[PATCH v2] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[PATCH v2] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Jan Kara]

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

Changes since v1:
* Added tags
* Removed unnecessary RCU protection

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

Re: [PATCH v2] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq

[Jens Axboe]

On Thu, 2 Jun 2022 10:12:42 +0200, Jan Kara wrote:
> 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.
> 
> [...]

Applied, thanks!

[1/1] block: fix bio_clone_blkg_association() to associate with proper blkcg_gq
      (no commit info)

Best regards,
-- 
Jens Axboe