block: fix default IO priority handling again

BugLink: https://bugs.launchpad.net/bugs/1989221

commit e589f46445960c274cc813a1cc8e2fc73b2a1849 upstream.

Commit e70344c ("block: fix default IO priority handling")
introduced an inconsistency in get_current_ioprio() that tasks without
IO context return IOPRIO_DEFAULT priority while tasks with freshly
allocated IO context will return 0 (IOPRIO_CLASS_NONE/0) IO priority.
Tasks without IO context used to be rare before 5a9d041ba2f6 ("block:
move io_context creation into where it's needed") but after this commit
they became common because now only BFQ IO scheduler setups task's IO
context. Similar inconsistency is there for get_task_ioprio() so this
inconsistency is now exposed to userspace and userspace will see
different IO priority for tasks operating on devices with BFQ compared
to devices without BFQ. Furthemore the changes done by commit
e70344c change the behavior when no IO priority is set for BFQ IO
scheduler which is also documented in ioprio_set(2) manpage:

"If no I/O scheduler has been set for a thread, then by default the I/O
priority will follow the CPU nice value (setpriority(2)).  In Linux
kernels before version 2.6.24, once an I/O priority had been set using
ioprio_set(), there was no way to reset the I/O scheduling behavior to
the default. Since Linux 2.6.24, specifying ioprio as 0 can be used to
reset to the default I/O scheduling behavior."

So make sure we default to IOPRIO_CLASS_NONE as used to be the case
before commit e70344c. Also cleanup alloc_io_context() to
explicitely set this IO priority for the allocated IO context to avoid
future surprises. Note that we tweak ioprio_best() to maintain
ioprio_get(2) behavior and make this commit easily backportable.

CC: [email protected]
Fixes: e70344c ("block: fix default IO priority handling")
Reviewed-by: Damien Le Moal <[email protected]>
Tested-by: Damien Le Moal <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Kamal Mostafa <[email protected]>
Signed-off-by: Stefan Bader <[email protected]>

Author: jankara

block/blk-ioc.c

@@ -265,6 +265,7 @@ int create_task_io_context(struct task_struct *task, gfp_t gfp_flags, int node)
 	INIT_RADIX_TREE(&ioc->icq_tree, GFP_ATOMIC);
 	INIT_HLIST_HEAD(&ioc->icq_list);
 	INIT_WORK(&ioc->release_work, ioc_release_fn);
+	ioc->ioprio = IOPRIO_DEFAULT;
 
 	/*
 	 * Try to install.  ioc shouldn't be installed if someone else

block/ioprio.c

@@ -189,9 +189,9 @@ static int get_task_ioprio(struct task_struct *p)
 int ioprio_best(unsigned short aprio, unsigned short bprio)
 {
 	if (!ioprio_valid(aprio))
-		aprio = IOPRIO_DEFAULT;
+		aprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_BE_NORM);
 	if (!ioprio_valid(bprio))
-		bprio = IOPRIO_DEFAULT;
+		bprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_BE_NORM);
 
 	return min(aprio, bprio);
 }

include/linux/ioprio.h

@@ -11,7 +11,7 @@
 /*
  * Default IO priority.
  */
-#define IOPRIO_DEFAULT	IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, IOPRIO_BE_NORM)
+#define IOPRIO_DEFAULT	IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0)
 
 /*
  * Check that a priority value has a valid class.