ioprio_set.2: Document meaning of ioprio==0

Reported-by: Jens Axboe <axboe@fb.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

diff --git a/man2/ioprio_set.2 b/man2/ioprio_set.2
index 6f70dc141f7dbb91575f08a98a2b53c39c1f04ef..9eecc33bf83fc889653dc3e55d05da5042eda7e1 100644 (file)
--- a/man2/ioprio_set.2
+++ b/man2/ioprio_set.2
@@ -137,7 +137,10 @@ value), this macro returns its priority
 component.
 .PP
 See the NOTES section for more
-information on scheduling classes and priorities.
+information on scheduling classes and priorities,
+as well as the meaning of specifying
+.I ioprio
+as 0.
 
 I/O priorities are supported for reads and for synchronous
 .RB ( O_DIRECT ,
@@ -222,6 +225,19 @@ These system calls have an effect only when used
 in conjunction with an I/O scheduler that supports I/O priorities.
 As at kernel 2.6.17 the only such scheduler is the Completely Fair Queuing
 (CFQ) I/O scheduler.
+
+If no I/O scheduler has been set for a thread,
+then by default the I/O priority will follow the CPU nice value
+.RB ( setpriority (2)).
+In Linux kernels before version 2.6.24,
+once an I/O priority had been set using
+.BR ioprio_set (),
+there was no way to reset the I/O scheduling behavior to the default.
+Since Linux 2.6.24,
+.\" commit 8ec680e4c3ec818efd1652f15199ed1c216ab550
+specifying
+.I ioprio
+as 0 can be used to reset to the default I/O scheduling behavior.
 .SS Selecting an I/O scheduler
 I/O schedulers are selected on a per-device basis via the special
 file