linux-kernel - Re: [PATCH v2 07/11] mm/hmm: add default fault flags to avoid the need to pre-fill pfns arrays.

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Date:   Thu, 28 Mar 2019 19:05:21 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...hat.com>, Ben Skeggs <bskeggs@...hat.com>
CC:     Ira Weiny <ira.weiny@...el.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 07/11] mm/hmm: add default fault flags to avoid the
 need to pre-fill pfns arrays.

On 3/28/19 6:59 PM, Jerome Glisse wrote:
>>>>>> [...]
>>>>> Indeed I did not realize there is an hmm "pfn" until I saw this function:
>>>>>
>>>>> /*
>>>>>  * hmm_pfn_from_pfn() - create a valid HMM pfn value from pfn
>>>>>  * @range: range use to encode HMM pfn value
>>>>>  * @pfn: pfn value for which to create the HMM pfn
>>>>>  * Returns: valid HMM pfn for the pfn
>>>>>  */
>>>>> static inline uint64_t hmm_pfn_from_pfn(const struct hmm_range *range,
>>>>>                                         unsigned long pfn)
>>>>>
>>>>> So should this patch contain some sort of helper like this... maybe?
>>>>>
>>>>> I'm assuming the "hmm_pfn" being returned above is the device pfn being
>>>>> discussed here?
>>>>>
>>>>> I'm also thinking calling it pfn is confusing.  I'm not advocating a new type
>>>>> but calling the "device pfn's" "hmm_pfn" or "device_pfn" seems like it would
>>>>> have shortened the discussion here.
>>>>>
>>>>
>>>> That helper is also use today by nouveau so changing that name is not that
>>>> easy it does require the multi-release dance. So i am not sure how much
>>>> value there is in a name change.
>>>>
>>>
>>> Once the dust settles, I would expect that a name change for this could go
>>> via Andrew's tree, right? It seems incredible to claim that we've built something
>>> that effectively does not allow any minor changes!
>>>
>>> I do think it's worth some *minor* trouble to improve the name, assuming that we
>>> can do it in a simple patch, rather than some huge maintainer-level effort.
>>
>> Change to nouveau have to go through nouveau tree so changing name means:

Yes, I understand the guideline, but is that always how it must be done? Ben (+cc)?

>>  -  release N add function with new name, maybe make the old function just
>>     a wrapper to the new function
>>  -  release N+1 update user to use the new name
>>  -  release N+2 remove the old name
>>
>> So it is do-able but it is painful so i rather do that one latter that now
>> as i am sure people will then complain again about some little thing and it
>> will post pone this whole patchset on that new bit. To avoid post-poning
>> RDMA and bunch of other patchset that build on top of that i rather get
>> this patchset in and then do more changes in the next cycle.
>>
>> This is just a capacity thing.
> 
> Also for clarity changes to API i am doing in this patchset is to make
> the ODP convertion easier and thus they bring a real hard value. Renaming
> those function is esthetic, i am not saying it is useless, i am saying it
> does not have the same value as those other changes and i would rather not
> miss another merge window just for esthetic changes.
> 

Agreed, that this minor point should not hold up this patch.

thanks,
-- 
John Hubbard
NVIDIA