[RFC PATCH 01/14] kernfs: add populate callbacks and KERNFS_LAZY flag for lazy dir population

[Pavol Sakac <sakacpav@amazon.de>]

Add populate() and populate_all() callbacks to struct
kernfs_syscall_ops:

  - populate() runs on a KERNFS_LAZY parent from
    kernfs_iop_lookup() before kernfs_rwsem is taken; the locked
    lookup then proceeds and its result is authoritative. The
    subsystem may materialize the named child during the call.

  - populate_all() runs from kernfs_fop_readdir() before
    kernfs_rwsem is taken. The subsystem may materialize all
    deferred children.

Both are invoked with the parent's active reference held by the
kernfs lookup path, so parent->priv stays live across the
(sleeping) callback. The callback must not drop this pin.

Add KERNFS_LAZY (0x8000) to enum kernfs_node_flag; non-lazy
directories skip the indirect call entirely.

Add kernfs_set_lazy() to mark a directory lazy between linking
into the parent tree and kernfs_activate(). It returns -EINVAL
for namespaced and non-directory nodes: populate dispatch does
not propagate the namespace tag, so a namespaced lazy node would
silently fail tagged lookups. The error code lets callers detect
contract violations without polluting dmesg with a stack trace
and lets KUnit cover the rejection paths via return-value
assertions rather than triggering WARN_ON_ONCE.

Add matching populate/populate_all fields to struct kobj_type.
These are sysfs-only; non-sysfs kernfs roots do not invoke them.

No behavior change: no subsystem sets these callbacks yet.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Rafael J. Wysocki <rafael@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-api@vger.kernel.org
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: driver-core@lists.linux.dev
Assisted-by: Claude:claude-opus-4.7
Signed-off-by: Pavol Sakac <sakacpav@amazon.de>
---
 fs/kernfs/dir.c         | 43 +++++++++++++++++++++++++++++++++++++++--
 include/linux/kernfs.h  | 28 +++++++++++++++++++++++++++
 include/linux/kobject.h | 10 ++++++++++
 3 files changed, 79 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 4f9ade82b08ab..9c5ec8b82d940 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -975,6 +975,20 @@ struct kernfs_node *kernfs_find_and_get_ns(struct kernfs_node *parent,
 }
 EXPORT_SYMBOL_GPL(kernfs_find_and_get_ns);
 
+int kernfs_set_lazy(struct kernfs_node *kn)
+{
+	struct kernfs_root *root = kernfs_root(kn);
+
+	if (kn->ns || (kn->flags & KERNFS_NS) ||
+	    kernfs_type(kn) != KERNFS_DIR)
+		return -EINVAL;
+
+	down_write(&root->kernfs_rwsem);
+	kn->flags |= KERNFS_LAZY;
+	up_write(&root->kernfs_rwsem);
+	return 0;
+}
+
 /**
  * kernfs_walk_and_get_ns - find and get kernfs_node with the given path
  * @parent: kernfs_node to search under
@@ -1255,6 +1269,19 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
 	struct kernfs_root *root;
 	struct inode *inode = NULL;
 	const struct ns_common *ns = NULL;
+	struct kernfs_syscall_ops *scops = kernfs_root(parent)->syscall_ops;
+	int populate_ret = 0;
+
+	/* Dispatch populate before taking kernfs_rwsem; kernfs_get_active()
+	 * pins @parent so @parent->priv stays live across the sleepable call.
+	 */
+	if (scops && scops->populate && (READ_ONCE(parent->flags) & KERNFS_LAZY)) {
+		if (kernfs_get_active(parent)) {
+			populate_ret = scops->populate(parent,
+						       dentry->d_name.name);
+			kernfs_put_active(parent);
+		}
+	}
 
 	root = kernfs_root(parent);
 	down_read(&root->kernfs_rwsem);
@@ -1285,6 +1312,10 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
 		kernfs_set_rev(parent, dentry);
 	up_read(&root->kernfs_rwsem);
 
+	/* Transient populate error: do not cache a negative dentry. */
+	if (!kn && populate_ret && populate_ret != -ENOENT)
+		return ERR_PTR(populate_ret);
+
 	/* instantiate and hash (possibly negative) dentry */
 	return d_splice_alias(inode, dentry);
 }
@@ -1973,13 +2004,21 @@ static int kernfs_fop_readdir(struct file *file, struct dir_context *ctx)
 	struct dentry *dentry = file->f_path.dentry;
 	struct kernfs_node *parent = kernfs_dentry_node(dentry);
 	struct kernfs_node *pos = file->private_data;
-	struct kernfs_root *root;
+	struct kernfs_root *root = kernfs_root(parent);
+	struct kernfs_syscall_ops *scops = root->syscall_ops;
 	const struct ns_common *ns = NULL;
 
 	if (!dir_emit_dots(file, ctx))
 		return 0;
 
-	root = kernfs_root(parent);
+	/* Same pinning invariant as kernfs_iop_lookup. */
+	if (scops && scops->populate_all && (READ_ONCE(parent->flags) & KERNFS_LAZY)) {
+		if (kernfs_get_active(parent)) {
+			scops->populate_all(parent);
+			kernfs_put_active(parent);
+		}
+	}
+
 	down_read(&root->kernfs_rwsem);
 
 	if (kernfs_ns_enabled(parent))
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index e21b2f7f4159f..cd529fd843831 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -113,6 +113,7 @@ enum kernfs_node_flag {
 	KERNFS_EMPTY_DIR	= 0x1000,
 	KERNFS_HAS_RELEASE	= 0x2000,
 	KERNFS_REMOVING		= 0x4000,
+	KERNFS_LAZY		= 0x8000,  /* subsystem defers child creation */
 };
 
 /* @flags for kernfs_create_root() */
@@ -239,6 +240,7 @@ struct kernfs_node {
  * kernfs_node parameter.
  */
 struct kernfs_syscall_ops {
+	/* Syscall-reactive callbacks */
 	int (*show_options)(struct seq_file *sf, struct kernfs_root *root);
 
 	int (*mkdir)(struct kernfs_node *parent, const char *name,
@@ -248,6 +250,12 @@ struct kernfs_syscall_ops {
 		      const char *new_name);
 	int (*show_path)(struct seq_file *sf, struct kernfs_node *kn,
 			 struct kernfs_root *root);
+
+	/* Lazy dispatch for KERNFS_LAZY directories; @parent is pinned
+	 * via kernfs_get_active() across the call. Both may sleep.
+	 */
+	int (*populate)(struct kernfs_node *parent, const char *name);
+	void (*populate_all)(struct kernfs_node *parent);
 };
 
 struct kernfs_node *kernfs_root_to_node(struct kernfs_root *root);
@@ -398,6 +406,24 @@ static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
 	return kn->flags & KERNFS_NS;
 }
 
+/**
+ * kernfs_set_lazy - mark a kernfs directory for lazy population
+ * @kn: directory kernfs_node (must not be namespaced)
+ *
+ * Sets KERNFS_LAZY under kernfs_rwsem to serialize with other
+ * kn->flags writers.  The flag is set once and never cleared.
+ *
+ * Populate dispatch does not propagate the namespace tag, so a
+ * namespaced lazy node would silently fail tagged lookups.  The
+ * function therefore rejects namespaced and non-directory nodes.
+ *
+ * Return:
+ * * %0           - on success.
+ * * %-EINVAL     - @kn is namespaced (@kn->ns non-NULL or KERNFS_NS
+ *                  flag set) or is not a directory.
+ */
+int kernfs_set_lazy(struct kernfs_node *kn);
+
 int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen);
 int kernfs_path_from_node(struct kernfs_node *kn_to, struct kernfs_node *kn_from,
 			  char *buf, size_t buflen);
@@ -481,6 +507,8 @@ static inline void kernfs_enable_ns(struct kernfs_node *kn) { }
 static inline bool kernfs_ns_enabled(struct kernfs_node *kn)
 { return false; }
 
+static inline int kernfs_set_lazy(struct kernfs_node *kn) { return 0; }
+
 static inline int kernfs_name(struct kernfs_node *kn, char *buf, size_t buflen)
 { return -ENOSYS; }
 
diff --git a/include/linux/kobject.h b/include/linux/kobject.h
index bcb5d4e320015..cec6de1720076 100644
--- a/include/linux/kobject.h
+++ b/include/linux/kobject.h
@@ -120,6 +120,16 @@ struct kobj_type {
 	const struct kobj_ns_type_operations *(*child_ns_type)(const struct kobject *kobj);
 	const struct ns_common *(*namespace)(const struct kobject *kobj);
 	void (*get_ownership)(const struct kobject *kobj, kuid_t *uid, kgid_t *gid);
+
+	/*
+	 * Lazy sysfs population. Invoked from kernfs lookup / readdir
+	 * paths only when the directory is flagged KERNFS_LAZY. May
+	 * sleep; @kobj is pinned across the call. populate() returns
+	 * 0 / -ENOENT / -errno (kernfs propagates non-ENOENT errors
+	 * to userspace). populate_all() is best-effort.
+	 */
+	int (*populate)(struct kobject *kobj, const char *name);
+	void (*populate_all)(struct kobject *kobj);
 };
 
 struct kobj_uevent_env {
-- 
2.47.3




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