Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

[David Woodhouse <dwmw2@infradead.org>]

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On Wed, 2026-08-12 at 15:38 +0100, David Woodhouse wrote:
> On Wed, 2026-08-12 at 11:26 -0300, Jason Gunthorpe wrote:
> > On Wed, Aug 12, 2026 at 03:05:43PM +0100, David Woodhouse wrote:
> > > the non_block_{start,end} guards around the MMU notifiers, which are
> > > basically never being called anyway and don't actually seem to protect
> > > against any real bugs.
> > 
> > If you think this blocked stuff is dead code then lets remove it, but
> > I'm pretty sure it is called and I remember seeing bug reports about
> > it being triggered in the wild. Vetter certainly added it because
> > their tests were actually triggering and they had bugs in their DRM
> > stack directly connected to this.
> > 
> > It might not trigger for your hypervisor case but we aren't here just
> > to make only kvm work now are we?
> 
> I'm sure there can be real-world use cases where it can trigger, but
> when I was *trying* to exercise the code path, to validate my belief
> (from source inspection) that even the plain spinlock would trigger the
> splat, I ended up having to hack the kernel to do so:
> 
> https://lore.kernel.org/all/787aa26cf62dfd361eea8ed19f384fc517892501.camel@infradead.org/
> 
> Perhaps in the past it was easier to trigger 'naturally'?

I did a bit more digging. On the one hand, I was wrong to claim that it
never triggers — there *are* a handful of reports; of my searches this
was probably the most fruitful:
https://lore.kernel.org/all/?q=%22non_block%3A+1%22

On the other hand, they are basically *all* false positives from the
point of view of what the check was actually trying to do. And in KVM.

It's causing more issues than it fixes. Including some bizarre MM
gymnastics: https://lore.kernel.org/all/amMZFqs1smKmMuj5@lucifer/

At this point I don't think I even *care* whether we manage to do the
atomic SRCU thing; I think this check should die *anyway*.

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