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

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Message-ID: <20190329010059.GB16680@redhat.com>
Date:   Thu, 28 Mar 2019 21:00:59 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     John Hubbard <jhubbard@...dia.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 09:57:09AM -0700, Ira Weiny wrote:
> 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...
> 
> :-(
> 

The get is mm_get_hmm() were you get a reference on HMM from a mm struct.
John in previous posting complained about me naming that function hmm_get()
and thus in this version i renamed it to mm_get_hmm() as we are getting
a reference on hmm from a mm struct.

The hmm_put() is just releasing the reference on the hmm struct.

Here i feel i am getting contradicting requirement from different people.
I don't think there is a way to please everyone here.

Cheers,
Jérôme