[PATCH] [2/2] Optimize touch_time too

[Andi Kleen <andi@firstfloor.org>]

Do a similar optimization as earlier for touch_atime. Getting
the lock in mnt_get_write is relatively costly, so try all
avenues to avoid it first.

This patch is careful to still only update inode fields
inside the lock region.

This didn't show up in benchmarks, but it's easy enough
to do.

Signed-off-by: Andi Kleen <ak@linux.intel.com>

---
 fs/inode.c |   43 +++++++++++++++++++++++--------------------
 1 file changed, 23 insertions(+), 20 deletions(-)

Index: linux-2.6.31-rc1-ak/fs/inode.c
===================================================================
--- linux-2.6.31-rc1-ak.orig/fs/inode.c
+++ linux-2.6.31-rc1-ak/fs/inode.c
@@ -1431,34 +1431,37 @@ void file_update_time(struct file *file)
 {
 	struct inode *inode = file->f_path.dentry->d_inode;
 	struct timespec now;
-	int sync_it = 0;
-	int err;
+	enum { S_MTIME = 1, S_CTIME = 2, S_VERSION = 4 } sync_it = 0;
 
+	/* First try to exhause all avenues to not sync */
 	if (IS_NOCMTIME(inode))
 		return;
 
-	err = mnt_want_write_file(file);
-	if (err)
-		return;
-
 	now = current_fs_time(inode->i_sb);
-	if (!timespec_equal(&inode->i_mtime, &now)) {
-		inode->i_mtime = now;
-		sync_it = 1;
-	}
+	if (!timespec_equal(&inode->i_mtime, &now))
+		sync_it = S_MTIME;
 
-	if (!timespec_equal(&inode->i_ctime, &now)) {
-		inode->i_ctime = now;
-		sync_it = 1;
-	}
+	if (!timespec_equal(&inode->i_ctime, &now))
+		sync_it |= S_CTIME;
 
-	if (IS_I_VERSION(inode)) {
-		inode_inc_iversion(inode);
-		sync_it = 1;
-	}
+	if (IS_I_VERSION(inode))
+		sync_it |= S_VERSION;
+
+	if (!sync_it)
+		return;
+
+	/* Finally allowed to write? Takes lock. */
+	if (!mnt_want_write_file(file))
+		return;
 
-	if (sync_it)
-		mark_inode_dirty_sync(inode);
+	/* Only change inode inside the lock region */
+	if (sync_it & S_VERSION)
+		inode_inc_iversion(inode);
+	if (sync_it & S_CTIME)
+		inode->i_ctime = now;
+	if (sync_it & S_MTIME)
+		inode->i_mtime = now;
+	mark_inode_dirty_sync(inode);
 	mnt_drop_write(file->f_path.mnt);
 }
 EXPORT_SYMBOL(file_update_time);