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

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

On Wed, Jan 14, 2026 at 02:12:04PM -0800, Andrew Morton wrote:
> On Wed, 14 Jan 2026 17:54:07 +0000 Lorenzo Stoakes <lorenzo.stoakes@oracle.com> wrote:
>
> > > >  {
> > > > @@ -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? :)
>
> Thanks, but I wouldn't have done that silently.

Yeah I didn't think you would!

>
> The later "mm/rmap: separate out fork-only logic on anon_vma_clone()" removed it:
>
> : @@ -281,17 +317,17 @@ static void cleanup_partial_anon_vmas(st
> :   *
> :   * Returns: 0 on success, -ENOMEM on failure.
> :   */
> : -int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
> : +int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src,
> : +		   enum vma_operation operation)
> :  {
> :  	struct anon_vma_chain *avc, *pavc;
> : +	struct anon_vma *active_anon_vma = src->anon_vma;
> :
> : -	check_anon_vma_clone(dst, src);
> : +	check_anon_vma_clone(dst, src, operation);
> :
> : -	if (!src->anon_vma)
> : +	if (!active_anon_vma)
> :  		return 0;
> :
> : -	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
>

Thanks yeah we figured it out, luckily it's completely benign so not a
bisection hazard so good to leave as is, but still also - doh! :)

Cheers, Lorenzo