Re: [PATCH 1/4] mm: Check if mmu notifier callbacks are allowed to fail

[Daniel Vetter]

On Tue, May 21, 2019 at 11:44:11AM -0400, Jerome Glisse wrote:
> On Mon, May 20, 2019 at 11:39:42PM +0200, 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.
> > 
> > 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.
> > 
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: "Christian König" <christian.koenig@xxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: "Jérôme Glisse" <jglisse@xxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Reviewed-by: Christian König <christian.koenig@xxxxxxx>
> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> 
> Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>

-mm folks, is this (entire series of 4 patches) planned to land in the 5.3
merge window? Or do you want more reviews/testing/polish?

I think with all the hmm rework going on, a bit more validation and checks
in this tricky area would help.

Thanks, Daniel

> 
> > ---
> >  mm/mmu_notifier.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c
> > index ee36068077b6..c05e406a7cd7 100644
> > --- a/mm/mmu_notifier.c
> > +++ b/mm/mmu_notifier.c
> > @@ -181,6 +181,9 @@ 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-" : "");
> > +				if (!mmu_notifier_range_blockable(range))
> > +					pr_warn("%pS callback failure not allowed\n",
> > +						mn->ops->invalidate_range_start);
> >  				ret = _ret;
> >  			}
> >  		}
> > -- 
> > 2.20.1
> > 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch