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

[Michal Hocko]

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@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
> > > >> 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.

[...]

> > 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.

-- 
Michal Hocko
SUSE Labs