[PATCH v2 3/6] kernfs: improve kernfs path resolution

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

Now that an rwsem is used by kernfs, take advantage of it to reduce
lookup overhead.

If there are many lookups (possibly many negative ones) there can
be a lot of overhead during path walks.

To reduce lookup overhead avoid allocating a new dentry where possible.

To do this stay in rcu-walk mode where possible and use the dentry cache
handling of negative hashed dentries to avoid allocating (and freeing
shortly after) new dentries on every negative lookup.

Signed-off-by: Ian Kent <raven@themaw.net>
---
fs/kernfs/dir.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++---------
1 file changed, 72 insertions(+), 15 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index 9b315f3b20ee..f4943329e578 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -1046,15 +1046,75 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
{
struct kernfs_node *kn;

- if (flags & LOOKUP_RCU)
+ if (flags & LOOKUP_RCU) {
+ kn = kernfs_dentry_node(dentry);
+ if (!kn) {
+ /* Negative hashed dentry, tell the VFS to switch to
+ * ref-walk mode and call us again so that node
+ * existence can be checked.
+ */
+ if (!d_unhashed(dentry))
+ return -ECHILD;
+
+ /* Negative unhashed dentry, this shouldn't happen
+ * because this case occurs in rcu-walk mode after
+ * dentry allocation which is followed by a call
+ * to ->loopup(). But if it does happen the dentry
+ * is surely invalid.
+ */
+ return 0;
+ }
+
+ /* Since the dentry is positive (we got the kernfs node) a
+ * kernfs node reference was held at the time. Now if the
+ * dentry reference count is still greater than 0 it's still
+ * positive so take a reference to the node to perform an
+ * active check.
+ */
+ if (d_count(dentry) <= 0 || !atomic_inc_not_zero(&kn->count))
+ return -ECHILD;
+
+ /* The kernfs node reference count was greater than 0, if
+ * it's active continue in rcu-walk mode.
+ */
+ if (kernfs_active_read(kn)) {
+ kernfs_put(kn);
+ return 1;
+ }
+
+ /* Otherwise, just tell the VFS to switch to ref-walk mode
+ * and call us again so the kernfs node can be validated.
+ */
+ kernfs_put(kn);
return -ECHILD;
+ }

- /* Always perform fresh lookup for negatives */
- if (d_really_is_negative(dentry))
- goto out_bad_unlocked;
+ down_read(&kernfs_rwsem);

kn = kernfs_dentry_node(dentry);
- down_read(&kernfs_rwsem);
+ 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.
+ */
+ parent = kernfs_dentry_node(dentry->d_parent);
+ if (parent) {
+ const void *ns = NULL;
+
+ if (kernfs_ns_enabled(parent))
+ ns = kernfs_info(dentry->d_parent->d_sb)->ns;
+ kn = kernfs_find_ns(parent, dentry->d_name.name, ns);
+ if (kn)
+ goto out_bad;
+ }
+
+ /* The kernfs node doesn't exist, leave the dentry negative
+ * and return success.
+ */
+ goto out;
+ }
+

/* The kernfs node has been deactivated */
if (!kernfs_active_read(kn))
@@ -1072,12 +1132,11 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
if (kn->parent && kernfs_ns_enabled(kn->parent) &&
kernfs_info(dentry->d_sb)->ns != kn->ns)
goto out_bad;
-
+out:
up_read(&kernfs_rwsem);
return 1;
out_bad:
up_read(&kernfs_rwsem);
-out_bad_unlocked:
return 0;
}

@@ -1092,7 +1151,7 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
struct dentry *ret;
struct kernfs_node *parent = dir->i_private;
struct kernfs_node *kn;
- struct inode *inode;
+ struct inode *inode = NULL;
const void *ns = NULL;

down_read(&kernfs_rwsem);
@@ -1102,11 +1161,9 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,

kn = kernfs_find_ns(parent, dentry->d_name.name, ns);

- /* no such entry */
- if (!kn || !kernfs_active(kn)) {
- ret = NULL;
- goto out_unlock;
- }
+ /* no such entry, retain as negative hashed dentry */
+ if (!kn || !kernfs_active(kn))
+ goto out_negative;

/* attach dentry and inode */
inode = kernfs_get_inode(dir->i_sb, kn);
@@ -1114,10 +1171,10 @@ static struct dentry *kernfs_iop_lookup(struct inode *dir,
ret = ERR_PTR(-ENOMEM);
goto out_unlock;
}
-
+out_negative:
/* instantiate and hash dentry */
ret = d_splice_alias(inode, dentry);
- out_unlock:
+out_unlock:
up_read(&kernfs_rwsem);
return ret;
}