block: Fix computation of merged request priority

commit ece9c72accdc45c3a9484dacb1125ce572647288 upstream.

Priority of a merged request is computed by ioprio_best(). If one of the
requests has undefined priority (IOPRIO_CLASS_NONE) and another request
has priority from IOPRIO_CLASS_BE, the function will return the
undefined priority which is wrong. Fix the function to properly return
priority of a request with the defined priority.

Fixes: d58cdfb89ce0c6bd5f81ae931a984ef298dbda20
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
[bwh: Backported to 3.2: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/fs/ioprio.c b/fs/ioprio.c
index f79dab8..5b55511 100644 (file)
--- a/fs/ioprio.c
+++ b/fs/ioprio.c
@@ -169,14 +169,16 @@ out:
 
 int ioprio_best(unsigned short aprio, unsigned short bprio)
 {
-       unsigned short aclass = IOPRIO_PRIO_CLASS(aprio);
-       unsigned short bclass = IOPRIO_PRIO_CLASS(bprio);
+       unsigned short aclass;
+       unsigned short bclass;
 
-       if (aclass == IOPRIO_CLASS_NONE)
-               aclass = IOPRIO_CLASS_BE;
-       if (bclass == IOPRIO_CLASS_NONE)
-               bclass = IOPRIO_CLASS_BE;
+       if (!ioprio_valid(aprio))
+               aprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
+       if (!ioprio_valid(bprio))
+               bprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_NORM);
 
+       aclass = IOPRIO_PRIO_CLASS(aprio);
+       bclass = IOPRIO_PRIO_CLASS(bprio);
        if (aclass == bclass)
                return min(aprio, bprio);
        if (aclass > bclass)