[PATCH v2 4/6] kernfs: use revision to identify directory node changes

Ian Kent
2020-06-17T07:38:32Z

If a kernfs directory node hasn't changed there's no need to search for
an added (or removed) child dentry.

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

Signed-off-by: Ian Kent <raven@themaw.net>
---
fs/kernfs/dir.c | 17 +++++++++++++++--
fs/kernfs/kernfs-internal.h | 24 ++++++++++++++++++++++++
include/linux/kernfs.h | 5 +++++
3 files changed, 44 insertions(+), 2 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index f4943329e578..03f4f179bbc4 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -383,6 +383,7 @@ 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;
}
@@ -405,6 +406,7 @@ 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);
@@ -1044,9 +1046,16 @@ struct kernfs_node *kernfs_create_empty_dir(struct kernfs_node *parent,

static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
{
+ struct kernfs_node *parent;
struct kernfs_node *kn;

if (flags & LOOKUP_RCU) {
+ /* Directory node changed? */
+ parent = kernfs_dentry_node(dentry->d_parent);
+
+ if (!kernfs_dir_changed(parent, dentry))
+ return 1;
+
kn = kernfs_dentry_node(dentry);
if (!kn) {
/* Negative hashed dentry, tell the VFS to switch to
@@ -1093,8 +1102,6 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)

kn = kernfs_dentry_node(dentry);
if (!kn) {
- struct kernfs_node *parent;
-
/* If the kernfs node can be found this is a stale negative
* hashed dentry so it must be discarded and the lookup redone.
*/
@@ -1102,6 +1109,10 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
if (parent) {
const void *ns = NULL;

+ /* Directory node changed? */
+ if (kernfs_dir_changed(parent, dentry))
+ goto out_bad;
+
if (kernfs_ns_enabled(parent))
ns = kernfs_info(dentry->d_parent->d_sb)->ns;
kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
@@ -1156,6 +1167,8 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,

down_read(&kernfs_rwsem);

+ kernfs_set_rev(dentry, parent);
+
if (kernfs_ns_enabled(parent))
ns = kernfs_info(dir->i_sb)->ns;

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 097c1a989aa4..a7b0e2074260 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -82,6 +82,30 @@ static inline struct kernfs_node *kernfs_dentry_node(struct dentry *dentry)
return d_inode(dentry)->i_private;
}

+static inline void kernfs_set_rev(struct dentry *dentry,
+ struct kernfs_node *kn)
+{
+ dentry->d_time = kn->dir.rev;
+}
+
+static inline void kernfs_inc_rev(struct kernfs_node *kn)
+{
+ if (kernfs_type(kn) == KERNFS_DIR) {
+ if (!++kn->dir.rev)
+ kn->dir.rev++;
+ }
+}
+
+static inline bool kernfs_dir_changed(struct kernfs_node *kn,
+ struct dentry *dentry)
+{
+ if (kernfs_type(kn) == KERNFS_DIR) {
+ if (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 89f6a4214a70..74727d98e380 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 {