[PATCH v6 14/14] KVM: s390: Move DAT table updates to kvm_arch_prepare_memory_region()

[Claudio Imbrenda]

Currently, when a memslot is changed, the changes in the page tables
are performed in kvm_arch_commit_memory_region(), which is assumed to
never fail, as it cannot return an error code.

If memory is tight, the allocation of page tables can fail, a warning
is printed, and the system is left in an inconsistent state.

Move the whole page table update into kvm_arch_prepare_memory_region()
instead. This function can fail and return an appropriate error code,
thus not leaving the system in an inconsistent state.

Fixes: 598841ca9919 ("[S390] use gmap address spaces for kvm guest images")
Fixes: e38c884df921 ("KVM: s390: Switch to new gmap")
Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
---
 arch/s390/kvm/kvm-s390.c | 87 ++++++++++++++++++----------------------
 1 file changed, 40 insertions(+), 47 deletions(-)

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 035476ec7fbc..668272dffa18 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -5761,11 +5761,30 @@ bool kvm_arch_irqchip_in_kernel(struct kvm *kvm)
 }
 
 /* Section: memory related */
+static long cmma_d_count_pte(union pte *ptep, gfn_t gfn, gfn_t next, struct dat_walk *walk)
+{
+	union pgste pgste;
+
+	pgste = pgste_get_lock(ptep);
+	if (pgste.cmma_d) {
+		pgste.cmma_d = 0;
+		atomic64_dec(walk->priv);
+	}
+	pgste_set_unlock(ptep, pgste);
+	return 0;
+}
+
 int kvm_arch_prepare_memory_region(struct kvm *kvm,
 				   const struct kvm_memory_slot *old,
 				   struct kvm_memory_slot *new,
 				   enum kvm_mr_change change)
 {
+	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 = -ENOMEM;
+
+	lockdep_assert_held(&kvm->slots_arch_lock);
+
 	if (kvm_is_ucontrol(kvm) && new && new->id < KVM_USER_MEM_SLOTS)
 		return -EINVAL;
 
@@ -5788,55 +5807,27 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 			return -EINVAL;
 	}
 
-	if (!kvm->arch.migration_mode)
-		return 0;
-
-	/*
-	 * Turn off migration mode when:
-	 * - userspace creates a new memslot with dirty logging off,
-	 * - userspace modifies an existing memslot (MOVE or FLAGS_ONLY) and
-	 *   dirty logging is turned off.
-	 * Migration mode expects dirty page logging being enabled to store
-	 * its dirty bitmap.
-	 */
-	if (change != KVM_MR_DELETE &&
-	    !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
-		WARN(kvm_s390_vm_stop_migration(kvm),
-		     "Failed to stop migration mode");
-
-	return 0;
-}
-
-static long cmma_d_count_pte(union pte *ptep, gfn_t gfn, gfn_t next, struct dat_walk *walk)
-{
-	union pgste pgste;
-
-	pgste = pgste_get_lock(ptep);
-	if (pgste.cmma_d) {
-		pgste.cmma_d = 0;
-		atomic64_dec(walk->priv);
+	if (kvm->arch.migration_mode) {
+		/*
+		 * Turn off migration mode when:
+		 * - userspace creates a new memslot with dirty logging off,
+		 * - userspace modifies an existing memslot (MOVE or FLAGS_ONLY)
+		 *   and dirty logging is turned off.
+		 * Migration mode expects dirty page logging being enabled to
+		 * store its dirty bitmap.
+		 */
+		if (change != KVM_MR_DELETE &&
+		    !(new->flags & KVM_MEM_LOG_DIRTY_PAGES))
+			WARN(kvm_s390_vm_stop_migration(kvm),
+			     "Failed to stop migration mode");
 	}
-	pgste_set_unlock(ptep, pgste);
-	return 0;
-}
-
-void kvm_arch_commit_memory_region(struct kvm *kvm,
-				struct kvm_memory_slot *old,
-				const struct kvm_memory_slot *new,
-				enum kvm_mr_change change)
-{
-	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 = -ENOMEM;
-
-	guard(mutex)(&kvm->slots_arch_lock);
 
 	if (change == KVM_MR_FLAGS_ONLY)
-		return;
+		return 0;
 
 	mc = kvm_s390_new_mmu_cache();
 	if (!mc)
-		goto out;
+		return -ENOMEM;
 retry:
 	scoped_guard(write_lock, &kvm->mmu_lock) {
 		if (kvm->arch.migration_mode && kvm->arch.use_cmma && old) {
@@ -5869,10 +5860,12 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
 		if (!rc)
 			goto retry;
 	}
-out:
-	if (KVM_BUG_ON(rc, kvm))
-		pr_warn("failed to commit memory region\n");
-	return;
+	return rc;
+}
+
+void kvm_arch_commit_memory_region(struct kvm *kvm, struct kvm_memory_slot *old,
+				   const struct kvm_memory_slot *new, enum kvm_mr_change change)
+{
 }
 
 /**
-- 
2.55.0