[RFC PATCH 1/5] fs: introduce notifier list for vfs inode

[Chengguang Xu <cgxu519@xxxxxxxxxxxx>]

Currently there is no notification api for kernel about modification
of vfs inode, in some use cases like overlayfs, this kind of notification
will be very helpful to implement containerized syncfs functionality.
As the first attempt, we introduce marking inode dirty notification so that
overlay's inode could mark itself dirty as well and then only sync dirty
overlay inode while syncfs.

Signed-off-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
---
 fs/fs-writeback.c  | 4 ++++
 fs/inode.c         | 5 +++++
 include/linux/fs.h | 6 ++++++
 3 files changed, 15 insertions(+)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1492271..657cceb 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2246,9 +2246,13 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 {
 	struct super_block *sb = inode->i_sb;
 	int dirtytime;
+	int copy_flags = flags;
 
 	trace_writeback_mark_inode_dirty(inode, flags);
 
+	blocking_notifier_call_chain(
+		&inode->notifier_lists[MARK_INODE_DIRTY_NOTIFIER],
+		0, &copy_flags);
 	/*
 	 * Don't do this for I_DIRTY_PAGES - that doesn't actually
 	 * dirty the inode itself
diff --git a/fs/inode.c b/fs/inode.c
index 72c4c34..84e82db 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -388,12 +388,17 @@ void address_space_init_once(struct address_space *mapping)
  */
 void inode_init_once(struct inode *inode)
 {
+	int i;
+
 	memset(inode, 0, sizeof(*inode));
 	INIT_HLIST_NODE(&inode->i_hash);
 	INIT_LIST_HEAD(&inode->i_devices);
 	INIT_LIST_HEAD(&inode->i_io_list);
 	INIT_LIST_HEAD(&inode->i_wb_list);
 	INIT_LIST_HEAD(&inode->i_lru);
+	for (i = 0; i < INODE_MAX_NOTIFIER; i++)
+		BLOCKING_INIT_NOTIFIER_HEAD(
+			&inode->notifier_lists[MARK_INODE_DIRTY_NOTIFIER]);
 	__address_space_init_once(&inode->i_data);
 	i_size_ordered_init(inode);
 }
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 7519ae0..42f0750 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -607,6 +607,11 @@ static inline void mapping_allow_writable(struct address_space *mapping)
 
 struct fsnotify_mark_connector;
 
+enum {
+	MARK_INODE_DIRTY_NOTIFIER,
+	INODE_MAX_NOTIFIER,
+};
+
 /*
  * Keep mostly read-only and often accessed (especially for
  * the RCU path lookup and 'stat' data) fields at the beginning
@@ -723,6 +728,7 @@ struct inode {
 #endif
 
 	void			*i_private; /* fs or device private pointer */
+	struct	blocking_notifier_head notifier_lists[INODE_MAX_NOTIFIER];
 } __randomize_layout;
 
 struct timespec64 timestamp_truncate(struct timespec64 t, struct inode *inode);
-- 
1.8.3.1