Re: [PATCH 1/3] KVM: Batch setting of per-page memory attributes to avoid soft lockup

[Sean Christopherson <seanjc@google.com>]

On Thu, Jun 05, 2025, Liam Merwick wrote:
> On 05/06/2025 16:57, Sean Christopherson wrote:
> > On Thu, Jun 05, 2025, Liam Merwick wrote:
> > > Limit the range of memory per operation when setting the attributes to
> > > avoid holding kvm->slots_lock for too long and causing a cpu soft lockup.
> > 
> > Holding slots_lock is totally fine.  Presumably the issue is that the CPU never
> > reschedules.
> > 
> > E.g. I would expect this to make the problem go away, though it's probably not a
> > complete fix (I'm guessing kvm_range_has_memory_attributes() can be made to yell
> > too).
> 
> That indeed works. I couldn't trigger anything in
> kvm_range_has_memory_attributes() but am limited to about 2TiB.  I'll do some
> more tracing before I send a v2 to see if there any more places that might be
> close to hitting the limit.

To get kvm_range_has_memory_attributes() to fail, I _think_ you would need to do
a large query when the attributes match a non-zero value, so that it needs to
perform its slower search.

Ah, actually, I wouldn't be at all surprised if the issue is limited to insertion,
or even just to the xa_reserve() path that allocates memory.