[PATCH] truncate speedup

This patch optimises the truncate of a zero-length file, which is a
sufficiently common case to justify the extra test-n-branch.

It does this by skipping the entire call into the fs if i_size is not
being altered.

The AIM9 `open_clo' test just loops, creating and unlinking a file.
This patch speeds it up 50% for ext2, 600% for reiserfs.

fs/attr.c

@@ -67,13 +67,20 @@ int inode_setattr(struct inode * inode, struct iattr * attr)
 	unsigned int ia_valid = attr->ia_valid;
 	int error = 0;
 
-	lock_kernel();
 	if (ia_valid & ATTR_SIZE) {
+		if (attr->ia_size == inode->i_size) {
+			if (ia_valid == ATTR_SIZE)
+				goto out;	/* we can skip lock_kernel() */
+		} else {
+			lock_kernel();
 			error = vmtruncate(inode, attr->ia_size);
+			unlock_kernel();
 			if (error)
 				goto out;
 		}
+	}
 
+	lock_kernel();
 	if (ia_valid & ATTR_UID)
 		inode->i_uid = attr->ia_uid;
 	if (ia_valid & ATTR_GID)
@@ -90,8 +97,8 @@ int inode_setattr(struct inode * inode, struct iattr * attr)
 			inode->i_mode &= ~S_ISGID;
 	}
 	mark_inode_dirty(inode);
-out:
 	unlock_kernel();
+out:
 	return error;
 }
 

mm/truncate.c

@@ -115,6 +115,9 @@ void truncate_inode_pages(struct address_space *mapping, loff_t lstart)
 	pgoff_t next;
 	int i;
 
+	if (mapping->nrpages == 0)
+		return;
+
 	pagevec_init(&pvec, 0);
 	next = start;
 	while (pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) {