[PATCH] libnvdimm: Clarify nd_pfn_init() flow

Dan Williams
2019-01-19T01:00:12Z

In recent days, 2 engineers, including the original author of
nd_pfn_init(), overlooked the internal call to nd_pfn_validate() and the
implications to memory allocation.

Clarify this situation to help anyone that reads through this code in
the future.

Reported-by: Wei Yang <richardw.yang@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
drivers/nvdimm/btt_devs.c | 5 +++++
drivers/nvdimm/dax_devs.c | 5 +++++
drivers/nvdimm/pfn_devs.c | 21 +++++++++++++++++++++
3 files changed, 31 insertions(+)

diff --git a/drivers/nvdimm/btt_devs.c b/drivers/nvdimm/btt_devs.c
index 795ad4ff35ca..e0a6f2491e57 100644
--- a/drivers/nvdimm/btt_devs.c
+++ b/drivers/nvdimm/btt_devs.c
@@ -354,6 +354,11 @@ int nd_btt_probe(struct device *dev, struct nd_namespace_common *ndns)
put_device(btt_dev);
}

+ /*
+ * Successful probe indicates to the caller that an nd_btt
+ * personality device has been registered and the caller can
+ * fail the probe of the baseline namespace device.
+ */
return rc;
}
EXPORT_SYMBOL(nd_btt_probe);
diff --git a/drivers/nvdimm/dax_devs.c b/drivers/nvdimm/dax_devs.c
index 0453f49dc708..65010d955fa7 100644
--- a/drivers/nvdimm/dax_devs.c
+++ b/drivers/nvdimm/dax_devs.c
@@ -136,6 +136,11 @@ int nd_dax_probe(struct device *dev, struct nd_namespace_common *ndns)
} else
__nd_device_register(dax_dev);

+ /*
+ * Successful probe indicates to the caller that a device-dax
+ * personality device has been registered and the caller can
+ * fail the probe of the baseline namespace device.
+ */
return rc;
}
EXPORT_SYMBOL(nd_dax_probe);
diff --git a/drivers/nvdimm/pfn_devs.c b/drivers/nvdimm/pfn_devs.c
index 6f22272e8d80..a8783b5a76ba 100644
--- a/drivers/nvdimm/pfn_devs.c
+++ b/drivers/nvdimm/pfn_devs.c
@@ -576,6 +576,11 @@ int nd_pfn_probe(struct device *dev, struct nd_namespace_common *ndns)
} else
__nd_device_register(pfn_dev);

+ /*
+ * Successful probe indicates to the caller that an nd_pfn
+ * personality device has been registered and the caller can
+ * fail the probe of the baseline namespace device.
+ */
return rc;
}
EXPORT_SYMBOL(nd_pfn_probe);
@@ -706,6 +711,22 @@ static int nd_pfn_init(struct nd_pfn *nd_pfn)
sig = DAX_SIG;
else
sig = PFN_SIG;
+
+ /*
+ * Check for an existing 'pfn' superblock before writing a new
+ * one. The intended flow is that on the first probe of an
+ * nd_{pfn,dax} device the superblock is calculated and written
+ * to the namespace. In this case nd_pfn_validate() returns
+ * -ENODEV because no valid superblock exists currently.
+ *
+ * On subsequent probes nd_pfn_validate() will find a valid
+ * superblock and return 0.
+ *
+ * If an assumption of the superblock has been violated, like a
+ * change to the physical alignment of the namespace,
+ * nd_pfn_validate() will return an error other than -ENODEV to
+ * fail probing.
+ */
rc = nd_pfn_validate(nd_pfn, sig);
if (rc != -ENODEV)
return rc;