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

[Andrew Morton]

On Wed, 14 Aug 2019 22:20:23 +0200 Daniel Vetter <daniel.vetter at 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.
> 
> ...
>
> --- a/mm/mmu_notifier.c
> +++ 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);