From: Ian Kent <raven@themaw.net>

Commit 895adbec302e92086359e6fd92611ac3be6d92c3 upstream.

Add a revision counter to kernfs directory nodes so it can be used
to detect if a directory node has changed during negative dentry
revalidation.

There's an assumption that sizeof(unsigned long) <= sizeof(pointer)
on all architectures and as far as I know that assumption holds.

So adding a revision counter to the struct kernfs_elem_dir variant of
the kernfs_node type union won't increase the size of the kernfs_node
struct. This is because struct kernfs_elem_dir is at least
sizeof(pointer) smaller than the largest union variant. It's tempting
to make the revision counter a u64 but that would increase the size of
kernfs_node on archs where sizeof(pointer) is smaller than the revision
counter.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Link: https://lore.kernel.org/r/162642769895.63632.8356662784964509867.stgit@web.messagingengine.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/kernfs/dir.c             |  2 ++
 fs/kernfs/kernfs-internal.h | 19 +++++++++++++++++++
 include/linux/kernfs.h      |  5 +++++
 3 files changed, 26 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 0ba056e06e48..9bc73c8b6e3f 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -380,6 +380,7 @@ static int kernfs_link_sibling(struct kernfs_node *kn)
 	/* successfully added, account subdir number */
 	if (kernfs_type(kn) == KERNFS_DIR)
 		kn->parent->dir.subdirs++;
+	kernfs_inc_rev(kn->parent);
 
 	return 0;
 }
@@ -402,6 +403,7 @@ static bool kernfs_unlink_sibling(struct kernfs_node *kn)
 
 	if (kernfs_type(kn) == KERNFS_DIR)
 		kn->parent->dir.subdirs--;
+	kernfs_inc_rev(kn->parent);
 
 	rb_erase(&kn->rb, &kn->parent->dir.children);
 	RB_CLEAR_NODE(&kn->rb);
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 7ee97ef59184..6a8d0ca26d03 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -81,6 +81,25 @@ static inline struct kernfs_node *kernfs_dentry_node(struct dentry *dentry)
 	return d_inode(dentry)->i_private;
 }
 
+static inline void kernfs_set_rev(struct kernfs_node *parent,
+				  struct dentry *dentry)
+{
+	dentry->d_time = parent->dir.rev;
+}
+
+static inline void kernfs_inc_rev(struct kernfs_node *parent)
+{
+	parent->dir.rev++;
+}
+
+static inline bool kernfs_dir_changed(struct kernfs_node *parent,
+				      struct dentry *dentry)
+{
+	if (parent->dir.rev != dentry->d_time)
+		return true;
+	return false;
+}
+
 extern const struct super_operations kernfs_sops;
 extern struct kmem_cache *kernfs_node_cache, *kernfs_iattrs_cache;
 
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 89f6a4214a70..195afa63ab1c 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -98,6 +98,11 @@ struct kernfs_elem_dir {
 	 * better directly in kernfs_node but is here to save space.
 	 */
 	struct kernfs_root	*root;
+	/*
+	 * Monotonic revision counter, used to identify if a directory
+	 * node has changed during negative dentry revalidation.
+	 */
+	unsigned long		rev;
 };
 
 struct kernfs_elem_symlink {
-- 
2.43.0

On Wed, May 21, 2025 at 09:53:31AM +0800, Qingfang Deng wrote:
> From: Ian Kent <raven@themaw.net>
> 
> Commit 895adbec302e92086359e6fd92611ac3be6d92c3 upstream.
> 
> Add a revision counter to kernfs directory nodes so it can be used
> to detect if a directory node has changed during negative dentry
> revalidation.
> 
> There's an assumption that sizeof(unsigned long) <= sizeof(pointer)
> on all architectures and as far as I know that assumption holds.
> 
> So adding a revision counter to the struct kernfs_elem_dir variant of
> the kernfs_node type union won't increase the size of the kernfs_node
> struct. This is because struct kernfs_elem_dir is at least
> sizeof(pointer) smaller than the largest union variant. It's tempting
> to make the revision counter a u64 but that would increase the size of
> kernfs_node on archs where sizeof(pointer) is smaller than the revision
> counter.
> 
> Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
> Signed-off-by: Ian Kent <raven@themaw.net>
> Link: https://lore.kernel.org/r/162642769895.63632.8356662784964509867.stgit@web.messagingengine.com
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

When forwarding on patches, you HAVE to sign off on them as well.

thanks,

greg k-h