[PATCH] mm/nvdimm: Pick the right alignment default when creating dax devices

[Aneesh Kumar K.V]

Hi Dan,

"Aneesh Kumar K.V" <aneesh.kumar at linux.ibm.com> writes:

> On 5/17/19 8:19 PM, Vaibhav Jain wrote:
>> Hi Aneesh,
>> 

....

>>
>>> +	/*
>>> +	 * Check whether the we support the alignment. For Dax if the
>>> +	 * superblock alignment is not matching, we won't initialize
>>> +	 * the device.
>>> +	 */
>>> +	if (!nd_supported_alignment(align) &&
>>> +	    memcmp(pfn_sb->signature, DAX_SIG, PFN_SIG_LEN)) {
>> Suggestion to change this check to:
>> 
>> if (memcmp(pfn_sb->signature, DAX_SIG, PFN_SIG_LEN) &&
>>     !nd_supported_alignment(align))
>> 
>> It would look  a bit more natural i.e. "If the device has dax signature and alignment is
>> not supported".
>> 
>
> I guess that should be !memcmp()? . I will send an updated patch with 
> the hash failure details in the commit message.
>

We need clarification on what the expected failure behaviour should be.
The nd_pmem_probe doesn't really have a failure behaviour in this
regard. For example.

I created a dax device with 16M alignment

{                                          
  "dev":"namespace0.0",
  "mode":"devdax",                         
  "map":"dev",                             
  "size":"9.98 GiB (10.72 GB)",
  "uuid":"ba62ef22-ebdf-4779-96f5-e6135383ed22",
  "raw_uuid":"7b2492f9-7160-4ee9-9c3d-2f547d9ef3ee",
  "daxregion":{                            
    "id":0,                                
    "size":"9.98 GiB (10.72 GB)",
    "align":16777216,
    "devices":[                            
      {                                    
        "chardev":"dax0.0",
        "size":"9.98 GiB (10.72 GB)"
      }                                    
    ]                                      
  },                                       
  "align":16777216,                        
  "numa_node":0,                           
  "supported_alignments":[
    65536,                                 
    16777216                               
  ]                                        
}      

Now what we want is to fail the initialization of the device when we
boot a kernel that doesn't support 16M page size. But with the
nd_pmem_probe failure behaviour we now end up with

[
  {
    "dev":"namespace0.0",
    "mode":"fsdax",
    "map":"mem",
    "size":10737418240,
    "uuid":"7b2492f9-7160-4ee9-9c3d-2f547d9ef3ee",
    "blockdev":"pmem0"
  }
]

So it did fallthrough the

	/* if we find a valid info-block we'll come back as that personality */
	if (nd_btt_probe(dev, ndns) == 0 || nd_pfn_probe(dev, ndns) == 0
			|| nd_dax_probe(dev, ndns) == 0)
		return -ENXIO;

	/* ...otherwise we're just a raw pmem device */
	return pmem_attach_disk(dev, ndns);


Is it ok if i update the code such that we don't do that default
pmem_atach_disk if we have a label area?

-aneesh