[PATCH v5 2/6] kernfs: add a revision to identify directory node changes

Ian Kent
2021-06-07T10:12:06Z

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

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 | 8 ++++++++
fs/kernfs/kernfs-internal.h | 24 ++++++++++++++++++++++++
include/linux/kernfs.h | 5 +++++
3 files changed, 37 insertions(+)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 33166ec90a112..b88432c48851f 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);
@@ -1105,6 +1107,12 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,

/* instantiate and hash dentry */
ret = d_splice_alias(inode, dentry);
+ if (!IS_ERR(ret)) {
+ if (unlikely(ret))
+ kernfs_set_rev(parent, ret);
+ else
+ kernfs_set_rev(parent, dentry);
+ }
out_unlock:
mutex_unlock(&kernfs_mutex);
return ret;
diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index ccc3b44f6306f..1536002584fc4 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -81,6 +81,30 @@ 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 *kn,
+ struct dentry *dentry)
+{
+ if (kernfs_type(kn) == KERNFS_DIR)
+ dentry->d_time = kn->dir.rev;
+}
+
+static inline void kernfs_inc_rev(struct kernfs_node *kn)
+{
+ if (kernfs_type(kn) == KERNFS_DIR)
+ kn->dir.rev++;
+}
+
+static inline bool kernfs_dir_changed(struct kernfs_node *kn,
+ struct dentry *dentry)
+{
+ if (kernfs_type(kn) == KERNFS_DIR) {
+ /* Not really a time bit it does what's needed */
+ if (time_after(kn->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..7947acb1163d7 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 revalidation.
+ */
+ unsigned long rev;
};

struct kernfs_elem_symlink {