[PATCH v2] nvme: scan sequentially only when list scan unsupported

[Uday Shankar <ushankar@purestorage.com>]

Currently, if nvme_scan_ns_list fails, nvme_scan_work will fall back to
a sequential scan. nvme_scan_ns_list can fail for a variety of reasons,
e.g. transient transport issue. And the resulting sequential scan can be
extremely expensive on controllers reporting an NN value close to the
maximum allowed (>4 billion). Avoid sequential scans wherever possible
by only falling back to them if nvme_scan_ns_list fails due to
controller non-support of Identify NS List. This breaks (noncompliant)
devices that claim to support version NVME_VS(1, 1, 0) or later, but
don't support Identify NS List. Such devices can be made to work again
using the existing NVME_QUIRK_IDENTIFY_CNS.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
---
Changes from v1:
- Move limited_cns check from nvme_scan_ns_list to nvme_scan_work
- Move note about devices that may break with this change into commit
  message

 drivers/nvme/host/core.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 0090dc0b3ae6..5abd8d4c6d9b 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -4425,9 +4425,6 @@ static int nvme_scan_ns_list(struct nvme_ctrl *ctrl)
 	u32 prev = 0;
 	int ret = 0, i;
 
-	if (nvme_ctrl_limited_cns(ctrl))
-		return -EOPNOTSUPP;
-
 	ns_list = kzalloc(NVME_IDENTIFY_DATA_SIZE, GFP_KERNEL);
 	if (!ns_list)
 		return -ENOMEM;
@@ -4535,8 +4532,10 @@ static void nvme_scan_work(struct work_struct *work)
 	}
 
 	mutex_lock(&ctrl->scan_lock);
-	if (nvme_scan_ns_list(ctrl) != 0)
+	if (nvme_ctrl_limited_cns(ctrl))
 		nvme_scan_ns_sequential(ctrl);
+	else
+		nvme_scan_ns_list(ctrl);
 	mutex_unlock(&ctrl->scan_lock);
 }
 

base-commit: d30a909f9bb5283e701a5fdfffac763ef57a3e7c
-- 
2.25.1