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

Paul E. McKenney
2020-08-13T16:04:46Z

On Thu, Aug 13, 2020 at 05:54:12PM +0200, Michal Hocko wrote:
> On Thu 13-08-20 08:41:59, Paul E. McKenney wrote:
> > On Thu, Aug 13, 2020 at 04:53:35PM +0200, Michal Hocko wrote:
> > > On Thu 13-08-20 16:34:57, 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.
> > >
> > > 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.
> >
> > That is a reasonable high-level summary.
> >
> > > 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.
> >
> > Unless I am missing something subtle, the problem with this approach
> > is that in production-environment CONFIG_PREEMPT_NONE=y kernels, there
> > is no way at runtime to distinguish between holding a spinlock on the
> > one hand and holding a raw spinlock on the other. Therefore, without
> > some sort of indication from the caller, this approach will not make
> > CONFIG_PREEMPT_NONE=y users happy.
>
> If the whole bailout is guarded by CONFIG_PREEMPT_RT specific atomicity
> check then there is no functional problem - GFP_RT_SAFE would still be
> GFP_NOWAIT so functional wise the allocator will still do the right
> thing.

Perhaps it was just me getting confused, early hour Pacific Time and
whatever other excuses might apply. But I thought that you still had
an objection to GFP_RT_SAFE based on changes in allocator semantics for
other users.

Of course, if it is just me being confused, by all means let's give this
a shot!!!

> [...]
>
> > > 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.
> >
> > Exactly, and avoiding changing this semantic for current users is
> > precisely why we are proposing some sort of indication to be passed
> > into the allocation request. In Uladzislau's patch, this was the
> > __GFP_NO_LOCKS flag, but whatever works.
>
> As I've tried to explain already, I would really hope we can do without
> any new gfp flags. We are running out of them and they tend to generate
> a lot of maintenance burden. There is a lot of abuse etc. We should also
> not expose such an implementation detail of the allocator to callers
> because that would make future changes even harder. The alias, on the
> othere hand already builds on top of existing NOWAIT semantic and it
> just helps the allocator to complain about a wrong usage while it
> doesn't expose any internals.

And I am plenty happy if an existing flag or set of flags (up to and
including the all-zeroes empty set) can be used in this case.

Thanx, Paul