block: fix default IO priority handling again

Commit 26330d5eb60a ("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 eaaf82e2eeab ("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
26330d5eb60a 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 26330d5eb60a. 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: 26330d5eb60a ("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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220623074840.5960-1-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index df9cfe4ca5328e48d7590ec74935712fada4ed02..63fc02042408274a8dd100d5214a44d293e6e130 100644 (file)
--- 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 2fe068fcaad585f0dfe069fa56c058b7ba3a420c..2a34cbca18aed951a4a75b3ced6055f91ab53300 100644 (file)
--- a/block/ioprio.c
+++ b/block/ioprio.c
@@ -157,9 +157,9 @@ out:
 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 3f53bc27a19bfc904a126b6be354b2e18dd985ed..3d088a88f83202c3523ceebe226ee9c2be1dc27b 100644 (file)
--- 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.