speed up writes

Speeds up generic_file_write() by not calling mark_inode_dirty() when the mtime and ctime didn't change.

There may be concerns over the fact that this restricts mtime and ctime updates to one-second resolution. But the interface doesn't support that anyway - all the filesystem knows is that its dirty_inode() superop was called. It doesn't know why.

So filesystems which support high-resolution timestamps already need to make their own arrangements. We need an update_mtime i_op to support those properly.

time to write a one megabyte file one-byte-at-a-time:

Before:

	ext3:		24.8 seconds
	ext2:		 4.9 seconds
	reiserfs:	17.0 seconds
After:
	ext3:		22.5 seconds
	ext2:		4.8  seconds
	reiserfs:	11.6 seconds

Not much improvement because we're also calling expensive mark_inode_dirty() functions when i_size is expanded. So compare the overwrite case:

time dd if=/dev/zero of=foo bs=1 count=1M conv=notrunc

ext3 before: 20.0 seconds
ext3 after: 9.7 seconds

=====================================

@@ -2098,6 +2098,7 @@ generic_file_write(struct file *file, co
 	ssize_t		written;
 	int		err;
 	unsigned	bytes;
+	time_t		time_now;
 
 	if (unlikely((ssize_t) count < 0))
 		return -EINVAL;

@@ -2195,9 +2196,12 @@ generic_file_write(struct file *file, co
 		goto out;
 
 	remove_suid(file->f_dentry);
-	inode->i_ctime = CURRENT_TIME;
-	inode->i_mtime = CURRENT_TIME;
-	mark_inode_dirty_sync(inode);
+	time_now = CURRENT_TIME;
+	if (inode->i_ctime != time_now || inode->i_mtime != time_now) {
+		inode->i_ctime = time_now;
+		inode->i_mtime = time_now;
+		mark_inode_dirty_sync(inode);
+	}
 
 	if (unlikely(file->f_flags & O_DIRECT)) {
 		written = generic_file_direct_IO(WRITE, file,

-