kernfs: add a revision to identify directory node changes

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>

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 33166ec..b3d1bc0 100644 (file)
--- 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);

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index ccc3b44..c2ae58f 100644 (file)
--- 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 9e8ca87..d68b4ad 100644 (file)
--- 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 {