On 8/14/19 3:14 PM, Andrew Morton wrote:
On Wed, 14 Aug 2019 22:20:23 +0200 Daniel Vetter <daniel.vetter@xxxxxxxx> 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);
This won't quite work. It is OK to have mmu_notifier_range_blockable(range) be true or false.
sync_cpu_device_pagetables() shouldn't return -EAGAIN unless blockable is true.
