Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

From: Michal Hocko
Date: Thu Aug 13 2020 - 10:53:40 EST

On Thu 13-08-20 16:34:57, Thomas Gleixner wrote:
> Michal Hocko <mhocko@xxxxxxxx> writes:
> > On Thu 13-08-20 15:22:00, Thomas Gleixner wrote:
> >> It basically requires to convert the wait queue to something else. Is
> >> the waitqueue strict single waiter?
> >
> > I would have to double check. From what I remember only kswapd should
> > ever sleep on it.
> That would make it trivial as we could simply switch it over to rcu_wait.
> >> So that should be:
> >>
> >> if (!preemptible() && gfp == GFP_RT_NOWAIT)
> >>
> >> which is limiting the damage to those callers which hand in
> >>
> >> lockdep will yell at invocations with gfp != GFP_RT_NOWAIT when it hits
> >> zone->lock in the wrong context. And we want to know about that so we
> >> can look at the caller and figure out how to solve it.
> >
> > Yes, that would have to somehow need to annotate the zone_lock to be ok
> > in those paths so that lockdep doesn't complain.
> That opens the worst of all cans of worms. If we start this here then
> Joe programmer and his dog will use these lockdep annotation to evade
> warnings and when exposed to RT it will fall apart in pieces. Just that
> at that point Joe programmer moved on to something else and the usual
> suspects can mop up the pieces. We've seen that all over the place and
> some people even disable lockdep temporarily because annotations don't
> help.

Hmm. I am likely missing something really important here. We have two
problems at hand:
1) RT will become broken as soon as this new RCU functionality which
requires an allocation from inside of raw_spinlock hits the RT tree
2) lockdep splats which are telling us that early because of the
raw_spinlock-> spin_lock dependency.

1) can be handled by handled by the bailing out whenever we have to use
zone->lock inside the buddy allocator - essentially even more strict
NOWAIT semantic than we have for RT tree - proposed (pseudo) patch is
trying to describe that.

2) would become a false positive if 1) is in place, right? RT wouldn't
do the illegal nesting and !RT would just work fine because
Why should we limit the functionality of the allocator for something
that is not a real problem?

> PeterZ might have opinions about that too I suspect.
> Really, if your primary lockless caches are empty then any allocation
> which comes from deep atomic context should simply always fail. Being
> stuck in an interrupt handler or even deeper for 200+ microseconds
> waiting for zone lock is just bonkers IMO.

That would require changing NOWAIT/ATOMIC allocations semantic quite
drastically for !RT kernels as well. I am not sure this is something we
can do. Or maybe I am just missing your point.
Michal Hocko