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Subject: Re: [PATCH v3 3/3] mm/hmm/test: add self tests for HMM
To:     Jason Gunthorpe <jgg@mellanox.com>
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From:   Ralph Campbell <rcampbell@nvidia.com>
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On 10/31/19 5:42 AM, Jason Gunthorpe wrote:
> On Wed, Oct 30, 2019 at 05:14:30PM -0700, Ralph Campbell wrote:
> 
>>> Well, that is good, is it also under drivers/char? It kind feels like
>>> it should not be there...
>>
>> I think most of the test modules live in lib/ but I wasn't sure that
>> was the right place for the HMM test driver.
>> If you think that is better, I can easily move it.
> 
> It would be good to get the various test people involved in this, I
> really don't know.

OK.
  
>>>>> It seems especially over-complicated to use a full page table layout
>>>>> for this, wouldn't something simple like an xarray be good enough for
>>>>> test purposes?
>>>>
>>>> Possibly. A page table is really just a lookup table from virtual address
>>>> to pfn/page. Part of the rationale was to mimic what a real device
>>>> might do.
>>>
>>> Well, but the details of the page table layout don't see really
>>> important to this testing, IMHO.
>>
>> One problem with XArray is that on 32-bit machines the value would
>> need to be u64 to hold a pfn which won't fit in a ULONG_MAX.
>> I guess we could make the driver 64-bit only.
> 
> Why would a 32 bit machine need a 64 bit pfn?
> 
> Jason
> 

On x86, Physical Address Extension (PAE) uses a 64 bit PTE.
See arch/x86/include/asm/pgtable_32_types.h which includes
arch/x86/include/asm/pgtable-3level_types.h.