FAILED: patch "[PATCH] blk-iocost: account flush sequence latency only once" failed to apply to 5.15-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 5.15-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 <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x e9804489e27dd6e33aec5511fe3780ff3009f57f
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261109202255-failed-5.15-e9804489e27d@kernel.org' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Thanks,
Sasha

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

From e9804489e27dd6e33aec5511fe3780ff3009f57f Mon Sep 17 00:00:00 2001
From: Zhe Liu <liuzhe1@kylinos.cn>
Date: Mon, 2 Nov 2026 14:18:02 +0800
Subject: [PATCH] blk-iocost: account flush sequence latency only once

A flush sequence with a DATA phase can complete its original request
twice. Both completions invoke ioc_rqos_done(), causing the request to
contribute two latency samples and adding the same queue wait time
twice. This gives such requests disproportionate weight compared to
ordinary writes, distorting the device-wide latency and queue wait
feedback used to adjust iocost's virtual time rate.

Skip requests marked RQF_FLUSH_SEQ in ioc_rqos_done(). The flag is set
when the request enters the flush machinery and is still set when the
data phase completes; blk_flush_restore_request() clears it before the
final completion, so accounting happens only once, after the whole
sequence finishes. The remaining sample covers the request's lifetime
through any postflush, and its queue wait time is accumulated only
once.

Fixes: 7caa47151ab2e ("blkcg: implement blk-iocost")
Cc: stable@vger.kernel.org
Signed-off-by: Zhe Liu <liuzhe1@kylinos.cn>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20261102061802.408173-1-liuzhe1@kylinos.cn
Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-iocost.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/blk-iocost.c b/block/blk-iocost.c
index 1c1fe07a96eaf..1c1025157d8e4 100644
--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -2936,6 +2936,9 @@ static void ioc_rqos_done(struct rq_qos *rqos, struct request *rq)
 	if (!ioc->enabled || !rq->alloc_time_ns || !rq->start_time_ns)
 		return;
 
+	if (unlikely(rq->rq_flags & RQF_FLUSH_SEQ))
+		return;
+
 	switch (req_op(rq)) {
 	case REQ_OP_READ:
 		pidx = QOS_RLAT;
-- 
2.53.0