Re: [DISCUSSION] anon_vma root lock contention and per anon_vma lock

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

On Mon, Sep 15, 2025 at 12:29:50PM +0200, David Hildenbrand wrote:

> > > MADV_UNSHARE we implemented simply took a range and one could apply it to
> > > the full process by supplying the full range.
> > >
> > > But yeah, the downside in any case is that you lose
> >
> > You just lose? :P I assume you forgot to finish this thought :>)
>
> "you lose the memory savings of COW" -- was still tired there.

You and me both, bud... :)

And right, yes agreed.

>
> >
> > I wonder from rmap point of view whether you could actually simply check to
> > see if you're fully CoW'd.
> >
> > E.g.:
> >
> > 	madvise(..., MADV_ISOLATE_COWED)
> >
> > And have it take the anon_vma write lock from root, have it walk the rmap,
> > go and check to see if every folio in the VMA is now CoW'd, and if so,
> > detatch the CoW'd anon_vma from its parent/root?
>
> TBH, this all feels like things we should be optimizing internally somehow.
>
> And don't get me started on
>
> MADV_ISOLATE_COWED eww
>
> NO_COWS eww
>
> COW NOW eww

Holy CoW! ;)

Yeah it'd be nice for us to do this automagically. But I suspect anything like
this will be quite painful.

A kthread to do this... hmmm... Doing it on fault? Expensive. Tracking it? Also
expensive.

++lorenzos_grand_rework_goals I guess...

>
> >
> > This would be a sort of after-the-fact thing, but maybe could be done
> > periodically.
> >
> > Of course then if you had one folio that was not yet CoW'd, that'd prevent
> > this from completing.
>
> [...]
>
> > > >
> > > > Another point in favour of COW_NOW or EAGER_COW is that we can choose to
> > > > allocate folios of the appropriate size at that time.  Unless something's
> > > > changed, I think we always COW individual pages rather than multiple
> > > > pages at once.
> > >
> > > Yes. khugepaged will soon starting fixing that up later asynchronously I
> > > hope.
> >
> > But only at mTHP granularity a. once the relevant series lands and b. if
> > mTHP is enabled (I mean for sub-PMD sized/aligned ranges) :>)
>
> Well, we need khugepaged in one form or the other form mTHP in any case :P
>
> And the glorious future will have all sizes enabled as default.

I look forward to this glorious future :)

>
> --
> Cheers
>
> David / dhildenb
>
>

Cheers, Lorenzo