Re: parallel nvme format

[Keith Busch <kbusch@kernel.org>]

On Thu, Dec 14, 2023 at 09:21:28PM +0000, Niklas Cassel wrote:
> Perhaps we could introduce a new bit, NVME_CTRL_NEEDS_SCAN or similar?
> (That gets cleared by nvme_scan_work(), after a scan has been performed.)

Good point. Updated patch below.
 
> I guess for format command specifically, we already have
> NVME_AER_NOTICE_NS_CHANGED, but that seems to be cleared before by
> nvme_scan_work() before the mutex is acquired.

The clearing should just re-arm the controller to send the AER again if
any other namespace changes. But we can't really depend on the AER
anyway.

> And I'm not sure if all command that have
> effects & (NVME_CMD_EFFECTS_NIC | NVME_CMD_EFFECTS_NCC)
> set will trigger a NVME_AER_NOTICE_NS_CHANGED AEN.

We have the "known_effects" for the format command to ensure the driver
will always request a rescan on a format no matter what a controller
reports, but yeah, the format could be followed up by another command
with NVME_CMD_EFFECTS_CSE_MASK set without NIC/NCC, so definitely need
to retain the need to restart the scan.

---
diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 590cd4f097c22..0b062a2268447 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -1120,6 +1120,8 @@ u32 nvme_passthru_start(struct nvme_ctrl *ctrl, struct nvme_ns *ns, u8 opcode)
 		nvme_start_freeze(ctrl);
 		nvme_wait_freeze(ctrl);
 	}
+	if (effects & (NVME_CMD_EFFECTS_NIC | NVME_CMD_EFFECTS_NCC))
+		set_bit(NVME_CTRL_NEEDS_RESCAN, &ctrl->flags);
 	return effects;
 }
 EXPORT_SYMBOL_NS_GPL(nvme_passthru_start, NVME_TARGET_PASSTHRU);
@@ -1140,7 +1142,7 @@ void nvme_passthru_end(struct nvme_ctrl *ctrl, struct nvme_ns *ns, u32 effects,
 "controller capabilities changed, reset may be required to take effect.\n");
 		}
 	}
-	if (effects & (NVME_CMD_EFFECTS_NIC | NVME_CMD_EFFECTS_NCC)) {
+	if (test_bit(NVME_CTRL_NEEDS_RESCAN, &ctrl->flags)) {
 		nvme_queue_scan(ctrl);
 		flush_work(&ctrl->scan_work);
 	}
@@ -3945,7 +3947,14 @@ static void nvme_scan_work(struct work_struct *work)
 		nvme_clear_changed_ns_log(ctrl);
 	}
 
-	mutex_lock(&ctrl->scan_lock);
+	/*
+	 * Failure to acquire scan_lock means another thread will requeue this
+	 * work shortly
+	 */
+	if (!mutex_trylock(&ctrl->scan_lock))
+		return;
+
+	clear_bit(NVME_CTRL_NEEDS_RESCAN, &ctrl->flags);
 	if (nvme_ctrl_limited_cns(ctrl)) {
 		nvme_scan_ns_sequential(ctrl);
 	} else {
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index 39a90b7cb1254..79a71b6ae64fe 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -251,6 +251,7 @@ enum nvme_ctrl_flags {
 	NVME_CTRL_STOPPED		= 3,
 	NVME_CTRL_SKIP_ID_CNS_CS	= 4,
 	NVME_CTRL_DIRTY_CAPABILITY	= 5,
+	NVME_CTRL_NEEDS_RESCAN		= 6,
 };
 
 struct nvme_ctrl {
--