sizeof_namespace_index() fails when NVDIMM devices have the minimum
1024 bytes label storage area.  nvdimm_num_label_slots() returns 3
slots while the area is only big enough for 2 slots.

Change nvdimm_num_label_slots() to calculate a number of label slots
according to UEFI 2.7 spec.

Signed-off-by: Toshi Kani <toshi.k...@hpe.com>
Cc: Dan Williams <dan.j.willi...@intel.com>
---
 drivers/nvdimm/label.c |   34 ++++++++++++++++++++++++----------
 1 file changed, 24 insertions(+), 10 deletions(-)

diff --git a/drivers/nvdimm/label.c b/drivers/nvdimm/label.c
index de66c02f6140..be3ccf7c5413 100644
--- a/drivers/nvdimm/label.c
+++ b/drivers/nvdimm/label.c
@@ -45,9 +45,27 @@ unsigned sizeof_namespace_label(struct nvdimm_drvdata *ndd)
        return ndd->nslabel_size;
 }
 
+static size_t __sizeof_namespace_index(u32 nslot)
+{
+       return ALIGN(sizeof(struct nd_namespace_index) + DIV_ROUND_UP(nslot, 8),
+                       NSINDEX_ALIGN);
+}
+
+static int __nvdimm_num_label_slots(struct nvdimm_drvdata *ndd,
+               size_t index_size)
+{
+       return (ndd->nsarea.config_size - index_size * 2) /
+                       sizeof_namespace_label(ndd);
+}
+
 int nvdimm_num_label_slots(struct nvdimm_drvdata *ndd)
 {
-       return ndd->nsarea.config_size / (sizeof_namespace_label(ndd) + 1);
+       u32 tmp_nslot, n;
+
+       tmp_nslot = ndd->nsarea.config_size / sizeof_namespace_label(ndd);
+       n = __sizeof_namespace_index(tmp_nslot) / NSINDEX_ALIGN;
+
+       return __nvdimm_num_label_slots(ndd, NSINDEX_ALIGN * n);
 }
 
 size_t sizeof_namespace_index(struct nvdimm_drvdata *ndd)
@@ -55,18 +73,14 @@ size_t sizeof_namespace_index(struct nvdimm_drvdata *ndd)
        u32 nslot, space, size;
 
        /*
-        * The minimum index space is 512 bytes, with that amount of
-        * index we can describe ~1400 labels which is less than a byte
-        * of overhead per label.  Round up to a byte of overhead per
-        * label and determine the size of the index region.  Yes, this
-        * starts to waste space at larger config_sizes, but it's
-        * unlikely we'll ever see anything but 128K.
+        * Per UEFI 2.7, the minimum size of the Label Storage Area is large
+        * enough to hold 2 index blocks and 2 labels.  The minimum index
+        * block size is 256 bytes, and the minimum label size is 256 bytes.
         */
        nslot = nvdimm_num_label_slots(ndd);
        space = ndd->nsarea.config_size - nslot * sizeof_namespace_label(ndd);
-       size = ALIGN(sizeof(struct nd_namespace_index) + DIV_ROUND_UP(nslot, 8),
-                       NSINDEX_ALIGN) * 2;
-       if (size <= space)
+       size = __sizeof_namespace_index(nslot) * 2;
+       if (size <= space && nslot >= 2)
                return size / 2;
 
        dev_err(ndd->dev, "label area (%d) too small to host (%d byte) 
labels\n",
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