[PATCH 4.19 050/338] block: dont merge across cgroup boundaries if blkcg is enabled

Greg Kroah-Hartman
2022-04-14T13:19:39Z

From: Tejun Heo <tj@kernel.org>

commit 6b2b04590b51aa4cf395fcd185ce439cab5961dc upstream.

blk-iocost and iolatency are cgroup aware rq-qos policies but they didn't
disable merges across different cgroups. This obviously can lead to
accounting and control errors but more importantly to priority inversions -
e.g. an IO which belongs to a higher priority cgroup or IO class may end up
getting throttled incorrectly because it gets merged to an IO issued from a
low priority cgroup.

Fix it by adding blk_cgroup_mergeable() which is called from merge paths and
rejects cross-cgroup and cross-issue_as_root merges.

Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: d70675121546 ("block: introduce blk-iolatency io controller")
Cc: stable@vger.kernel.org # v4.19+
Cc: Josef Bacik <jbacik@fb.com>
Link: https://lore.kernel.org/r/Yi/eE/6zFNyWJ+qd@slm.duckdns.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/blk-merge.c | 12 ++++++++++++
include/linux/blk-cgroup.h | 17 +++++++++++++++++
2 files changed, 29 insertions(+)

--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -7,6 +7,8 @@
#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/scatterlist.h>
+#include <linux/blkdev.h>
+#include <linux/blk-cgroup.h>

#include <trace/events/block.h>

@@ -486,6 +488,9 @@ static inline int ll_new_hw_segment(stru
if (req->nr_phys_segments + nr_phys_segs > queue_max_segments(q))
goto no_merge;

+ if (!blk_cgroup_mergeable(req, bio))
+ goto no_merge;
+
if (blk_integrity_merge_bio(q, req, bio) == false)
goto no_merge;

@@ -609,6 +614,9 @@ static int ll_merge_requests_fn(struct r
if (total_phys_segments > queue_max_segments(q))
return 0;

+ if (!blk_cgroup_mergeable(req, next->bio))
+ return 0;
+
if (blk_integrity_merge_rq(q, req, next) == false)
return 0;

@@ -843,6 +851,10 @@ bool blk_rq_merge_ok(struct request *rq,
if (rq->rq_disk != bio->bi_disk || req_no_special_merge(rq))
return false;

+ /* don't merge across cgroup boundaries */
+ if (!blk_cgroup_mergeable(rq, bio))
+ return false;
+
/* only merge integrity protected bio into ditto rq */
if (blk_integrity_merge_bio(rq->q, rq, bio) == false)
return false;
--- a/include/linux/blk-cgroup.h
+++ b/include/linux/blk-cgroup.h
@@ -21,6 +21,7 @@
#include <linux/blkdev.h>
#include <linux/atomic.h>
#include <linux/kthread.h>
+#include <linux/blkdev.h>

/* percpu_counter batch for blkg_[rw]stats, per-cpu drift doesn't matter */
#define BLKG_STAT_CPU_BATCH (INT_MAX / 2)
@@ -844,6 +845,21 @@ static inline void blkcg_use_delay(struc
atomic_inc(&blkg->blkcg->css.cgroup->congestion_count);
}

+/**
+ * blk_cgroup_mergeable - Determine whether to allow or disallow merges
+ * @rq: request to merge into
+ * @bio: bio to merge
+ *
+ * @bio and @rq should belong to the same cgroup and their issue_as_root should
+ * match. The latter is necessary as we don't want to throttle e.g. a metadata
+ * update because it happens to be next to a regular IO.
+ */
+static inline bool blk_cgroup_mergeable(struct request *rq, struct bio *bio)
+{
+ return rq->bio->bi_blkg == bio->bi_blkg &&
+ bio_issue_as_root_blkg(rq->bio) == bio_issue_as_root_blkg(bio);
+}
+
static inline int blkcg_unuse_delay(struct blkcg_gq *blkg)
{
int old = atomic_read(&blkg->use_delay);
@@ -947,6 +963,7 @@ static inline struct request_list *blk_r

static inline bool blkcg_bio_issue_check(struct request_queue *q,
struct bio *bio) { return true; }
+static inline bool blk_cgroup_mergeable(struct request *rq, struct bio *bio) { return true; }

#define blk_queue_for_each_rl(rl, q) \
for ((rl) = &(q)->root_rl; (rl); (rl) = NULL)