3/8] mm/rmap: remove unnecessary root lock dance in anon_vma clone, unmap

[Lorenzo Stoakes]

On Wed, Jan 14, 2026 at 08:55:58AM -0800, Suren Baghdasaryan wrote:
> On Tue, Jan 6, 2026 at 7:04 AM Lorenzo Stoakes
> <lorenzo.stoakes@xxxxxxxxxx> wrote:
> >
> > The root anon_vma of all anon_vma's linked to a VMA must by definition be
> > the same - a VMA and all of its descendants/ancestors must exist in the
> > same CoW chain.
> >
> > Commit bb4aa39676f7 ("mm: avoid repeated anon_vma lock/unlock sequences in
> > anon_vma_clone()") introduced paranoid checking of the root anon_vma
> > remaining the same throughout all AVC's in 2011.
> >
> > I think 15 years later we can safely assume that this is always the case.
> >
> > Additionally, since unfaulted VMAs being cloned from or unlinked are
> > no-op's, we can simply lock the anon_vma's associated with this rather than
> > doing any specific dance around this.
> >
> > This removes unnecessary checks and makes it clear that the root anon_vma
> > is shared between all anon_vma's in a given VMA's anon_vma_chain.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> > ---
> >  mm/rmap.c | 51 +++++++++++++++------------------------------------
> >  1 file changed, 15 insertions(+), 36 deletions(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index de2cbe860566..6ac42671bedd 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -232,32 +232,6 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
> >         return -ENOMEM;
> >  }
> >
> > -/*
> > - * This is a useful helper function for locking the anon_vma root as
> > - * we traverse the vma->anon_vma_chain, looping over anon_vma's that
> > - * have the same vma.
> > - *
> > - * Such anon_vma's should have the same root, so you'd expect to see
> > - * just a single mutex_lock for the whole traversal.
> > - */
> > -static inline struct anon_vma *lock_anon_vma_root(struct anon_vma *root, struct anon_vma *anon_vma)
> > -{
> > -       struct anon_vma *new_root = anon_vma->root;
> > -       if (new_root != root) {
> > -               if (WARN_ON_ONCE(root))
> > -                       up_write(&root->rwsem);
> > -               root = new_root;
> > -               down_write(&root->rwsem);
> > -       }
> > -       return root;
> > -}
> > -
> > -static inline void unlock_anon_vma_root(struct anon_vma *root)
> > -{
> > -       if (root)
> > -               up_write(&root->rwsem);
> > -}
> > -
> >  static void check_anon_vma_clone(struct vm_area_struct *dst,
> >                                  struct vm_area_struct *src)
> >  {
> > @@ -310,26 +284,28 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma);
> >  int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> >  {
> >         struct anon_vma_chain *avc, *pavc;
> > -       struct anon_vma *root = NULL;
> >
> >         check_anon_vma_clone(dst, src);
> >
> >         if (!src->anon_vma)
> >                 return 0;
> >
> > +       check_anon_vma_clone(dst, src);
>
> check_anon_vma_clone() was already called 4 lines above. I'm guessing
> this was accidentally carried over from the previous version?
>

Yeah I don't know why this is here, in the tree it isn't, maybe Andrew noticed
and elided? :)

But yeah the check above is correct so current in-tree state is correct.

Thanks, Lorenzo