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[209.132.183.28]) by mx.google.com with ESMTPS id w14si349285qkf.159.2019.03.28.19.25.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 28 Mar 2019 19:25:23 -0700 (PDT) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id ECE5E88306; Fri, 29 Mar 2019 02:25:22 +0000 (UTC) Received: from redhat.com (ovpn-121-118.rdu2.redhat.com [10.10.121.118]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 00C1C60851; Fri, 29 Mar 2019 02:25:21 +0000 (UTC) Date: Thu, 28 Mar 2019 22:25:19 -0400 From: Jerome Glisse To: Ira Weiny Cc: John Hubbard , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , Dan Williams Subject: Re: [PATCH v2 02/11] mm/hmm: use reference counting for HMM struct v2 Message-ID: <20190329022519.GJ16680@redhat.com> References: <20190328110719.GA31324@iweiny-DESK2.sc.intel.com> <20190328191122.GA5740@redhat.com> <20190328212145.GA13560@redhat.com> <20190328165708.GH31324@iweiny-DESK2.sc.intel.com> <20190329010059.GB16680@redhat.com> <55dd8607-c91b-12ab-e6d7-adfe6d9cb5e2@nvidia.com> <20190329015003.GE16680@redhat.com> <20190328182100.GJ31324@iweiny-DESK2.sc.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190328182100.GJ31324@iweiny-DESK2.sc.intel.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Fri, 29 Mar 2019 02:25:23 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 28, 2019 at 11:21:00AM -0700, Ira Weiny wrote: > On Thu, Mar 28, 2019 at 09:50:03PM -0400, Jerome Glisse wrote: > > On Thu, Mar 28, 2019 at 06:18:35PM -0700, John Hubbard wrote: > > > On 3/28/19 6:00 PM, Jerome Glisse wrote: > > > > 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 > > > >>> [...] > > > >>>>>>>> @@ -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. > > > > > > Well, that's not what I recommended, though. The actual conversation went like > > > this [1]: > > > > > > --------------------------------------------------------------- > > > >> So for this, hmm_get() really ought to be symmetric with > > > >> hmm_put(), by taking a struct hmm*. And the null check is > > > >> not helping here, so let's just go with this smaller version: > > > >> > > > >> static inline struct hmm *hmm_get(struct hmm *hmm) > > > >> { > > > >> if (kref_get_unless_zero(&hmm->kref)) > > > >> return hmm; > > > >> > > > >> return NULL; > > > >> } > > > >> > > > >> ...and change the few callers accordingly. > > > >> > > > > > > > > What about renaning hmm_get() to mm_get_hmm() instead ? > > > > > > > > > > For a get/put pair of functions, it would be ideal to pass > > > the same argument type to each. It looks like we are passing > > > around hmm*, and hmm retains a reference count on hmm->mm, > > > so I think you have a choice of using either mm* or hmm* as > > > the argument. I'm not sure that one is better than the other > > > here, as the lifetimes appear to be linked pretty tightly. > > > > > > Whichever one is used, I think it would be best to use it > > > in both the _get() and _put() calls. > > > --------------------------------------------------------------- > > > > > > Your response was to change the name to mm_get_hmm(), but that's not > > > what I recommended. > > > > Because i can not do that, hmm_put() can _only_ take hmm struct as > > input while hmm_get() can _only_ get mm struct as input. > > > > hmm_put() can only take hmm because the hmm we are un-referencing > > might no longer be associated with any mm struct and thus i do not > > have a mm struct to use. > > > > hmm_get() can only get mm as input as we need to be careful when > > accessing the hmm field within the mm struct and thus it is better > > to have that code within a function than open coded and duplicated > > all over the place. > > The input value is not the problem. The problem is in the naming. > > obj = get_obj( various parameters ); > put_obj(obj); > > > The problem is that the function is named hmm_register() either "gets" a > reference to _or_ creates and gets a reference to the hmm object. > > What John is probably ready to submit is something like. > > struct hmm *get_create_hmm(struct mm *mm); > void put_hmm(struct hmm *hmm); > > > So when you are reading the code you see... > > foo(...) { > struct hmm *hmm = get_create_hmm(mm); > > if (!hmm) > error... > > do stuff... > > put_hmm(hmm); > } > > Here I can see a very clear get/put pair. The name also shows that the hmm is > created if need be as well as getting a reference. > You only need to create HMM when you either register a mirror or register a range. So they two pattern: average_foo() { struct hmm *hmm = mm_get_hmm(mm); ... hmm_put(hmm); } register_foo() { struct hmm *hmm = hmm_register(mm); ... return 0; error: ... hmm_put(hmm); } Cheers, Jérôme