From 22b106e5355d6e7a9c3b5cb5ed4ef22ae585ea94 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Thu, 2 Jun 2022 10:12:42 +0200
Subject: [PATCH] block: fix bio_clone_blkg_association() to associate with
 proper blkcg_gq
Git-commit: 22b106e5355d6e7a9c3b5cb5ed4ef22ae585ea94
Patch-mainline: v5.19-rc1
References: bsc#1200259

Commit d92c370a16cb ("block: really clone the block cgroup in
bio_clone_blkg_association") changed bio_clone_blkg_association() to
just clone bio->bi_blkg reference from source to destination bio. This
is however wrong if the source and destination bios are against
different block devices because struct blkcg_gq is different for each
bdev-blkcg pair. This will result in IOs being accounted (and throttled
as a result) multiple times against the same device (src bdev) while
throttling of the other device (dst bdev) is ignored. In case of BFQ the
inconsistency can even result in crashes in bfq_bic_update_cgroup().
Fix the problem by looking up correct blkcg_gq for the cloned bio.

Reported-by: Logan Gunthorpe <logang@deltatee.com>
Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
Fixes: d92c370a16cb ("block: really clone the block cgroup in bio_clone_blkg_association")
CC: stable@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220602081242.7731-1-jack@suse.cz
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Acked-by: Jan Kara <jack@suse.cz>

---
 block/blk-cgroup.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 40161a3f68d0..764e740b0c0f 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1974,12 +1974,8 @@ EXPORT_SYMBOL_GPL(bio_associate_blkg);
  */
 void bio_clone_blkg_association(struct bio *dst, struct bio *src)
 {
-	if (src->bi_blkg) {
-		if (dst->bi_blkg)
-			blkg_put(dst->bi_blkg);
-		blkg_get(src->bi_blkg);
-		dst->bi_blkg = src->bi_blkg;
-	}
+	if (src->bi_blkg)
+		bio_associate_blkg_from_css(dst, bio_blkcg_css(src));
 }
 EXPORT_SYMBOL_GPL(bio_clone_blkg_association);
 
-- 
2.35.3