[PATCH v4 11/13] KVM: s390: Fix kvm_arch_commit_memory_region() when low on memory

[Claudio Imbrenda]

When low on memory, kvm_arch_commit_memory_region() might not perform
the required action completely, and instead just print a warning and
return. This allows the VM to continue running with an inconsistent
state.

Fix by refilling the struct kvm_s390_mmu_cache and trying again. A true
failure only happens if the refill fails, or if the cache cannot be
allocated at all; in such case print a warning and kill the VM. Such a
failure cannot currently happen.

Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
---
 arch/s390/kvm/kvm-s390.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 59f784f8c01b..b151d25a4a4c 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -5817,7 +5817,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 {
 	const struct dat_walk_ops ops = { .pte_entry = cmma_d_count_pte, };
 	struct kvm_s390_mmu_cache *mc __free(kvm_s390_mmu_cache) = NULL;
-	int rc = 0;
+	int rc = -ENOMEM;
 
 	guard(mutex)(&kvm->slots_arch_lock);
 
@@ -5825,11 +5825,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 		return;
 
 	mc = kvm_s390_new_mmu_cache();
-	if (!mc) {
-		rc = -ENOMEM;
+	if (!mc)
 		goto out;
-	}
-
+retry:
 	scoped_guard(write_lock, &kvm->mmu_lock) {
 		if (kvm->arch.migration_mode && kvm->arch.use_cmma && old) {
 			_dat_walk_gfn_range(old->base_gfn, old->base_gfn + old->npages,
@@ -5852,11 +5850,17 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 		case KVM_MR_FLAGS_ONLY:
 			break;
 		default:
+			rc = 0;
 			WARN(1, "Unknown KVM MR CHANGE: %d\n", change);
 		}
 	}
+	if (rc == -ENOMEM) {
+		rc = kvm_s390_mmu_cache_topup(mc);
+		if (!rc)
+			goto retry;
+	}
 out:
-	if (rc)
+	if (KVM_BUG_ON(rc, kvm))
 		pr_warn("failed to commit memory region\n");
 	return;
 }
-- 
2.55.0