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Subject: Re: [RFC PATCH 1/4] libnvdimm/namespace: Make namespace size
 validation arch dependent
To: Dan Williams <dan.j.williams@intel.com>
References: <20191028094825.21448-1-aneesh.kumar@linux.ibm.com>
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From: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Message-ID: <7e9a19c1-992f-a92a-172d-bcbad1298c41@linux.ibm.com>
Date: Wed, 20 Nov 2019 08:49:02 +0530
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On 11/19/19 11:28 PM, Dan Williams wrote:
> On Mon, Nov 18, 2019 at 1:52 AM Aneesh Kumar K.V
> <aneesh.kumar@linux.ibm.com> wrote:
>>
>> Dan Williams <dan.j.williams@intel.com> writes:
>>
>>> On Sat, Nov 16, 2019 at 4:15 AM Aneesh Kumar K.V
>>> <aneesh.kumar@linux.ibm.com> wrote:
>>>>
>>
>> ....
>>
>>
>>>>
>>>> Considering the direct-map map size is not going to be user selectable,
>>>> do you agree that we can skip the above step 0 configuration you
>>>> suggested.
>>>>
>>>> The changes proposed in the patch series essentially does the rest.
>>>>
>>>> 1) It validate the size against the arch specific limit during
>>>> namespace creation. (part of step 1)
>>>
>>> This validation is a surprise failure to ndctl.
>>>
>>>> 2) It also disable initializing a region if it find the size not
>>>> correctly aligned as per the platform requirement.
>>>
>>> There needs to be a way for the user to discover the correct alignment
>>> that the kernel will accept.
>>>
>>>> 3) Direct map  mapping size is different from supported_alignment for a
>>>> namespace. The supported alignment controls what possible PAGE SIZE user want the
>>>> namespace to be mapped to user space.
>>>
>>> No, the namespace alignment is different than the page mapping size.
>>> The alignment is only interpreted as a mapping size at the device-dax
>>> level, otherwise at the raw namespace level it's just an arbitrary
>>> alignment.
>>>
>>>> With the above do you think the current patch series is good?
>>>
>>> I don't think we've quite converged on a solution.
>>
>> How about we make it a property of seed device. ie,
>> we add `supported_size_align` RO attribute to the seed device. ndctl can
>> use this to validate the size value. So this now becomes step0
>>
>> sys/bus/nd/devices/region0> cat btt0.0/supported_size_align
>> 16777216
>> /sys/bus/nd/devices/region0> cat pfn0.0/supported_size_align
>> 16777216
>> /sys/bus/nd/devices/region0> cat dax0.0/supported_size_align
>> 16777216
> 
> Why on those devices and not namespace0.0?

sure.

> 
>> We follow that up with validating the size value written to size
>> attribute(step 1).
>>
>> While initializing the namespaces already present in a region we again
>> validate the size and if not properly aligned we mark the region
>> disabled.
> 
> The region might have a mix of namespaces, some aligned and some not,
> only the misaligned namespaces should fail to enable. The region
> should otherwise enable successfully.
> 

One misaligned namespace would mean, we get other namespace resource 
start addr wrongly aligned. If we allow regions to be enabled with 
namespace with wrong size, user would find further namespace creation in 
that regions failing due to wrongly aligned resource start. IMHO that is 
a confusing user experience.

-aneesh