Re: [PATCH 7/8] mm/rmap: allocate anon_vma_chain objects unlocked when possible

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

On Tue, Dec 30, 2025 at 01:35:41PM -0800, Suren Baghdasaryan wrote:
> On Wed, Dec 17, 2025 at 4:27 AM Lorenzo Stoakes
> <lorenzo.stoakes@oracle.com> wrote:
> >
> > There is no reason to allocate the anon_vma_chain under the anon_vma write
> > lock when cloning - we can in fact assign these to the destination VMA
> > safely as we hold the exclusive mmap lock and therefore preclude anybody
> > else accessing these fields.
> >
> > We only need take the anon_vma write lock when we link rbtree edges from
> > the anon_vma to the newly established AVCs.
> >
> > This also allows us to eliminate the weird GFP_NOWAIT, GFP_KERNEL dance
> > introduced in commit dd34739c03f2 ("mm: avoid anon_vma_chain allocation
> > under anon_vma lock"), further simplifying this logic.
> >
> > This should reduce lock anon_vma contention, and clarifies exactly where
> > the anon_vma lock is required.
> >
> > We cannot adjust __anon_vma_prepare() in the same way as this is only
> > protected by VMA read lock, so we have to perform the allocation here under
> > the anon_vma write lock and page_table_lock (to protect against racing
> > threads), and we wish to retain the lock ordering.
> >
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
>
> One nit but otherwise nice cleanup.
>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks!

One nice thing with the separate cleanup_partial_anon_vmas()'s function
introduced as part of this review (thanks for the good spot!) is we can now
simplify this _even further_ since we don't even insert anything into the
interval tree at the point of allocation, and so freeing is just a case of
freeing up AVC's.

>
> > ---
> >  mm/rmap.c | 49 +++++++++++++++++++++++++++++--------------------
> >  1 file changed, 29 insertions(+), 20 deletions(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 60134a566073..de9de6d71c23 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -146,14 +146,13 @@ static void anon_vma_chain_free(struct anon_vma_chain *anon_vma_chain)
> >         kmem_cache_free(anon_vma_chain_cachep, anon_vma_chain);
> >  }
> >
> > -static void anon_vma_chain_link(struct vm_area_struct *vma,
> > -                               struct anon_vma_chain *avc,
> > -                               struct anon_vma *anon_vma)
> > +static void anon_vma_chain_assign(struct vm_area_struct *vma,
> > +                                 struct anon_vma_chain *avc,
> > +                                 struct anon_vma *anon_vma)
> >  {
> >         avc->vma = vma;
> >         avc->anon_vma = anon_vma;
> >         list_add(&avc->same_vma, &vma->anon_vma_chain);
> > -       anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> >  }
> >
> >  /**
> > @@ -210,7 +209,8 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
> >         spin_lock(&mm->page_table_lock);
> >         if (likely(!vma->anon_vma)) {
> >                 vma->anon_vma = anon_vma;
> > -               anon_vma_chain_link(vma, avc, anon_vma);
> > +               anon_vma_chain_assign(vma, avc, anon_vma);
> > +               anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> >                 anon_vma->num_active_vmas++;
> >                 allocated = NULL;
> >                 avc = NULL;
> > @@ -287,20 +287,28 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> >
> >         check_anon_vma_clone(dst, src);
> >
> > +       /*
> > +        * Allocate AVCs. We don't need an anon_vma lock for this as we
> > +        * are not updating the anon_vma rbtree nor are we changing
> > +        * anon_vma statistics.
> > +        *
> > +        * We hold the mmap write lock so there's no possibliity of
>
> To be more specific, we are holding src's mmap write lock. I think
> clarifying that will avoid any confusion.

Well, it's the same mm for both right? :) and actually the observations
would be made around dst no? As that's where the unlinked AVC's are being
established.

I think more clear is 'We hold the exclusive mmap write lock' just to
highlight that it excludes anybody else from accessing these fields in the
VMA.

>
> > +        * the unlinked AVC's being observed yet.
> > +        */
> > +       list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
> > +               avc = anon_vma_chain_alloc(GFP_KERNEL);
> > +               if (!avc)
> > +                       goto enomem_failure;
> > +
> > +               anon_vma_chain_assign(dst, avc, pavc->anon_vma);
> > +       }
> > +
> > +       /* Now link the anon_vma's back to the newly inserted AVCs. */
> >         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)) {
> > -                       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);
> > -               }
> > -               anon_vma = pavc->anon_vma;
> > -               anon_vma_chain_link(dst, avc, anon_vma);
> > +       list_for_each_entry_reverse(avc, &dst->anon_vma_chain, same_vma) {
> > +               struct anon_vma *anon_vma = avc->anon_vma;
> > +
> > +               anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> >
> >                 /*
> >                  * Reuse existing anon_vma if it has no vma and only one
> > @@ -316,7 +324,6 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> >         }
> >         if (dst->anon_vma)
> >                 dst->anon_vma->num_active_vmas++;
> > -
> >         anon_vma_unlock_write(src->anon_vma);
> >         return 0;
> >
> > @@ -385,8 +392,10 @@ int anon_vma_fork(struct vm_area_struct *vma, struct vm_area_struct *pvma)
> >         get_anon_vma(anon_vma->root);
> >         /* Mark this anon_vma as the one where our new (COWed) pages go. */
> >         vma->anon_vma = anon_vma;
> > +       anon_vma_chain_assign(vma, avc, anon_vma);
> > +       /* Now let rmap see it. */
> >         anon_vma_lock_write(anon_vma);
> > -       anon_vma_chain_link(vma, avc, anon_vma);
> > +       anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
> >         anon_vma->parent->num_children++;
> >         anon_vma_unlock_write(anon_vma);
> >
> > --
> > 2.52.0
> >