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

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Date:   Thu, 28 Mar 2019 09:57:09 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jerome Glisse <jglisse@...hat.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 Thu, Mar 28, 2019 at 05:39:26PM -0700, John Hubbard wrote:
> 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.

No I don't have anything.

I was just really digging into these this time around and I was about to
comment on the lack of "get's" for some "puts" when I realized that
"hmm_register" _was_ the get...

:-(

Ira

> 
> 
> thanks,
> -- 
> John Hubbard
> NVIDIA
> 
>