Messages in this thread | | | Subject | Re: [PATCH 1/5] mm: Check if mmu notifier callbacks are allowed to fail | From | Ralph Campbell <> | Date | Wed, 14 Aug 2019 16:34:58 -0700 |
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On 8/14/19 3:14 PM, Andrew Morton wrote: > On Wed, 14 Aug 2019 22:20:23 +0200 Daniel Vetter <daniel.vetter@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); >
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.
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