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

[peterz@infradead.org]

On Thu, Aug 13, 2020 at 04:34:57PM +0200, Thomas Gleixner wrote:
> Michal Hocko <mhocko@suse.com> 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
> >> GFP_RT_NOWAIT.
> >> 
> >> 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.
> 
> PeterZ might have opinions about that too I suspect.

PeterZ is mightily confused by all of this -- also heat induced brain
melt.

I thought the rule was:

 - No allocators (alloc/free) inside raw_spinlock_t, full-stop.

Why are we trying to craft an exception?