[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>

4 files changed, 11 insertions, 10 deletions

diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
index fca83e28edcf..385bed09b0d8 100644
--- a/fs/autofs4/autofs_i.h
+++ b/fs/autofs4/autofs_i.h
@@ -209,7 +209,7 @@ static inline int simple_empty_nolock(struct dentry *dentry)
         struct dentry *child;
         int ret = 0;
 
-        list_for_each_entry(child, &dentry->d_subdirs, d_child)
+        list_for_each_entry(child, &dentry->d_subdirs, d_u.d_child)
                 if (simple_positive(child))
                         goto out;
         ret = 1;
diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index feb6ac427d05..dc39589df165 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -105,7 +105,7 @@ repeat:
         next = this_parent->d_subdirs.next;
 resume:
         while (next != &this_parent->d_subdirs) {
-                struct dentry *dentry = list_entry(next, struct dentry, d_child);
+                struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);
 
                 /* Negative dentry - give up */
                 if (!simple_positive(dentry)) {
@@ -138,7 +138,7 @@ resume:
         }
 
         if (this_parent != top) {
-                next = this_parent->d_child.next;
+                next = this_parent->d_u.d_child.next;
                 this_parent = this_parent->d_parent;
                 goto resume;
         }
@@ -163,7 +163,7 @@ repeat:
         next = this_parent->d_subdirs.next;
 resume:
         while (next != &this_parent->d_subdirs) {
-                struct dentry *dentry = list_entry(next, struct dentry, d_child);
+                struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);
 
                 /* Negative dentry - give up */
                 if (!simple_positive(dentry)) {
@@ -199,7 +199,7 @@ cont:
         }
 
         if (this_parent != parent) {
-                next = this_parent->d_child.next;
+                next = this_parent->d_u.d_child.next;
                 this_parent = this_parent->d_parent;
                 goto resume;
         }
@@ -238,7 +238,7 @@ static struct dentry *autofs4_expire(struct super_block *sb,
         /* On exit from the loop expire is set to a dgot dentry
          * to expire or it's NULL */
         while ( next != &root->d_subdirs ) {
-                struct dentry *dentry = list_entry(next, struct dentry, d_child);
+                struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);
 
                 /* Negative dentry - give up */
                 if ( !simple_positive(dentry) ) {
@@ -302,7 +302,7 @@ next:
                         expired, (int)expired->d_name.len, expired->d_name.name);
                 spin_lock(&dcache_lock);
                 list_del(&expired->d_parent->d_subdirs);
-                list_add(&expired->d_parent->d_subdirs, &expired->d_child);
+                list_add(&expired->d_parent->d_subdirs, &expired->d_u.d_child);
                 spin_unlock(&dcache_lock);
                 return expired;
         }
diff --git a/fs/autofs4/inode.c b/fs/autofs4/inode.c
index 818b37be5153..2d3082854a29 100644
--- a/fs/autofs4/inode.c
+++ b/fs/autofs4/inode.c
@@ -91,7 +91,7 @@ repeat:
         next = this_parent->d_subdirs.next;
 resume:
         while (next != &this_parent->d_subdirs) {
-                struct dentry *dentry = list_entry(next, struct dentry, d_child);
+                struct dentry *dentry = list_entry(next, struct dentry, d_u.d_child);
 
                 /* Negative dentry - don`t care */
                 if (!simple_positive(dentry)) {
@@ -117,7 +117,7 @@ resume:
         if (this_parent != sbi->root) {
                 struct dentry *dentry = this_parent;
 
-                next = this_parent->d_child.next;
+                next = this_parent->d_u.d_child.next;
                 this_parent = this_parent->d_parent;
                 spin_unlock(&dcache_lock);
                 DPRINTK("parent dentry %p %.*s",
diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
index 2a771ec66956..2241405ffc41 100644
--- a/fs/autofs4/root.c
+++ b/fs/autofs4/root.c
@@ -143,7 +143,8 @@ static int autofs4_dcache_readdir(struct file * filp, void * dirent, filldir_t f
                         }
 
                         while(1) {
-                                struct dentry *de = list_entry(list, struct dentry, d_child);
+                                struct dentry *de = list_entry(list,
+                                                struct dentry, d_u.d_child);
 
                                 if (!d_unhashed(de) && de->d_inode) {
                                         spin_unlock(&dcache_lock);