Re: [PATCH 4/4] mm, notifier: Catch sleeping/blocking for !blockable

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Aug 20, 2019 at 05:18:10PM +0200, Daniel Vetter wrote:
> On Tue, Aug 20, 2019 at 10:34:18AM -0300, Jason Gunthorpe wrote:
> > On Tue, Aug 20, 2019 at 10:19:02AM +0200, Daniel Vetter wrote:
> > > We need to make sure implementations don't cheat and don't have a
> > > possible schedule/blocking point deeply burried where review can't
> > > catch it.
> > > 
> > > I'm not sure whether this is the best way to make sure all the
> > > might_sleep() callsites trigger, and it's a bit ugly in the code flow.
> > > But it gets the job done.
> > > 
> > > Inspired by an i915 patch series which did exactly that, because the
> > > rules haven't been entirely clear to us.
> > > 
> > > v2: Use the shiny new non_block_start/end annotations instead of
> > > abusing preempt_disable/enable.
> > > 
> > > v3: Rebase on top of Glisse's arg rework.
> > > 
> > > v4: Rebase on top of more Glisse rework.
> > > 
> > > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > > Cc: Andrew Morton <akpm@linux-foundation.org>
> > > Cc: Michal Hocko <mhocko@suse.com>
> > > Cc: David Rientjes <rientjes@google.com>
> > > Cc: "Christian König" <christian.koenig@amd.com>
> > > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > > Cc: "Jérôme Glisse" <jglisse@redhat.com>
> > > Cc: linux-mm@kvack.org
> > > Reviewed-by: Christian König <christian.koenig@amd.com>
> > > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> > > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > >  mm/mmu_notifier.c | 8 +++++++-
> > >  1 file changed, 7 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > > index 538d3bb87f9b..856636d06ee0 100644
> > > +++ b/mm/mmu_notifier.c
> > > @@ -181,7 +181,13 @@ int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
> > >  	id = srcu_read_lock(&srcu);
> > >  	hlist_for_each_entry_rcu(mn, &range->mm->mmu_notifier_mm->list, hlist) {
> > >  		if (mn->ops->invalidate_range_start) {
> > > -			int _ret = mn->ops->invalidate_range_start(mn, range);
> > > +			int _ret;
> > > +
> > > +			if (!mmu_notifier_range_blockable(range))
> > > +				non_block_start();
> > > +			_ret = mn->ops->invalidate_range_start(mn, range);
> > > +			if (!mmu_notifier_range_blockable(range))
> > > +				non_block_end();
> > 
> > If someone Acks all the sched changes then I can pick this for
> > hmm.git, but I still think the existing pre-emption debugging is fine
> > for this use case.
> 
> Ok, I'll ping Peter Z. for an ack, iirc he was involved.
> 
> > Also, same comment as for the lockdep map, this needs to apply to the
> > non-blocking range_end also.
> 
> Hm, I thought the page table locks we're holding there already prevent any
> sleeping, so would be redundant? But reading through code I think that's
> not guaranteed, so yeah makes sense to add it for invalidate_range_end
> too. I'll respin once I have the ack/nack from scheduler people.

So I started to look into this, and I'm a bit confused. There's no
_nonblock version of this, so does this means blocking is never allowed,
or always allowed?

From a quick look through implementations I've only seen spinlocks, and
one up_read. So I guess I should wrape this callback in some unconditional
non_block_start/end, but I'm not sure.

Thanks, Daniel


> > Anyhow, since this series has conflicts with hmm.git it would be best
> > to flow through the whole thing through that tree. If there are no
> > remarks on the first two patches I'll grab them in a few days.
> 
> Thanks, Daniel
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch