nvdimm/pfn_dev: Prevent the creation of zero-sized namespaces

nvdimm/pfn_dev: Prevent the creation of zero-sized namespaces

On architectures that have different page size values used for kernel
direct mapping and userspace mappings, the user can end up creating zero-sized
namespaces as shown below

:/sys/bus/nd/devices/region1# cat align
0x1000000
/sys/bus/nd/devices/region1# echo 0x200000 > align
/sys/bus/nd/devices/region1/dax1.0# cat supported_alignments
65536 16777216
 $ ndctl create-namespace -r region1 -m devdax -s 18M --align 64K
{
  "dev":"namespace1.0",
  "mode":"devdax",
  "map":"dev",
  "size":0,
  "uuid":"3094329a-0c66-4905-847e-357223e56ab0",
  "daxregion":{
    "id":1,
    "size":0,
    "align":65536
  },
  "align":65536
}
similarily for fsdax

 $ ndctl create-namespace -r region1 -m fsdax  -s 18M --align 64K
{
  "dev":"namespace1.0",
  "mode":"fsdax",
  "map":"dev",
  "size":0,
  "uuid":"45538a6f-dec7-405d-b1da-2a4075e06232",
  "sector_size":512,
  "align":65536,
  "blockdev":"pmem1"
}

In commit 9ffc1d19fc4a ("mm/memremap_pages: Introduce memremap_compat_align()")
memremap_compat_align was added to make sure the kernel always creates
namespaces with 16MB alignment. But the user can still override the
region alignment and no input validation is done in the region alignment
values to retain the flexibility user had before. However, the kernel
ensures that only part of the namespace that can be mapped via kernel
direct mapping page size is enabled. This is achieved by tracking the
unmapped part of the namespace in pfn_sb->end_trunc. The kernel also
ensures that the start address of the namespace is also aligned to the
kernel direct mapping page size.

Depending on the user request, the kernel implements userspace mapping
alignment by updating pfn device alignment attribute and this value is
used to adjust the start address for userspace mappings. This is tracked
in pfn_sb->dataoff. Hence the available size for userspace mapping is:

usermapping_size = size of the namespace - pfn_sb->end_trun - pfn_sb->dataoff

If the kernel finds the user mapping size zero then don't allow the
creation of namespace.

After fix:
$ ndctl create-namespace -f  -r region1 -m devdax  -s 18M --align 64K
libndctl: ndctl_dax_enable: dax1.1: failed to enable
  Error: namespace1.2: failed to enable

failed to create namespace: No such device or address

And existing zero sized namespace will be marked disabled.
root@ltczz75-lp2:/home/kvaneesh# ndctl  list -N -r region1 -i
[
  {
    "dev":"namespace1.0",
    "mode":"raw",
    "size":18874368,
    "uuid":"94a90fb0-8e78-4fb6-a759-ffc62f9fa181",
    "sector_size":512,
    "state":"disabled"
  },

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Link: https://lore.kernel.org/r/20230809053512.350660-1-aneesh.kumar@linux.ibm.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>