Currently some kernfs files (e.g. cgroup.events, memory.events) support
inotify watches for IN_MODIFY, but unlike with regular filesystems, they
do not receive IN_DELETE_SELF or IN_IGNORED events when they are
removed.

This creates a problem for processes monitoring cgroups. For example, a
service monitoring memory.events for memory.high breaches needs to know
when a cgroup is removed to clean up its state. Where it's known that a
cgroup is removed when all processes die, without IN_DELETE_SELF the
service must resort to inefficient workarounds such as:
1.  Periodically scanning procfs to detect process death (wastes CPU and
    is susceptible to PID reuse).
2.  Placing an additional IN_DELETE watch on the parent directory
    (wastes resources managing double the watches).
3.  Holding a pidfd for every monitored cgroup (can exhaust file
    descriptors).

This patch enables kernfs to send IN_DELETE_SELF and IN_IGNORED events.
This allows applications to rely on a single existing watch on the file
of interest (e.g. memory.events) to receive notifications for both
modifications and the eventual removal of the file, as well as automatic
watch descriptor cleanup, simplifying userspace logic and improving
resource efficiency.

Implementation details:
The kernfs notification worker is updated to handle file deletion.
fsnotify handles sending MODIFY events to both a watched file and its
parent, but it does not handle sending a DELETE event to the parent and
a DELETE_SELF event to the watched file in a single call. Therefore,
separate fsnotify calls are made: one for the parent (DELETE) and one
for the child (DELETE_SELF), while retaining the optimized single call
for MODIFY events.

Signed-off-by: T.J. Mercier <tjmercier@google.com>
---
 fs/kernfs/dir.c             | 21 +++++++++++++++++++++
 fs/kernfs/file.c            | 29 +++++++++++++++++++++++------
 fs/kernfs/kernfs-internal.h |  3 +++
 3 files changed, 47 insertions(+), 6 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 29baeeb97871..74a4c347b78a 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -9,6 +9,7 @@
 
 #include <linux/sched.h>
 #include <linux/fs.h>
+#include <linux/fsnotify_backend.h>
 #include <linux/namei.h>
 #include <linux/idr.h>
 #include <linux/slab.h>
@@ -1471,6 +1472,23 @@ void kernfs_show(struct kernfs_node *kn, bool show)
 	up_write(&root->kernfs_rwsem);
 }
 
+static void kernfs_notify_file_deleted(struct kernfs_node *kn)
+{
+	static DECLARE_WORK(kernfs_notify_deleted_work,
+			    kernfs_notify_workfn);
+
+	guard(spinlock_irqsave)(&kernfs_notify_lock);
+	/* may overwite already pending FS_MODIFY events */
+	kn->attr.notify_event = FS_DELETE;
+
+	if (!kn->attr.notify_next) {
+		kernfs_get(kn);
+		kn->attr.notify_next = kernfs_notify_list;
+		kernfs_notify_list = kn;
+		schedule_work(&kernfs_notify_deleted_work);
+	}
+}
+
 static void __kernfs_remove(struct kernfs_node *kn)
 {
 	struct kernfs_node *pos, *parent;
@@ -1520,6 +1538,9 @@ static void __kernfs_remove(struct kernfs_node *kn)
 			struct kernfs_iattrs *ps_iattr =
 				parent ? parent->iattr : NULL;
 
+			if (kernfs_type(kn) == KERNFS_FILE)
+				kernfs_notify_file_deleted(pos);
+
 			/* update timestamps on the parent */
 			down_write(&kernfs_root(kn)->kernfs_iattr_rwsem);
 
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index e978284ff983..3e813b09ab05 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -37,8 +37,8 @@ struct kernfs_open_node {
  */
 #define KERNFS_NOTIFY_EOL			((void *)&kernfs_notify_list)
 
-static DEFINE_SPINLOCK(kernfs_notify_lock);
-static struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;
+DEFINE_SPINLOCK(kernfs_notify_lock);
+struct kernfs_node *kernfs_notify_list = KERNFS_NOTIFY_EOL;
 
 static inline struct mutex *kernfs_open_file_mutex_ptr(struct kernfs_node *kn)
 {
@@ -909,12 +909,21 @@ static loff_t kernfs_fop_llseek(struct file *file, loff_t offset, int whence)
 	return ret;
 }
 
-static void kernfs_notify_workfn(struct work_struct *work)
+static int fsnotify_self_event(int event)
+{
+	if (event == FS_DELETE)
+		return FS_DELETE_SELF;
+
+	return event;
+}
+
+void kernfs_notify_workfn(struct work_struct *work)
 {
 	struct kernfs_node *kn;
 	struct kernfs_super_info *info;
 	struct kernfs_root *root;
 	u32 notify_event;
+	u32 self_event;
 repeat:
 	/* pop one off the notify_list */
 	spin_lock_irq(&kernfs_notify_lock);
@@ -929,6 +938,8 @@ static void kernfs_notify_workfn(struct work_struct *work)
 	kn->attr.notify_event = 0;
 	spin_unlock_irq(&kernfs_notify_lock);
 
+	self_event = fsnotify_self_event(notify_event);
+
 	root = kernfs_root(kn);
 	/* kick fsnotify */
 
@@ -959,15 +970,21 @@ static void kernfs_notify_workfn(struct work_struct *work)
 			if (p_inode) {
 				fsnotify(notify_event | FS_EVENT_ON_CHILD,
 					 inode, FSNOTIFY_EVENT_INODE,
-					 p_inode, &name, inode, 0);
+					 p_inode, &name,
+					 (notify_event == self_event) ?
+						inode : NULL, 0);
 				iput(p_inode);
 			}
 
 			kernfs_put(parent);
 		}
 
-		if (!p_inode)
-			fsnotify_inode(inode, notify_event);
+		if (!p_inode || self_event != notify_event)
+			fsnotify_inode(inode, self_event);
+
+		/* For IN_IGNORED, and automatic watch descriptor removal */
+		if (self_event == FS_DELETE_SELF)
+			fsnotify_inode_delete(inode);
 
 		iput(inode);
 	}
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 6061b6f70d2a..cf4b21f4f3b6 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -199,6 +199,8 @@ struct kernfs_node *kernfs_new_node(struct kernfs_node *parent,
  * file.c
  */
 extern const struct file_operations kernfs_file_fops;
+extern struct kernfs_node *kernfs_notify_list;
+extern void kernfs_notify_workfn(struct work_struct *work);
 
 bool kernfs_should_drain_open_files(struct kernfs_node *kn);
 void kernfs_drain_open_files(struct kernfs_node *kn);
@@ -212,4 +214,5 @@ extern const struct inode_operations kernfs_symlink_iops;
  * kernfs locks
  */
 extern struct kernfs_global_locks *kernfs_locks;
+extern spinlock_t kernfs_notify_lock;
 #endif	/* __KERNFS_INTERNAL_H */
-- 
2.53.0.rc2.204.g2597b5adb4-goog