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Subject: [PATCH v7 2/6] kernfs: add a revision to identify directory node
 changes
From:   Ian Kent <raven@themaw.net>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Tejun Heo <tj@kernel.org>
Cc:     Eric Sandeen <sandeen@sandeen.net>, Fox Chen <foxhlchen@gmail.com>,
        Brice Goglin <brice.goglin@gmail.com>,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Rick Lindsley <ricklind@linux.vnet.ibm.com>,
        David Howells <dhowells@redhat.com>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Carlos Maiolino <cmaiolino@redhat.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:   Tue, 15 Jun 2021 18:26:07 +0800
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In-Reply-To: <162375263398.232295.14755578426619198534.stgit@web.messagingengine.com>
References: <162375263398.232295.14755578426619198534.stgit@web.messagingengine.com>
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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.

Signed-off-by: Ian Kent <raven@themaw.net>
---
 fs/kernfs/dir.c             |    3 +++
 fs/kernfs/kernfs-internal.h |   19 +++++++++++++++++++
 include/linux/kernfs.h      |    5 +++++
 3 files changed, 27 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 33166ec90a112..48704c5b6a072 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -372,6 +372,7 @@ 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;
 }
@@ -394,6 +395,7 @@ 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);
@@ -1573,6 +1575,7 @@ int kernfs_rename_ns(struct kernfs_node *kn, struct kernfs_node *new_parent,
 	 */
 	kernfs_unlink_sibling(kn);
 	kernfs_get(new_parent);
+	kernfs_inc_rev(new_parent);
 
 	/* rename_lock protects ->parent and ->name accessors */
 	spin_lock_irq(&kernfs_rename_lock);
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index ccc3b44f6306f..6a38897b4d02c 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 9e8ca8743c268..d68b4ad095737 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 {