Re: [PATCH] mm/mmap_lock: Reset maple state on lock_vma_under_rcu() retry

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Nov 21, 2025 at 10:08:19AM +0100, Vlastimil Babka wrote:
> (sorry for the late reply)

And me for missing this!

> On 11/13/25 12:05, Lorenzo Stoakes wrote:
> >> > > I think one source of confusion for me with maple tree operations is - what
> >> > > to do if we are in a position where some kind of reset is needed?
> >> > >
> >> > > So even if I'd realised 'aha we need to reset this' it wouldn't be obvious
> >> > > to me that we ought to set to the address.
> >> >
> >> > I think that's a separate problem.
> >> >
> >> > > > +++ b/mm/mmap_lock.c
> >> > > > @@ -257,6 +257,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm,
> >> > > >  		if (PTR_ERR(vma) == -EAGAIN) {
> >> > > >  			count_vm_vma_lock_event(VMA_LOCK_MISS);
> >> > > >  			/* The area was replaced with another one */
> >> > > > +			mas_set(&mas, address);
> >> > >
> >> > > I wonder if we could detect that the RCU lock was released (+ reacquired) in
> >> > > mas_walk() in a debug mode, like CONFIG_VM_DEBUG_MAPLE_TREE?
> >> >
> >> > Dropping and reacquiring the RCU read lock should have been a big red
> >> > flag.  I didn't have time to review the patches, but if I had, I would
> >> > have suggested passing the mas down to the routine that drops the rcu
> >> > read lock so it can be invalidated before dropping the readlock.
> >>
> >> There has been some academic efforts to check for RCU-protected pointers
> >> leaking from one RCU read-side critical section to another, but nothing
> >> useful has come from this.  :-/
> > 
> > Ugh a pity. I was hoping we could do (in debug mode only obv) something
> > absolutely roughly like:
> > 
> > On init:
> > 
> > mas->rcu_critical_section = rcu_get_critical_section_blah();
> 
> AFAICT, get_state_synchronize_rcu()?

This would get a grace-period counter, and I believe that Lorenzo wants
a per-task count of RCU read-side critical sections.  In theory, this
is easy, but it does add overhead.  And in practice, there are a lot
of different types of RCU readers, and keeping them all straight would
be quite challenging.  But if you only care about a debug-only facility
for preemptible RCU's rcu_read_lock() and rcu_read_unlock(), something
could be done.

Besides, I didn't understand what Vlastimil was getting at...

> > ...
> > 
> > On walk:
> > 
> > 	VM_WARN_ON(rcu_critical_section_blah() != mas->rcu_critical_section);
> 
> And here, poll_state_synchronize_rcu()?
> 
> It wouldn't detect directly that we dropped and reacquired the rcu read
> lock, but with enough testing, that would at some point translate to a new
> grace period between the first and second read lock, and we'd catch it then?

Ah, good point.  And CONFIG_RCU_STRICT_GRACE_PERIOD=y would make it more
likely to happen.  As would booting with rcupdate.rcu_expedited=1.

> > But sounds like that isn't feasible.
> I don't think what Paul says means your suggestion is not feasible. I think
> he says there are no known ways to do this checking automagicallt. But your
> suggestion is doing it manually for a specific case. I guess it depends on
> how many maple tree functions we'd have to change and how ugly it would be.

All good points!

							Thanx, Paul

> > I always like the idea of us having debug stuff that helps highlight dumb
> > mistakes very quickly, no matter how silly they might be :)
> > 
> >>
> >> But rcu_pointer_handoff() and unrcu_pointer() are intended not only for
> >> documentation, but also to suppress the inevitable false positives should
> >> anyone figure out how to detect leaking of RCU-protected pointers.
> >>
> >> 							Thanx, Paul
> > 
> > Cheers, Lorenzo
>