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

[Daniel Vetter]

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@xxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: "Christian König" <christian.koenig@xxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> > Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> >  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.

> 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