Patch "nvme: double KA polling frequency to avoid KATO with TBKAS on" has been added to the 6.3-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    nvme: double KA polling frequency to avoid KATO with TBKAS on

to the 6.3-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     nvme-double-ka-polling-frequency-to-avoid-kato-with-.patch
and it can be found in the queue-6.3 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit eea549472e554aed5d6c31d952c36d1620b21d9e
Author: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
Date:   Thu May 25 12:22:02 2023 -0600

    nvme: double KA polling frequency to avoid KATO with TBKAS on
    
    [ Upstream commit ea4d453b9ec9ea279c39744cd0ecb47ef48ede35 ]
    
    With TBKAS on, the completion of one command can defer sending a
    keep alive for up to twice the delay between successive runs of
    nvme_keep_alive_work. The current delay of KATO / 2 thus makes it
    possible for one command to defer sending a keep alive for up to
    KATO, which can result in the controller detecting a KATO. The following
    trace demonstrates the issue, taking KATO = 8 for simplicity:
    
    1. t = 0: run nvme_keep_alive_work, no keep-alive sent
    2. t = ε: I/O completion seen, set comp_seen = true
    3. t = 4: run nvme_keep_alive_work, see comp_seen == true,
              skip sending keep-alive, set comp_seen = false
    4. t = 8: run nvme_keep_alive_work, see comp_seen == false,
              send a keep-alive command.
    
    Here, there is a delay of 8 - ε between receiving a command completion
    and sending the next command. With ε small, the controller is likely to
    detect a keep alive timeout.
    
    Fix this by running nvme_keep_alive_work with a delay of KATO / 4
    whenever TBKAS is on. Going through the above trace now gives us a
    worst-case delay of 4 - ε, which is in line with the recommendation of
    sending a command every KATO / 2 in the NVMe specification.
    
    Reported-by: Costa Sapuntzakis <costa@xxxxxxxxxxxxxxx>
    Reported-by: Randy Jennings <randyj@xxxxxxxxxxxxxxx>
    Signed-off-by: Uday Shankar <ushankar@xxxxxxxxxxxxxxx>
    Reviewed-by: Hannes Reinecke <hare@xxxxxxx>
    Reviewed-by: Sagi Grimberg <sagi@xxxxxxxxxxx>
    Reviewed-by: Christoph Hellwig <hch@xxxxxx>
    Signed-off-by: Keith Busch <kbusch@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index f12109230dc8d..3838a6622c34b 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1163,9 +1163,25 @@ EXPORT_SYMBOL_NS_GPL(nvme_passthru_end, NVME_TARGET_PASSTHRU);
  *   The host should send Keep Alive commands at half of the Keep Alive Timeout
  *   accounting for transport roundtrip times [..].
  */
+static unsigned long nvme_keep_alive_work_period(struct nvme_ctrl *ctrl)
+{
+	unsigned long delay = ctrl->kato * HZ / 2;
+
+	/*
+	 * When using Traffic Based Keep Alive, we need to run
+	 * nvme_keep_alive_work at twice the normal frequency, as one
+	 * command completion can postpone sending a keep alive command
+	 * by up to twice the delay between runs.
+	 */
+	if (ctrl->ctratt & NVME_CTRL_ATTR_TBKAS)
+		delay /= 2;
+	return delay;
+}
+
 static void nvme_queue_keep_alive_work(struct nvme_ctrl *ctrl)
 {
-	queue_delayed_work(nvme_wq, &ctrl->ka_work, ctrl->kato * HZ / 2);
+	queue_delayed_work(nvme_wq, &ctrl->ka_work,
+			   nvme_keep_alive_work_period(ctrl));
 }
 
 static enum rq_end_io_ret nvme_keep_alive_end_io(struct request *rq,