X io notes: Link to minor pagefault cost

t/neo/storage/fs1/notes_io.txt

@@ -80,7 +80,9 @@ mmap
 
 The kernel handles both disk I/O and caching (in pagecache).
 
-Cost ~ α (XXX recheck) is spent on first-time access.
+XXX the cost of minor pagefault is ~ 5.5·α   http://marc.info/?l=linux-kernel&m=149002565506733&w=2
+
+Cost ~ α (FIXME see ^^^) is spent on first-time access.
 Future accesses to page, given it is still in page-cache, does not incur α cost.
 
 However I/O errors are reported as SIGBUS on memory access. Thus if for read

t/neo/storage/fs1/xbufio_test.go

-// TODO copyright / license
+// Copyright (C) 2017  Nexedi SA and Contributors.
+//                     Kirill Smelkov <kirr@nexedi.com>
+//
+// This program is free software: you can Use, Study, Modify and Redistribute
+// it under the terms of the GNU General Public License version 2, or (at your
+// option) any later version, as published by the Free Software Foundation.
+//
+// This program is distributed WITHOUT ANY WARRANTY; without even the implied
+// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+//
+// See COPYING file for full licensing terms.
 
 // XXX move -> xbufio