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: stable@vger.kernel.org
Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
Tested-by: "yukuai (C)" <yukuai3@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220401102752.8599-3-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 33 insertions, 3 deletions

diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
index 420eda2589c0..9352f3cc2377 100644
--- a/block/bfq-cgroup.c
+++ b/block/bfq-cgroup.c
@@ -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;