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

[Daniel Vetter <daniel.vetter@ffwll.ch>]

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.

v2: Drop the full WARN_ON backtrace in favour of just a pr_warn for
the problematic case (Michal Hocko).

v3: Rebase on top of Glisse's arg rework.

v4: More rebase on top of Glisse reworking everything.

v5: Fixup rebase damage and also catch failures != EAGAIN for
!blockable (Jason). Also go back to WARN_ON as requested by Jason, so
automatic checkers can easily catch bugs by setting panic_on_warn.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Jérôme Glisse" <jglisse@redhat.com>
Cc: linux-mm@kvack.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 mm/mmu_notifier.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
index b5670620aea0..16f1cbc775d0 100644
--- 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;
 			}
 		}
-- 
2.22.0