Re: [PATCH v2 1/8] mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts

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

On Wed, Jan 14, 2026 at 01:59:11PM -0800, Andrew Morton wrote:
> On Wed, 14 Jan 2026 19:02:20 +0000 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
>
> > Can you apply the below fix-patch to this to fix up a rather silly
> > failure-to-unlock mistake that Suren picked up on?
> >
> > Luckily this partial unmap function is unlikely to ever be triggerable in real
> > life, AND more to the point - a later patch completely eliminates the locking -
> > but to avoid bisection hazard let's fix this.
> >
> > Note that there is a conflict at 'mm/rmap: allocate anon_vma_chain objects
> > unlocked when possible', please resolve it by just taking that patch and
> > dropping _everything_ from this one _including_ the trailing 'if (root) ...'
> > code.
>
> No probs.
> mm-rmap-allocate-anon_vma_chain-objects-unlocked-when-possible.patch is
> now

LGTM, appreciate it thanks! :)

Cheers, Lorenzo

>
> --- a/mm/rmap.c~mm-rmap-allocate-anon_vma_chain-objects-unlocked-when-possible
> +++ a/mm/rmap.c
> @@ -147,14 +147,13 @@ static void anon_vma_chain_free(struct a
>  	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);
>  }
>
>  /**
> @@ -211,7 +210,8 @@ int __anon_vma_prepare(struct vm_area_st
>  	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;
> @@ -292,21 +292,31 @@ int anon_vma_clone(struct vm_area_struct
>
>  	check_anon_vma_clone(dst, src);
>
> -	/* All anon_vma's share the same root. */
> +	/*
> +	 * 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 exclusive mmap write lock so there's no possibliity of
> +	 * 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.
> +	 * Note that all anon_vma's share the same root.
> +	 */
>  	anon_vma_lock_write(src->anon_vma);
> -	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
> -		struct anon_vma *anon_vma;
> +	list_for_each_entry_reverse(avc, &dst->anon_vma_chain, same_vma) {
> +		struct anon_vma *anon_vma = avc->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);
> +		anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
>
>  		/*
>  		 * Reuse existing anon_vma if it has no vma and only one
> @@ -322,7 +332,6 @@ int anon_vma_clone(struct vm_area_struct
>  	}
>  	if (dst->anon_vma)
>  		dst->anon_vma->num_active_vmas++;
> -
>  	anon_vma_unlock_write(src->anon_vma);
>  	return 0;
>
> @@ -384,8 +393,10 @@ int anon_vma_fork(struct vm_area_struct
>  	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);
>
> @@ -402,40 +413,18 @@ int anon_vma_fork(struct vm_area_struct
>   * In the unfortunate case of anon_vma_clone() failing to allocate memory we
>   * have to clean things up.
>   *
> - * On clone we hold the exclusive mmap write lock, so we can't race
> - * unlink_anon_vmas(). Since we're cloning, we know we can't have empty
> - * anon_vma's, since existing anon_vma's are what we're cloning from.
> - *
> - * So this function needs only traverse the anon_vma_chain and free each
> - * allocated anon_vma_chain.
> + * Since we allocate anon_vma_chain's before we insert them into the interval
> + * trees, we simply have to free up the AVC's and remove the entries from the
> + * VMA's anon_vma_chain.
>   */
>  static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
>  {
>  	struct anon_vma_chain *avc, *next;
> -	struct anon_vma *root = NULL;
> -
> -	/*
> -	 * We exclude everybody else from being able to modify anon_vma's
> -	 * underneath us.
> -	 */
> -	mmap_assert_locked(vma->vm_mm);
>
>  	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
> -		struct anon_vma *anon_vma = avc->anon_vma;
> -
> -		/* All anon_vma's share the same root. */
> -		if (!root) {
> -			root = anon_vma->root;
> -			anon_vma_lock_write(root);
> -		}
> -
> -		anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
>  		list_del(&avc->same_vma);
>  		anon_vma_chain_free(avc);
>  	}
> -
> -	if (root)
> -		anon_vma_unlock_write(root);
>  }
>
>  /**
> _
>