clone.2: Document CLONE_IO (new in Linux 2.6.25)

Some text also by mtk.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/clone.2

@@ -35,13 +35,12 @@
 .\" 2008-11-18, mtk, document CLONE_NEWPID
 .\" 2008-11-19, mtk, document CLONE_NEWUTS
 .\" 2008-11-19, mtk, document CLONE_NEWIPC
+.\" 2008-11-19, mtk, document CLONE_IO
 .\"
 .\" FIXME Document CLONE_NEWUSER, which is new in 2.6.23
 .\"       (also supported for unshare()?)
 .\" FIXME 2.6.25 marks the unused CLONE_STOPPED as obsolete, and it will
 .\"       probably be removed in the future.
-.\" FIXME 2.6.25: CLONE_IO flag to clone() causes I/O contexts (used in the
-.\"       CFQ block I/O scheduler) to be shared with the new child process.
 .\"
 .TH CLONE 2 2008-11-19 "Linux" "Linux Programmer's Manual"
 .SH NAME
@@ -224,6 +223,36 @@ Calls to
 .BR umask (2)
 performed later by one of the processes do not affect the other process.
 .TP
+.BR CLONE_IO " (since Linux 2.4.25)"
+If
+.B CLONE_IO
+is set, then the new process shares an I/O context with
+the calling process.
+If this flag is not set, then (as with
+.BR fork (2))
+the new process has its own I/O context.
+
+.\" The following based on text from Jens Axboe
+The I/O context is the I/O scope of the disk scheduler (i.e, 
+what the I/O scheduler uses to model scheduling of a process's I/O).
+If processes share the same I/O context,
+they are treated as one by the I/O scheduler.
+As a consequence, they get to share disk time.
+For some I/O schedulers,
+.\" the anticipatory and CFQ scheduler
+if two processes share an I/O context,
+they will be allowed to interleave their disk access.
+If several threads are doing I/O on behalf of the same process
+.RB ( aio_read (3),
+for instance), they should employ
+.BR CLONE_IO
+to get better I/O performance.
+.\" with CFQ and AS.
+
+If the kernel is not configured with the
+.B CONFIG_BLOCK
+option, this flag is a no-op.
+.TP
 .BR CLONE_NEWIPC " (since Linux 2.4.19)"
 If
 .B CLONE_NEWIPC