[PATCH] reduce sizeof(struct file)

Now that RCU applied on 'struct file' seems stable, we can place f_rcuhead
in a memory location that is not anymore used at call_rcu(&f->f_rcuhead,
file_free_rcu) time, to reduce the size of this critical kernel object.

The trick I used is to move f_rcuhead and f_list in an union called f_u

The callers are changed so that f_rcuhead becomes f_u.fu_rcuhead and f_list
becomes f_u.f_list

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 8 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index f83d997c5582..6d6226732c93 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -574,7 +574,14 @@ struct file_ra_state {
 #define RA_FLAG_INCACHE 0x02    /* file is already in cache */
 
 struct file {
-        struct list_head        f_list;
+        /*
+         * fu_list becomes invalid after file_free is called and queued via
+         * fu_rcuhead for RCU freeing
+         */
+        union {
+                struct list_head        fu_list;
+                struct rcu_head         fu_rcuhead;
+        } f_u;
         struct dentry           *f_dentry;
         struct vfsmount         *f_vfsmnt;
         struct file_operations  *f_op;
@@ -598,7 +605,6 @@ struct file {
         spinlock_t              f_ep_lock;
 #endif /* #ifdef CONFIG_EPOLL */
         struct address_space    *f_mapping;
-        struct rcu_head         f_rcuhead;
 };
 extern spinlock_t files_lock;
 #define file_list_lock() spin_lock(&files_lock);