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

[David Woodhouse]

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@xxxxxxxxxxxxx/

Perhaps in the past it was easier to trigger 'naturally'?

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