Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Dec 19, 2024 at 10:55 AM Liam R. Howlett
<Liam.Howlett@oracle.com> wrote:
>
> * Peter Zijlstra <peterz@infradead.org> [241219 13:47]:
> > On Thu, Dec 19, 2024 at 01:18:23PM -0500, Liam R. Howlett wrote:
> >
> > > > For RCU lookups only the mas tree matters -- and its left present there.
> > > >
> > > > If you really want to block RCU readers, I would suggest punching a hole
> > > > in the mm_mt. All the traditional code won't notice anyway, this is all
> > > > with mmap_lock held for writing.
> > >
> > > We don't want to block all rcu readers, we want to block the rcu readers
> > > that would see the problem - that is, anyone trying to read a particular
> > > area.
> > >
> > > Right now we can page fault in unpopulated vmas while writing other vmas
> > > to the tree.  We are also moving more users to rcu reading to use the
> > > vmas they need without waiting on writes to finish.
> > >
> > > Maybe I don't understand your suggestion, but I would think punching a
> > > hole would lose this advantage?
> >
> > My suggestion was to remove the range stuck in mas_detach from mm_mt.
> > That is exactly the affected range, no?
>
> Yes.
>
> But then looping over the vmas will show a gap where there should not be
> a gap.
>
> If we stop rcu readers entirely we lose the advantage.
>
> This is exactly the issue that the locking dance was working around :)

Sorry for not participating in the discussion, folks. I'm down with a
terrible flu and my brain is not working well. I'll catch up once I
get better.

>