[PATCH] shrink dentry struct

Some long time ago, dentry struct was carefully tuned so that on 32 bits
UP, sizeof(struct dentry) was exactly 128, ie a power of 2, and a multiple
of memory cache lines.

Then RCU was added and dentry struct enlarged by two pointers, with nice
results for SMP, but not so good on UP, because breaking the above tuning
(128 + 8 = 136 bytes)

This patch reverts this unwanted side effect, by using an union (d_u),
where d_rcu and d_child are placed so that these two fields can share their
memory needs.

At the time d_free() is called (and d_rcu is really used), d_child is known
to be empty and not touched by the dentry freeing.

Lockless lookups only access d_name, d_parent, d_lock, d_op, d_flags (so
the previous content of d_child is not needed if said dentry was unhashed
but still accessed by a CPU because of RCU constraints)

As dentry cache easily contains millions of entries, a size reduction is
worth the extra complexity of the ugly C union.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: Maneesh Soni <maneesh@in.ibm.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Ian Kent <raven@themaw.net>
Cc: Paul Jackson <pj@sgi.com>
Cc: Al Viro <viro@ftp.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: Neil Brown <neilb@cse.unsw.edu.au>
Cc: James Morris <jmorris@namei.org>
Cc: Stephen Smalley <sds@epoch.ncsc.mil>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 7 insertions, 2 deletions

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 46a2ba617595..a3ed5e059d47 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -95,14 +95,19 @@ struct dentry {
         struct qstr d_name;
 
         struct list_head d_lru;         /* LRU list */
-        struct list_head d_child;       /* child of parent list */
+        /*
+         * d_child and d_rcu can share memory
+         */
+        union {
+                struct list_head d_child;       /* child of parent list */
+                struct rcu_head d_rcu;
+        } d_u;
         struct list_head d_subdirs;     /* our children */
         struct list_head d_alias;       /* inode alias list */
         unsigned long d_time;           /* used by d_revalidate */
         struct dentry_operations *d_op;
         struct super_block *d_sb;       /* The root of the dentry tree */
         void *d_fsdata;                 /* fs-specific data */
-        struct rcu_head d_rcu;
         struct dcookie_struct *d_cookie; /* cookie, if any */
         int d_mounted;
         unsigned char d_iname[DNAME_INLINE_LEN_MIN];    /* small names */