FAILED: patch "[PATCH] bfq: Split shared queues on move between cgroups" 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]>.

thanks,

greg k-h

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

From 3bc5e683c67d94bd839a1da2e796c15847b51b69 Mon Sep 17 00:00:00 2001
From: Jan Kara <[email protected]>
Date: Fri, 1 Apr 2022 12:27:44 +0200
Subject: [PATCH] bfq: Split shared queues on move between cgroups

When bfqq is shared by multiple processes it can happen that one of the processes gets moved to a different cgroup (or just starts submitting IO for different cgroup). In case that happens we need to split the merged bfqq as otherwise we will have IO for multiple cgroups in one bfqq and we will just account IO time to wrong entities etc.

Similarly if the bfqq is scheduled to merge with another bfqq but the merge didn't happen yet, cancel the merge as it need not be valid anymore.

CC [email protected]

Fixes e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")

Tested-by "yukuai (C)" <[email protected]>

Signed-off-by Jan Kara <[email protected]>

Reviewed-by Christoph Hellwig <[email protected]>

Link https://lore.kernel.org/r/[email protected] open here

Signed-off-by Jens Axboe <[email protected]>

block/bfq-cgroup.c +33 -3

index 420eda2589c0..9352f3cc2377 100644
@@ -743,9 +743,39 @@ static struct bfq_group *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
 	}
 
 	if (sync_bfqq) {
-		entity = &sync_bfqq->entity;
-		if (entity->sched_data != &bfqg->sched_data)
-			bfq_bfqq_move(bfqd, sync_bfqq, bfqg);
+		if (!sync_bfqq->new_bfqq && !bfq_bfqq_coop(sync_bfqq)) {
+			/* We are the only user of this bfqq, just move it */
+			if (sync_bfqq->entity.sched_data != &bfqg->sched_data)
+				bfq_bfqq_move(bfqd, sync_bfqq, bfqg);
+		} else {
+			struct bfq_queue *bfqq;
+
+			/*
+			 * The queue was merged to a different queue. Check
+			 * that the merge chain still belongs to the same
+			 * cgroup.
+			 */
+			for (bfqq = sync_bfqq; bfqq; bfqq = bfqq->new_bfqq)
+				if (bfqq->entity.sched_data !=
+				    &bfqg->sched_data)
+					break;
+			if (bfqq) {
+				/*
+				 * Some queue changed cgroup so the merge is
+				 * not valid anymore. We cannot easily just
+				 * cancel the merge (by clearing new_bfqq) as
+				 * there may be other processes using this
+				 * queue and holding refs to all queues below
+				 * sync_bfqq->new_bfqq. Similarly if the merge
+				 * already happened, we need to detach from
+				 * bfqq now so that we cannot merge bio to a
+				 * request from the old cgroup.
+				 */
+				bfq_put_cooperator(sync_bfqq);
+				bfq_release_process_ref(bfqd, sync_bfqq);
+				bic_set_bfqq(bic, NULL, 1);
+			}
+		}
 	}
 
 	return bfqg;

block/bfq-iosched.c +1 -1

index 7d00b21ebe5d..89fe3f85eb3c 100644
@@ -5315,7 +5315,7 @@ static void bfq_put_stable_ref(struct bfq_queue *bfqq)
 	bfq_put_queue(bfqq);
 }
 
-static void bfq_put_cooperator(struct bfq_queue *bfqq)
+void bfq_put_cooperator(struct bfq_queue *bfqq)
 {
 	struct bfq_queue *__bfqq, *next;
 

block/bfq-iosched.h +1 -0

index 3b83e3d1c2e5..a56763045d19 100644
@@ -979,6 +979,7 @@ void bfq_weights_tree_remove(struct bfq_data *bfqd,
 void bfq_bfqq_expire(struct bfq_data *bfqd, struct bfq_queue *bfqq,
 		     bool compensate, enum bfqq_expiration reason);
 void bfq_put_queue(struct bfq_queue *bfqq);
+void bfq_put_cooperator(struct bfq_queue *bfqq);
 void bfq_end_wr_async_queues(struct bfq_data *bfqd, struct bfq_group *bfqg);
 void bfq_release_process_ref(struct bfq_data *bfqd, struct bfq_queue *bfqq);
 void bfq_schedule_dispatch(struct bfq_data *bfqd);

Hello Greg!

I've seen this patch failed to apply to 4.19 and 4.14 stable trees but you've still merged (some) of the patches following this one from the same series. Honestly, I would not consider the result very trustworthy. The code involved is rather complex with subtle interactions across subsystems. So please remove also upstream commits:

ea591cd4eb27 ("bfq: Update cgroup information before merging bio") fc84e1f941b9 ("bfq: Drop pointless unlock-lock pair")
5f550ede5edf ("bfq: Remove pointless bfq_init_rq() calls") 09f871868080 ("bfq: Track whether bfq_group is still online") 4e54a2493e58 ("bfq: Get rid of __bio_blkcg() usage")
075a53b78b81 ("bfq: Make sure bfqg for which we are queueing requests is online")

from 4.14 and 4.19 stable queues. If someone will need these fixes there, he'll need to do proper backport with targetted testing etc... Thanks!