FAILED: patch "[PATCH] blk-wbt: call rq_qos_add() after wb_normal is initialized" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

Possible dependencies:

8c5035dfbb94 ("blk-wbt: call rq_qos_add() after wb_normal is initialized") 5a20d073ec54 ("block: wbt: Remove unnecessary invoking of wbt_update_limits in wbt_init") 4d89e1d112a9 ("blk-wbt: rename __wbt_update_limits to wbt_update_limits") 9677a3e01f83 ("block/rq_qos: implement rq_qos_ops->queue_depth_changed()") d3e65ffff61c ("block/rq_qos: add rq_qos_merge()")
14ccb66b3f58 ("block: remove the bi_phys_segments field in struct bio") f924cddebc90 ("block: remove blk_init_request_from_bio")
0c8cf8c2a553 ("block: initialize the write priority in blk_rq_bio_prep") 47cdee29ef9d ("block: move blk_exit_queue into __blk_release_queue") 6869875fbc04 ("block: remove the bi_seg_{front,back}_size fields in struct bio") eded341c085b ("block: don't decrement nr_phys_segments for physically contigous segments") dcdca753c152 ("block: clean up __bio_add_pc_page a bit")
5c61ee2cd586 ("Merge tag 'v5.1-rc6' into for-5.2/block")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 8c5035dfbb9475b67c82b3fdb7351236525bf52b Mon Sep 17 00:00:00 2001
From: Yu Kuai <[email protected]>
Date: Tue, 13 Sep 2022 18:57:49 +0800
Subject: [PATCH] blk-wbt: call rq_qos_add() after wb_normal is initialized
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
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Our test found a problem that wbt inflight counter is negative, which will cause io hang(noted that this problem doesn't exist in mainline):

t1: device create t2: issue io
add_disk
blk_register_queue
wbt_enable_default
wbt_init

    rq_qos_add
    // wb_normal is still 0
			/*
			 * in mainline, disk can't be opened before
			 * bdev_add(), however, in old kernels, disk
			 * can be opened before blk_register_queue().
			 */
			blkdev_issue_flush
                        // disk size is 0, however, it's not checked
                         submit_bio_wait
                          submit_bio
                           blk_mq_submit_bio
                            rq_qos_throttle
                             wbt_wait
			      bio_to_wbt_flags
                               rwb_enabled
			       // wb_normal is 0, inflight is not increased

    wbt_queue_depth_changed(&rwb->rqos);
     wbt_update_limits
     // wb_normal is initialized
                            rq_qos_track
                             wbt_track
                              rq->wbt_flags |= bio_to_wbt_flags(rwb, bio);
			      // wb_normal is not 0，wbt_flags will be set

t3: io completion
blk_mq_free_request
rq_qos_done
wbt_done
wbt_is_tracked
// return true
__wbt_done

    wbt_rqw_done
     atomic_dec_return(&rqw->inflight);
     // inflight is decreased

commit 8235b5c1e8c1 ("block: call bdev_add later in device_add_disk") can avoid this problem, however it's better to fix this problem in wbt:

1) Lower kernel can't backport this patch due to lots of refactor.
2) Root cause is that wbt call rq_qos_add() before wb_normal is initialized.

index a9982000b667..246467926253 100644
@@ -843,6 +843,10 @@ int wbt_init(struct request_queue *q)
 	rwb->enable_state = WBT_STATE_ON_DEFAULT;
 	rwb->wc = 1;
 	rwb->rq_depth.default_depth = RWB_DEF_DEPTH;
+	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
+
+	wbt_queue_depth_changed(&rwb->rqos);
+	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
 
 	/*
 	 * Assign rwb and add the stats callback.

@@ -853,11 +857,6 @@ int wbt_init(struct request_queue *q)
 
 	blk_stat_add_callback(q, rwb->cb);
 
-	rwb->min_lat_nsec = wbt_default_latency_nsec(q);
-
-	wbt_queue_depth_changed(&rwb->rqos);
-	wbt_set_write_cache(q, test_bit(QUEUE_FLAG_WC, &q->queue_flags));
-
 	return 0;
 
 err_free: