Re: [PATCH] KVM: mmu_notifier: make mn_invalidate_lock non-sleeping for non-blocking invalidations

[Paolo Bonzini]

On 3/12/26 20:24, shaikh kamaluddin wrote:
> > > Alternatively, if you think this needs to be addressed in
> > > mmu_notifiers(eg. how non_block_start() is applied), I'm happy to
> > > redirect my efforts there-Please advise.
> >
> > Have you considered a "OOM entered" callback for MMU notifiers?  KVM's MMU
> > notifier can just remove itself for example, in fact there is code in
> > kvm_destroy_vm() to do that even if invalidations are unbalanced.
> >
> > Paolo
> Thanks for the suggestion! That's a much cleaner approach than what I was considering.
>
> If I understand correctly, the idea would be:
> 1. Add a new MMU notifier callback (e.g., .oom_entered or .release_on_oom)
> 2. Have KVM implement it to unregister the notifier when OOM reaper starts
> 3. Leverage the existing kvm_destroy_vm() logic that already handles unbalanced invalidations

Yes pretty much.  Essentially, move the existing logic to the new 
callback and invoke it from kvm_destroy_vm().

> This avoids the whole "convert locks to raw" problem and the complexity of deferring work.
>
> I have questions on Testing part:
> ------------------------------------
> I tried to reproduce the bug scenario using the virtme-ng then running
> the stress-ng putting memory pressure on VM, but not able to reproduce
> the scenario.
> I tried this way ..
> vng -v -r ./arch/x86/boot/bzImage
> VM is up, then running the stress-ng as below
> stress-ng --vm 2 --vm-bytes 95% --timeout 20s & sleep 5 & dmesg | tail -30 | grep "sleeping function"
> OOM Killer is triggered, but exact bug not able to reproduce, Please
> suggest how to reproduce this bug, even we need to verify after code
> changes which you have suggested.

I don't know, sorry.  But with this new approach there will always be a 
call to the new callback from the OOM killer, so it's easier to test.

Thanks,

Paolo