From: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Wed, 14 Aug 2019 22:20:23 +0200
Subject: mm/mmu_notifiers: check if mmu notifier callbacks are allowed to fail
Patch-mainline: v5.4-rc1
Git-commit: 8402ce61bec28a1b73ec467b4c146f1e3f587044
References: jsc#SLE-15176, jsc#SLE-16387

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.

Link: https://lore.kernel.org/r/20190814202027.18735-2-daniel.vetter@ffwll.ch
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 mm/mmu_notifier.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/mm/mmu_notifier.c
+++ b/mm/mmu_notifier.c
@@ -179,6 +179,8 @@ int __mmu_notifier_invalidate_range_star
 				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;
 			}
 		}