diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c
index 35d78cb3c38a..64c5205e2b5e 100644
--- a/fs/mnt_idmapping.c
+++ b/fs/mnt_idmapping.c
@@ -9,8 +9,16 @@
 
 #include "internal.h"
 
+/*
+ * Outside of this file vfs{g,u}id_t are always created from k{g,u}id_t,
+ * never from raw values. These are just internal helpers.
+ */
+#define VFSUIDT_INIT_RAW(val) (vfsuid_t){ val }
+#define VFSGIDT_INIT_RAW(val) (vfsgid_t){ val }
+
 struct mnt_idmap {
-	struct user_namespace *owner;
+	struct uid_gid_map uid_map;
+	struct uid_gid_map gid_map;
 	refcount_t count;
 };
 
@@ -20,7 +28,6 @@ struct mnt_idmap {
  * mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...].
  */
 struct mnt_idmap nop_mnt_idmap = {
-	.owner	= &init_user_ns,
 	.count	= REFCOUNT_INIT(1),
 };
 EXPORT_SYMBOL_GPL(nop_mnt_idmap);
@@ -65,7 +72,6 @@ vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
 		     kuid_t kuid)
 {
 	uid_t uid;
-	struct user_namespace *mnt_userns = idmap->owner;
 
 	if (idmap == &nop_mnt_idmap)
 		return VFSUIDT_INIT(kuid);
@@ -75,7 +81,7 @@ vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
 		uid = from_kuid(fs_userns, kuid);
 	if (uid == (uid_t)-1)
 		return INVALID_VFSUID;
-	return VFSUIDT_INIT(make_kuid(mnt_userns, uid));
+	return VFSUIDT_INIT_RAW(map_id_down(&idmap->uid_map, uid));
 }
 EXPORT_SYMBOL_GPL(make_vfsuid);
 
@@ -103,7 +109,6 @@ vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
 		     struct user_namespace *fs_userns, kgid_t kgid)
 {
 	gid_t gid;
-	struct user_namespace *mnt_userns = idmap->owner;
 
 	if (idmap == &nop_mnt_idmap)
 		return VFSGIDT_INIT(kgid);
@@ -113,7 +118,7 @@ vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
 		gid = from_kgid(fs_userns, kgid);
 	if (gid == (gid_t)-1)
 		return INVALID_VFSGID;
-	return VFSGIDT_INIT(make_kgid(mnt_userns, gid));
+	return VFSGIDT_INIT_RAW(map_id_down(&idmap->gid_map, gid));
 }
 EXPORT_SYMBOL_GPL(make_vfsgid);
 
@@ -132,11 +137,10 @@ kuid_t from_vfsuid(struct mnt_idmap *idmap,
 		   struct user_namespace *fs_userns, vfsuid_t vfsuid)
 {
 	uid_t uid;
-	struct user_namespace *mnt_userns = idmap->owner;
 
 	if (idmap == &nop_mnt_idmap)
 		return AS_KUIDT(vfsuid);
-	uid = from_kuid(mnt_userns, AS_KUIDT(vfsuid));
+	uid = map_id_up(&idmap->uid_map, __vfsuid_val(vfsuid));
 	if (uid == (uid_t)-1)
 		return INVALID_UID;
 	if (initial_idmapping(fs_userns))
@@ -160,11 +164,10 @@ kgid_t from_vfsgid(struct mnt_idmap *idmap,
 		   struct user_namespace *fs_userns, vfsgid_t vfsgid)
 {
 	gid_t gid;
-	struct user_namespace *mnt_userns = idmap->owner;
 
 	if (idmap == &nop_mnt_idmap)
 		return AS_KGIDT(vfsgid);
-	gid = from_kgid(mnt_userns, AS_KGIDT(vfsgid));
+	gid = map_id_up(&idmap->gid_map, __vfsgid_val(vfsgid));
 	if (gid == (gid_t)-1)
 		return INVALID_GID;
 	if (initial_idmapping(fs_userns))
@@ -195,16 +198,91 @@ int vfsgid_in_group_p(vfsgid_t vfsgid)
 #endif
 EXPORT_SYMBOL_GPL(vfsgid_in_group_p);
 
+static int copy_mnt_idmap(struct uid_gid_map *map_from,
+			  struct uid_gid_map *map_to)
+{
+	struct uid_gid_extent *forward, *reverse;
+	u32 nr_extents = READ_ONCE(map_from->nr_extents);
+	/* Pairs with smp_wmb() when writing the idmapping. */
+	smp_rmb();
+
+	/*
+	 * Don't blindly copy @map_to into @map_from if nr_extents is
+	 * smaller or equal to UID_GID_MAP_MAX_BASE_EXTENTS. Since we
+	 * read @nr_extents someone could have written an idmapping and
+	 * then we might end up with inconsistent data. So just don't do
+	 * anything at all.
+	 */
+	if (nr_extents == 0)
+		return 0;
+
+	/*
+	 * Here we know that nr_extents is greater than zero which means
+	 * a map has been written. Since idmappings can't be changed
+	 * once they have been written we know that we can safely copy
+	 * from @map_to into @map_from.
+	 */
+
+	if (nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
+		*map_to = *map_from;
+		return 0;
+	}
+
+	forward = kmemdup(map_from->forward,
+			  nr_extents * sizeof(struct uid_gid_extent),
+			  GFP_KERNEL_ACCOUNT);
+	if (!forward)
+		return -ENOMEM;
+
+	reverse = kmemdup(map_from->reverse,
+			  nr_extents * sizeof(struct uid_gid_extent),
+			  GFP_KERNEL_ACCOUNT);
+	if (!reverse) {
+		kfree(forward);
+		return -ENOMEM;
+	}
+
+	/*
+	 * The idmapping isn't exposed anywhere so we don't need to care
+	 * about ordering between extent pointers and @nr_extents
+	 * initialization.
+	 */
+	map_to->forward = forward;
+	map_to->reverse = reverse;
+	map_to->nr_extents = nr_extents;
+	return 0;
+}
+
+static void free_mnt_idmap(struct mnt_idmap *idmap)
+{
+	if (idmap->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+		kfree(idmap->uid_map.forward);
+		kfree(idmap->uid_map.reverse);
+	}
+	if (idmap->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
+		kfree(idmap->gid_map.forward);
+		kfree(idmap->gid_map.reverse);
+	}
+	kfree(idmap);
+}
+
 struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
 {
 	struct mnt_idmap *idmap;
+	int ret;
 
 	idmap = kzalloc(sizeof(struct mnt_idmap), GFP_KERNEL_ACCOUNT);
 	if (!idmap)
 		return ERR_PTR(-ENOMEM);
 
-	idmap->owner = get_user_ns(mnt_userns);
 	refcount_set(&idmap->count, 1);
+	ret = copy_mnt_idmap(&mnt_userns->uid_map, &idmap->uid_map);
+	if (!ret)
+		ret = copy_mnt_idmap(&mnt_userns->gid_map, &idmap->gid_map);
+	if (ret) {
+		free_mnt_idmap(idmap);
+		idmap = ERR_PTR(ret);
+	}
 	return idmap;
 }
 
@@ -234,9 +312,7 @@ EXPORT_SYMBOL_GPL(mnt_idmap_get);
  */
 void mnt_idmap_put(struct mnt_idmap *idmap)
 {
-	if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count)) {
-		put_user_ns(idmap->owner);
-		kfree(idmap);
-	}
+	if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count))
+		free_mnt_idmap(idmap);
 }
 EXPORT_SYMBOL_GPL(mnt_idmap_put);
diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
index b0542cd11aeb..415a7ca2b882 100644
--- a/include/linux/uidgid.h
+++ b/include/linux/uidgid.h
@@ -17,6 +17,7 @@
 
 struct user_namespace;
 extern struct user_namespace init_user_ns;
+struct uid_gid_map;
 
 typedef struct {
 	uid_t val;
@@ -138,6 +139,9 @@ static inline bool kgid_has_mapping(struct user_namespace *ns, kgid_t gid)
 	return from_kgid(ns, gid) != (gid_t) -1;
 }
 
+u32 map_id_down(struct uid_gid_map *map, u32 id);
+u32 map_id_up(struct uid_gid_map *map, u32 id);
+
 #else
 
 static inline kuid_t make_kuid(struct user_namespace *from, uid_t uid)
@@ -186,6 +190,15 @@ static inline bool kgid_has_mapping(struct user_namespace *ns, kgid_t gid)
 	return gid_valid(gid);
 }
 
+static inline u32 map_id_down(struct uid_gid_map *map, u32 id)
+{
+	return id;
+}
+
+static inline u32 map_id_up(struct uid_gid_map *map, u32 id)
+{
+	return id;
+}
 #endif /* CONFIG_USER_NS */
 
 #endif /* _LINUX_UIDGID_H */
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 625101249e4d..ce4d99df5f0e 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -332,7 +332,7 @@ static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
 	return id;
 }
 
-static u32 map_id_down(struct uid_gid_map *map, u32 id)
+u32 map_id_down(struct uid_gid_map *map, u32 id)
 {
 	return map_id_range_down(map, id, 1);
 }
@@ -375,7 +375,7 @@ map_id_up_max(unsigned extents, struct uid_gid_map *map, u32 id)
 		       sizeof(struct uid_gid_extent), cmp_map_id);
 }
 
-static u32 map_id_up(struct uid_gid_map *map, u32 id)
+u32 map_id_up(struct uid_gid_map *map, u32 id)
 {
 	struct uid_gid_extent *extent;
 	unsigned extents = map->nr_extents;