[RFC PATCH v1 02/26] KVM: Introduce kvm_gmem_get_pfn_locked(), which retains the folio lock

[Fuad Tabba <tabba@google.com>]

Create a new variant of kvm_gmem_get_pfn(), which retains the
folio lock if it returns successfully.

Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/linux/kvm_host.h | 11 +++++++++++
 virt/kvm/guest_memfd.c   | 21 ++++++++++++++++++---
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 4cacf2a9a5d5..b96abeeb2b65 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -2381,6 +2381,8 @@ static inline bool kvm_mem_is_private(struct kvm *kvm, gfn_t gfn)
 #ifdef CONFIG_KVM_PRIVATE_MEM
 int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
+int kvm_gmem_get_pfn_locked(struct kvm *kvm, struct kvm_memory_slot *slot,
+			      gfn_t gfn, kvm_pfn_t *pfn, int *max_order);
 #else
 static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 				   struct kvm_memory_slot *slot, gfn_t gfn,
@@ -2389,6 +2391,15 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
 	KVM_BUG_ON(1, kvm);
 	return -EIO;
 }
+
+static inline int kvm_gmem_get_pfn_locked(struct kvm *kvm,
+					  struct kvm_memory_slot *slot,
+					  gfn_t gfn, kvm_pfn_t *pfn,
+					  int *max_order)
+{
+	KVM_BUG_ON(1, kvm);
+	return -EIO;
+}
 #endif /* CONFIG_KVM_PRIVATE_MEM */
 
 #endif
diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
index 0f4e0cf4f158..7e3ea7a3f086 100644
--- a/virt/kvm/guest_memfd.c
+++ b/virt/kvm/guest_memfd.c
@@ -482,8 +482,8 @@ void kvm_gmem_unbind(struct kvm_memory_slot *slot)
 	fput(file);
 }
 
-int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
-		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
+int kvm_gmem_get_pfn_locked(struct kvm *kvm, struct kvm_memory_slot *slot,
+			    gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
 {
 	pgoff_t index = gfn - slot->base_gfn + slot->gmem.pgoff;
 	struct kvm_gmem *gmem;
@@ -523,10 +523,25 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
 	r = 0;
 
 out_unlock:
-	folio_unlock(folio);
+	if (r)
+		folio_unlock(folio);
 out_fput:
 	fput(file);
 
 	return r;
 }
+EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn_locked);
+
+int kvm_gmem_get_pfn(struct kvm *kvm, struct kvm_memory_slot *slot,
+		     gfn_t gfn, kvm_pfn_t *pfn, int *max_order)
+{
+	int r;
+
+	r = kvm_gmem_get_pfn_locked(kvm, slot, gfn, pfn, max_order);
+	if (r)
+		return r;
+
+	unlock_page(pfn_to_page(*pfn));
+	return 0;
+}
 EXPORT_SYMBOL_GPL(kvm_gmem_get_pfn);
-- 
2.44.0.rc1.240.g4c46232300-goog