linux-kernel - Re: [PATCH v2 02/11] mm/hmm: use reference counting for HMM struct v2

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Message-ID: <fcb7be01-38c1-ed1f-70a0-d03dc9260473@nvidia.com>
Date:   Thu, 28 Mar 2019 17:39:26 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jerome Glisse <jglisse@...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 02/11] mm/hmm: use reference counting for HMM struct v2

On 3/28/19 2:21 PM, Jerome Glisse wrote:
> On Thu, Mar 28, 2019 at 01:43:13PM -0700, John Hubbard wrote:
>> On 3/28/19 12:11 PM, Jerome Glisse wrote:
>>> On Thu, Mar 28, 2019 at 04:07:20AM -0700, Ira Weiny wrote:
>>>> On Mon, Mar 25, 2019 at 10:40:02AM -0400, Jerome Glisse wrote:
>>>>> From: Jérôme Glisse <jglisse@...hat.com>
[...]
>>>>> @@ -67,14 +78,9 @@ struct hmm {
>>>>>   */
>>>>>  static struct hmm *hmm_register(struct mm_struct *mm)
>>>>>  {
>>>>> -	struct hmm *hmm = READ_ONCE(mm->hmm);
>>>>> +	struct hmm *hmm = mm_get_hmm(mm);
>>>>
>>>> FWIW: having hmm_register == "hmm get" is a bit confusing...
>>>
>>> The thing is that you want only one hmm struct per process and thus
>>> if there is already one and it is not being destroy then you want to
>>> reuse it.
>>>
>>> Also this is all internal to HMM code and so it should not confuse
>>> anyone.
>>>
>>
>> Well, it has repeatedly come up, and I'd claim that it is quite 
>> counter-intuitive. So if there is an easy way to make this internal 
>> HMM code clearer or better named, I would really love that to happen.
>>
>> And we shouldn't ever dismiss feedback based on "this is just internal
>> xxx subsystem code, no need for it to be as clear as other parts of the
>> kernel", right?
> 
> Yes but i have not seen any better alternative that present code. If
> there is please submit patch.
> 

Ira, do you have any patch you're working on, or a more detailed suggestion there?
If not, then I might (later, as it's not urgent) propose a small cleanup patch 
I had in mind for the hmm_register code. But I don't want to duplicate effort 
if you're already thinking about it.


thanks,
-- 
John Hubbard
NVIDIA