fs: optimize out 16 bytes worth of padding in struct inode

Rearrange the fields in struct inode so that on an x86_64 system,
fields that require 8-byte alignment don't end up causing 4-byte holes
in the structure.  It reduces the size of struct inode from 568 bytes
to 552 bytes.

Also move the fields protected by i_lock (i_blocks, i_bytes, and
i_size) into the same cache line as i_lock.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 178cdb4..fbda9a6 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -771,12 +771,13 @@ struct inode {
        unsigned long           i_ino;
        unsigned int            i_nlink;
        dev_t                   i_rdev;
-       loff_t                  i_size;
        struct timespec         i_atime;
        struct timespec         i_mtime;
        struct timespec         i_ctime;
-       unsigned int            i_blkbits;
+       spinlock_t              i_lock; /* i_blocks, i_bytes, maybe i_size */
+       unsigned short          i_bytes;
        blkcnt_t                i_blocks;
+       loff_t                  i_size;
 
 #ifdef __NEED_I_SIZE_ORDERED
        seqcount_t              i_size_seqcount;
@@ -784,7 +785,6 @@ struct inode {
 
        /* Misc */
        unsigned long           i_state;
-       spinlock_t              i_lock; /* i_blocks, i_bytes, maybe i_size */
        struct mutex            i_mutex;
 
        unsigned long           dirtied_when;   /* jiffies of first dirtying */
@@ -798,9 +798,10 @@ struct inode {
                struct rcu_head         i_rcu;
        };
        atomic_t                i_count;
+       unsigned int            i_blkbits;
        u64                     i_version;
-       unsigned short          i_bytes;
        atomic_t                i_dio_count;
+       atomic_t                i_writecount;
        const struct file_operations    *i_fop; /* former ->i_op->default_file_ops */
        struct file_lock        *i_flock;
        struct address_space    i_data;
@@ -824,7 +825,6 @@ struct inode {
 #ifdef CONFIG_IMA
        atomic_t                i_readcount; /* struct files open RO */
 #endif
-       atomic_t                i_writecount;
        void                    *i_private; /* fs or device private pointer */
 };