[PATCH 3/6] vfs: Add mount change counter [ver #15]

[David Howells <dhowells@xxxxxxxxxx>]

Add a change counter on each mount object so that the user can easily check
to see if a mount has changed its attributes or its children.

Future patches will:

 (1) Provide this value through fsinfo() attributes.

 (2) Implement a watch_mount() system call to provide a notification
     interface for userspace monitoring.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 fs/mount.h     |   22 ++++++++++++++++++++++
 fs/namespace.c |   11 +++++++++++
 2 files changed, 33 insertions(+)

diff --git a/fs/mount.h b/fs/mount.h
index 6250de544760..65cb51f47c8c 100644
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -70,6 +70,7 @@ struct mount {
 	struct hlist_head mnt_pins;
 	struct fs_pin mnt_umount;
 	struct dentry *mnt_ex_mountpoint;
+	atomic_t mnt_change_counter;	/* Number of changed applied */
 } __randomize_layout;
 
 #define MNT_NS_INTERNAL ERR_PTR(-EINVAL) /* distinct from any mnt_namespace */
@@ -151,3 +152,24 @@ static inline bool is_anon_ns(struct mnt_namespace *ns)
 {
 	return ns->seq == 0;
 }
+
+/*
+ * Type of mount topology change notification.
+ */
+enum mount_notification_subtype {
+	NOTIFY_MOUNT_NEW_MOUNT	= 0, /* New mount added */
+	NOTIFY_MOUNT_UNMOUNT	= 1, /* Mount removed manually */
+	NOTIFY_MOUNT_EXPIRY	= 2, /* Automount expired */
+	NOTIFY_MOUNT_READONLY	= 3, /* Mount R/O state changed */
+	NOTIFY_MOUNT_SETATTR	= 4, /* Mount attributes changed */
+	NOTIFY_MOUNT_MOVE_FROM	= 5, /* Mount moved from here */
+	NOTIFY_MOUNT_MOVE_TO	= 6, /* Mount moved to here (compare op_id) */
+};
+
+static inline void notify_mount(struct mount *changed,
+				struct mount *aux,
+				enum mount_notification_subtype subtype,
+				u32 info_flags)
+{
+	atomic_inc(&changed->mnt_change_counter);
+}
diff --git a/fs/namespace.c b/fs/namespace.c
index d96bc1dfab03..c306e9362604 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -513,6 +513,8 @@ static int mnt_make_readonly(struct mount *mnt)
 	smp_wmb();
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
 	unlock_mount_hash();
+	if (ret == 0)
+		notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY, 0x10000);
 	return ret;
 }
 
@@ -521,6 +523,7 @@ static int __mnt_unmake_readonly(struct mount *mnt)
 	lock_mount_hash();
 	mnt->mnt.mnt_flags &= ~MNT_READONLY;
 	unlock_mount_hash();
+	notify_mount(mnt, NULL, NOTIFY_MOUNT_READONLY, 0);
 	return 0;
 }
 
@@ -833,6 +836,7 @@ static void umount_mnt(struct mount *mnt)
 {
 	/* old mountpoint will be dropped when we can do that */
 	mnt->mnt_ex_mountpoint = mnt->mnt_mountpoint;
+	notify_mount(mnt->mnt_parent, mnt, NOTIFY_MOUNT_UNMOUNT, 0);
 	unhash_mnt(mnt);
 }
 
@@ -1472,6 +1476,7 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
 		p = list_first_entry(&tmp_list, struct mount, mnt_list);
 		list_del_init(&p->mnt_expire);
 		list_del_init(&p->mnt_list);
+
 		ns = p->mnt_ns;
 		if (ns) {
 			ns->mounts--;
@@ -2095,7 +2100,10 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		lock_mount_hash();
 	}
 	if (parent_path) {
+		notify_mount(source_mnt->mnt_parent, source_mnt,
+			     NOTIFY_MOUNT_MOVE_FROM, 0);
 		detach_mnt(source_mnt, parent_path);
+		notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_MOVE_TO, 0);
 		attach_mnt(source_mnt, dest_mnt, dest_mp);
 		touch_mnt_namespace(source_mnt->mnt_ns);
 	} else {
@@ -2104,6 +2112,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 			list_del_init(&source_mnt->mnt_ns->list);
 		}
 		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
+		notify_mount(dest_mnt, source_mnt, NOTIFY_MOUNT_NEW_MOUNT, 0);
 		commit_tree(source_mnt);
 	}
 
@@ -2480,6 +2489,7 @@ static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 	mnt->mnt.mnt_flags = mnt_flags;
 	touch_mnt_namespace(mnt->mnt_ns);
 	unlock_mount_hash();
+	notify_mount(mnt, NULL, NOTIFY_MOUNT_SETATTR, 0);
 }
 
 /*
@@ -2878,6 +2888,7 @@ void mark_mounts_for_expiry(struct list_head *mounts)
 		if (!xchg(&mnt->mnt_expiry_mark, 1) ||
 			propagate_mount_busy(mnt, 1))
 			continue;
+		notify_mount(mnt, NULL, NOTIFY_MOUNT_EXPIRY, 0);
 		list_move(&mnt->mnt_expire, &graveyard);
 	}
 	while (!list_empty(&graveyard)) {