[PATCH 2/8] fs/dcache: use rcu_ptr instead of rcu_head for external names

["Harry Yoo (Oracle)" <harry@kernel.org>]

When a file name length exceeds 31 (DCACHE_INLINE_LEN-1),
struct external_name is dynamically allocated. Because only kfree_rcu()
is used to free the objects, struct rcu_ptr is enough and saves a pointer
per object.

Under the author's home directory, there are 230k unique file names that
are longer than 31. Some memory saving benefit is expected.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Harry Yoo (Oracle) <harry@kernel.org>
---
 fs/dcache.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 7ba1801d8132..fa37e3964b38 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -325,12 +325,12 @@ static inline int dentry_cmp(const struct dentry *dentry, const unsigned char *c
 /*
  * long names are allocated separately from dentry and never modified.
  * Refcounted, freeing is RCU-delayed.  See take_dentry_name_snapshot()
- * for the reason why ->count and ->head can't be combined into a union.
+ * for the reason why ->count and ->rcu can't be combined into a union.
  * dentry_string_cmp() relies upon ->name[] being word-aligned.
  */
 struct external_name {
 	atomic_t count;
-	struct rcu_head head;
+	struct rcu_ptr rcu;
 	unsigned char name[] __aligned(sizeof(unsigned long));
 };
 
@@ -393,7 +393,7 @@ void release_dentry_name_snapshot(struct name_snapshot *name)
 		struct external_name *p;
 		p = container_of(name->name.name, struct external_name, name[0]);
 		if (unlikely(atomic_dec_and_test(&p->count)))
-			kfree_rcu(p, head);
+			kfree_rcu(p, rcu);
 	}
 }
 EXPORT_SYMBOL(release_dentry_name_snapshot);
@@ -2863,7 +2863,7 @@ static void copy_name(struct dentry *dentry, struct dentry *target)
 		dentry->__d_name.hash_len = target->__d_name.hash_len;
 	}
 	if (old_name && likely(atomic_dec_and_test(&old_name->count)))
-		kfree_rcu(old_name, head);
+		kfree_rcu(old_name, rcu);
 }
 
 /*
-- 
2.43.0