Re: [PATCH v6 06/14] KVM: s390: Fix race in __do_essa()

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] System stall / SRCU stall warning due to holding `kvm->srcu` across a long-running page table walk.
--

--- Patch [6]: [PATCH v6 06/14] KVM: s390: Fix race in __do_essa() ---
commit 8a0e7e6fefae191da0effc8b1bcdd717dfb5216b
Author: Claudio Imbrenda <[email protected]>

KVM: s390: Fix race in __do_essa()

This commit modifies the locking around KVM s390 migration and CMMA handling,
replacing slots_lock with slots_arch_lock to fix a race with __do_essa(). It
also adds a srcu guard to kvm_s390_vm_set_migration().

> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index de3cb6111bd44..24c3bfcd5db26 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
[ ... ]
> @@ -1300,7 +1300,9 @@ static int kvm_s390_vm_set_migration(struct kvm *kvm,
>  {
>  	int res = -ENXIO;
>  
> -	mutex_lock(&kvm->slots_lock);
> +	guard(srcu)(&kvm->srcu);
> +	guard(mutex)(&kvm->slots_arch_lock);
> +
>  	switch (attr->attr) {
>  	case KVM_S390_VM_MIGRATION_START:
>  		res = kvm_s390_vm_start_migration(kvm);

[Severity: High]
Will holding kvm->srcu across the entire migration start/stop cause an SRCU
stall for large VMs?

Looking at kvm_s390_vm_start_migration(), it eventually calls
gmap_set_cmma_all_dirty() (and kvm_s390_vm_stop_migration() calls
gmap_set_cmma_all_clean()), which iterates over the entire guest address space:

kvm_s390_vm_set_migration()
  kvm_s390_vm_start_migration()
    gmap_set_cmma_all_dirty()

While the page table walk yields with cond_resched() to prevent soft lockups,
yielding inside an SRCU read-side critical section does not allow the SRCU
grace period to advance.

Could this stall the SRCU grace period and block concurrent memslot updates
across the system?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=6