On Mon 10-12-18 11:36:38, Daniel Vetter 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.
OK, fair enough. If this is going to help with testing then I do not
have any objections of course.
> v2: Drop the full WARN_ON backtrace in favour of just a pr_warn for
> the problematic case (Michal Hocko).
Thanks!
> Cc: Andrew Morton <a...@linux-foundation.org>
> Cc: Michal Hocko <mho...@suse.com>
> Cc: "Christian König" <christian.koe...@amd.com>
> Cc: David Rientjes <rient...@google.com>
> Cc: Daniel Vetter <daniel.vet...@ffwll.ch>
> Cc: "Jérôme Glisse" <jgli...@redhat.com>
> Cc: linux...@kvack.org
> Cc: Paolo Bonzini <pbonz...@redhat.com>
> Signed-off-by: Daniel Vetter <daniel.vet...@intel.com>
> ---
> mm/mmu_notifier.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> index 5119ff846769..ccc22f21b735 100644
> --- a/mm/mmu_notifier.c
> +++ b/mm/mmu_notifier.c
> @@ -190,6 +190,9 @@ int __mmu_notifier_invalidate_range_start(struct
> mm_struct *mm,
> pr_info("%pS callback failed with %d in
> %sblockable context.\n",
>
> mn->ops->invalidate_range_start, _ret,
> !blockable ? "non-" : "");
> + if (blockable)
> + pr_warn("%pS callback failure not
> allowed\n",
> +
> mn->ops->invalidate_range_start);
> ret = _ret;
> }
> }
> --
> 2.20.0.rc1
>
--
Michal Hocko
SUSE Labs
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