Re: [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to fail

Jason Gunthorpe
2019-08-14T23:22:42Z

On Wed, Aug 14, 2019 at 03:14:47PM -0700, Andrew Morton wrote:
> On Wed, 14 Aug 2019 22:20:23 +0200 Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> > Just a bit of paranoia, since if we start pushing this deep into
> > callchains it's hard to spot all places where an mmu notifier
> > implementation might fail when it's not allowed to.
> >
> > Inspired by some confusion we had discussing i915 mmu notifiers and
> > whether we could use the newly-introduced return value to handle some
> > corner cases. Until we realized that these are only for when a task
> > has been killed by the oom reaper.
> >
> > An alternative approach would be to split the callback into two
> > versions, one with the int return value, and the other with void
> > return value like in older kernels. But that's a lot more churn for
> > fairly little gain I think.
> >
> > Summary from the m-l discussion on why we want something at warning
> > level: This allows automated tooling in CI to catch bugs without
> > humans having to look at everything. If we just upgrade the existing
> > pr_info to a pr_warn, then we'll have false positives. And as-is, no
> > one will ever spot the problem since it's lost in the massive amounts
> > of overall dmesg noise.
> >
> > ...
> >
> > +++ b/mm/mmu_notifier.c
> > @@ -179,6 +179,8 @@ int __mmu_notifier_invalidate_range_start(struct mmu_notifier_range *range)
> > pr_info("%pS callback failed with %d in %sblockable context.\n",
> > mn->ops->invalidate_range_start, _ret,
> > !mmu_notifier_range_blockable(range) ? "non-" : "");
> > + WARN_ON(mmu_notifier_range_blockable(range) ||
> > + ret != -EAGAIN);
> > ret = _ret;
> > }
> > }
>
> A problem with WARN_ON(a || b) is that if it triggers, we don't know
> whether it was because of a or because of b. Or both. So I'd suggest
>
> WARN_ON(a);
> WARN_ON(b);
>

Well, we did just make a pr_info right above with the value of
blockable, that seems enough to tell the cases apart?

But you are generally right, the full logic:

if (_ret) {
if (WARN_ON(mmu_notifier_range_blockable(range)))
continue;
WARN_ON(_ret != -EAGAIN);
ret = -EAGAIN;
break;
}

would force correct API contract up the call chain once we detect a
broken driver..

But at some point it does feel like a bit much debugging logic to have
in a production code path, as this should never happen and is just to
discourage wrong driver behaviors during driver development.

If we like this version then:

Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>

Also - I have a bunch of other patches to mmu notifiers for hmm.git,
so when everyone agrees I can grab this to avoid conflicts.

Thanks,
Jason