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

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

On Mon, Dec 29, 2025 at 02:17:53PM -0800, Suren Baghdasaryan wrote:
> On Wed, Dec 17, 2025 at 4:27 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> 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@oracle.com>
> > ---
> >  mm/rmap.c | 48 ++++++++++++------------------------------------
> >  1 file changed, 12 insertions(+), 36 deletions(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 9332d1cbc643..60134a566073 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -231,32 +231,6 @@ 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)
> >  {
> > @@ -307,26 +281,25 @@ static void check_anon_vma_clone(struct vm_area_struct *dst,
> >  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;
> >
> >         if (!src->anon_vma)
> >                 return 0;
> >
> >         check_anon_vma_clone(dst, src);
> >
> > +       anon_vma_lock_write(src->anon_vma);
> >         list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
> >                 struct anon_vma *anon_vma;
> >
> >                 avc = anon_vma_chain_alloc(GFP_NOWAIT);
> >                 if (unlikely(!avc)) {
> > -                       unlock_anon_vma_root(root);
> > -                       root = NULL;
> > +                       anon_vma_unlock_write(src->anon_vma);
> >                         avc = anon_vma_chain_alloc(GFP_KERNEL);
> >                         if (!avc)
> >                                 goto enomem_failure;
> > +                       anon_vma_lock_write(src->anon_vma);
>
> So, we drop and then reacquire src->anon_vma->root->rwsem, expecting
> src->anon_vma and src->anon_vma->root to be the same. And IIUC

I mean did you read the commit message? :)

We're not expecting that, they _have_ to be the same. It simply makes no sense
for them _not_ to be the same.

This is kind of the entire point of the patch.

> src->vm_mm's mmap lock is what guarantees all this. If so, could you
> please add a clarifying comment here?

No that's not what guarantees it? I don't understand what you mean?

I mean in a sense, if you had a totally broken situation where you didn't take
exclusive locks and could do some horribly broken racing here, then sure you
might end up with something broken, but I think it's super confusing to say 'oh
this lock guarantees it', well no it guarantees that you aren't completely
broken, what guarantees the shared root is how anon_vma_fork() works, which is
to:

- Clone.
- If not reused an anon_vma (which by recursion would also have same root)
  allocate new anon_vma.
- If allocated new, set root to source VMA's anon_vma, which by definition also
  has to be in its anon_vma_chain and have the same root (itself, if we're
  cloning from the ultimate parent).

But I don't think it'd be helpful to document all this, or we get into _adding_
confusion by putting _too much_ in a comment.

So I guess I'll just say,a s I do in the newly introduced
clenaup_partial_anon_vmas():

	/* All anon_vma's share the same root. */

>
> >                 }
> >                 anon_vma = pavc->anon_vma;
> > -               root = lock_anon_vma_root(root, anon_vma);
> >                 anon_vma_chain_link(dst, avc, anon_vma);
> >
> >                 /*
> > @@ -343,7 +316,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> >         }
> >         if (dst->anon_vma)
> >                 dst->anon_vma->num_active_vmas++;
> > -       unlock_anon_vma_root(root);
> > +
> > +       anon_vma_unlock_write(src->anon_vma);
> >         return 0;
> >
> >   enomem_failure:
> > @@ -438,15 +412,17 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> >  void unlink_anon_vmas(struct vm_area_struct *vma)
> >  {
> >         struct anon_vma_chain *avc, *next;
> > -       struct anon_vma *root = NULL;
> > +       struct anon_vma *active_anon_vma = vma->anon_vma;
> >
> >         /* Always hold mmap lock, read-lock on unmap possibly. */
> >         mmap_assert_locked(vma->vm_mm);
> >
> >         /* Unfaulted is a no-op. */
> > -       if (!vma->anon_vma)
> > +       if (!active_anon_vma)
> >                 return;
> >
> > +       anon_vma_lock_write(active_anon_vma);
> > +
> >         /*
> >          * Unlink each anon_vma chained to the VMA.  This list is ordered
> >          * from newest to oldest, ensuring the root anon_vma gets freed last.
> > @@ -454,7 +430,6 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> >         list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> >                 struct anon_vma *anon_vma = avc->anon_vma;
> >
> > -               root = lock_anon_vma_root(root, anon_vma);
> >                 anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
> >
> >                 /*
> > @@ -470,13 +445,14 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
> >                 anon_vma_chain_free(avc);
> >         }
> >
> > -       vma->anon_vma->num_active_vmas--;
> > +       active_anon_vma->num_active_vmas--;
> >         /*
> >          * vma would still be needed after unlink, and anon_vma will be prepared
> >          * when handle fault.
> >          */
> >         vma->anon_vma = NULL;
> > -       unlock_anon_vma_root(root);
> > +       anon_vma_unlock_write(active_anon_vma);
> > +
> >
> >         /*
> >          * Iterate the list once more, it now only contains empty and unlinked
> > --
> > 2.52.0
> >