For the synchronous I/O path case (read(), write() etc system calls), a
BIO I/O priority is not initialized until the execution of
blk_init_request_from_bio() when the BIO is submitted and a request
initialized for the BIO execution. This is due to the ki_ioprio field of
the struct kiocb defined on stack being always initialized to
IOPRIO_CLASS_NONE, regardless of the calling process I/O context ioprio
value set with ioprio_set(). This late initialization can result in the
BIO being merged to pending requests even when the I/O priorities
differ.

Fix this by initializing the ki_iopriority field of on stack struct
kiocb using the get_current_ioprio() helper, ensuring that all BIOs
allocated and submitted for the system call execution see the correct
intended I/O priority early. With this, since a BIO I/O priority is
always set to the intended effective value for both the sync and async
path, blk_init_request_from_bio() can be simplified.

Signed-off-by: Damien Le Moal <redacted>
---
 block/blk-core.c   | 5 +----
 include/linux/fs.h | 2 +-
 2 files changed, 2 insertions(+), 5 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index dde30b08aa14..04f5be473638 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -814,10 +814,7 @@ void blk_init_request_from_bio(struct request *req, struct bio *bio)
 		req->cmd_flags |= REQ_FAILFAST_MASK;
 
 	req->__sector = bio->bi_iter.bi_sector;
-	if (ioprio_valid(bio_prio(bio)))
-		req->ioprio = bio_prio(bio);
-	else
-		req->ioprio = get_current_ioprio();
+	req->ioprio = bio_prio(bio);
 	req->write_hint = bio->bi_write_hint;
 	blk_rq_bio_prep(req->q, req, bio);
 }

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c95c0807471f..a1ab233e6469 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h

@@ -2021,7 +2021,7 @@ static inline void init_sync_kiocb(struct kiocb *kiocb, struct file *filp)
 		.ki_filp = filp,
 		.ki_flags = iocb_flags(filp),
 		.ki_hint = ki_hint_validate(file_write_hint(filp)),
-		.ki_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0),
+		.ki_ioprio = get_current_ioprio(),
 	};
 }
 

-- 
2.19.1