Commit e70344c05995 ("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 e70344c05995 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 e70344c05995. 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: stable@vger.kernel.org Fixes: e70344c05995 ("block: fix default IO priority handling") Reviewed-by: Damien Le Moal damien.lemoal@opensource.wdc.com Tested-by: Damien Le Moal damien.lemoal@opensource.wdc.com Signed-off-by: Jan Kara jack@suse.cz --- block/blk-ioc.c | 2 ++ block/ioprio.c | 4 ++-- include/linux/ioprio.h | 2 +- 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/block/blk-ioc.c b/block/blk-ioc.c index df9cfe4ca532..63fc02042408 100644 --- a/block/blk-ioc.c +++ b/block/blk-ioc.c @@ -247,6 +247,8 @@ static struct io_context *alloc_io_context(gfp_t gfp_flags, int node) INIT_HLIST_HEAD(&ioc->icq_list); INIT_WORK(&ioc->release_work, ioc_release_fn); #endif + ioc->ioprio = IOPRIO_DEFAULT; + return ioc; }

diff --git a/block/ioprio.c b/block/ioprio.c index 2fe068fcaad5..2a34cbca18ae 100644 --- a/block/ioprio.c +++ b/block/ioprio.c @@ -157,9 +157,9 @@ 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); } diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h index 3f53bc27a19b..3d088a88f832 100644 --- a/include/linux/ioprio.h +++ b/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.