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Tue, 09 Feb 2021 07:56:03 -0800 (PST) Received: from google.com ([2620:15c:211:201:d107:fbfb:a7c8:913e]) by smtp.gmail.com with ESMTPSA id 25sm14762989pfh.199.2021.02.09.07.56.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Feb 2021 07:56:01 -0800 (PST) Date: Tue, 9 Feb 2021 07:55:59 -0800 From: Minchan Kim To: Greg KH Cc: John Hubbard , Andrew Morton , linux-mm , LKML , surenb@google.com, joaodias@google.com, willy@infradead.org Subject: Re: [PATCH v2] mm: cma: support sysfs Message-ID: References: <20210208180142.2765456-1-minchan@kernel.org> <43cd6fc4-5bc5-50ec-0252-ffe09afd68ea@nvidia.com> <7cc229f4-609c-71dd-9361-063ef1bf7c73@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: X-Stat-Signature: nff8nk79h9a7orbobi5whaohwy8r13gz X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 129D4C0001ED Received-SPF: none (gmail.com>: No applicable sender policy available) receiver=imf03; identity=mailfrom; envelope-from=""; helo=mail-pj1-f49.google.com; client-ip=209.85.216.49 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1612886162-203693 Content-Transfer-Encoding: quoted-printable 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 Tue, Feb 09, 2021 at 07:56:30AM +0100, Greg KH wrote: > On Mon, Feb 08, 2021 at 10:34:51PM -0800, John Hubbard wrote: > > On 2/8/21 10:27 PM, John Hubbard wrote: > > > On 2/8/21 10:13 PM, Greg KH wrote: > > > > On Mon, Feb 08, 2021 at 05:57:17PM -0800, John Hubbard wrote: > > > > > On 2/8/21 3:36 PM, Minchan Kim wrote: > > > > > ... > > > > > > > > =A0=A0=A0=A0=A0=A0=A0 char name[CMA_MAX_NAME]; > > > > > > > > +#ifdef CONFIG_CMA_SYSFS > > > > > > > > +=A0=A0=A0 struct cma_stat=A0=A0=A0 *stat; > > > > > > >=20 > > > > > > > This should not be a pointer. By making it a pointer, you'v= e added a bunch of pointless > > > > > > > extra code to the implementation. > > > > > >=20 > > > > > > Originally, I went with the object lifetime with struct cma a= s you > > > > > > suggested to make code simple. However, Greg KH wanted to hav= e > > > > > > release for kobj_type since it is consistent with other kboje= ct > > > > > > handling. > > > > >=20 > > > > > Are you talking about the kobj in your new struct cma_stat? Tha= t seems > > > > > like circular logic if so. I'm guessing Greg just wanted kobj m= ethods > > > > > to be used *if* you are dealing with kobjects. That's a narrowe= r point. > > > > >=20 > > > > > I can't imagine that he would have insisted on having additiona= l > > > > > allocations just so that kobj freeing methods could be used. :) > > > >=20 > > > > Um, yes, I was :) > > > >=20 > > > > You can not add a kobject to a structure and then somehow think y= ou can > > > > just ignore the reference counting issues involved.=A0 If a kobje= ct is > > > > part of a structure then the kobject is responsible for controlin= g the > > > > lifespan of the memory, nothing else can be. > > > >=20 > > > > So by making the kobject dynamic, you properly handle that memory > > > > lifespan of the object, instead of having to worry about the life= span of > > > > the larger object (which the original patch was not doing.) > > > >=20 > > > > Does that make sense? > > > >=20 > > > That part makes sense, yes, thanks. The part that I'm trying to str= aighten > > > out is, why was kobject even added to the struct cma_stat in the fi= rst > > > place? Why not just leave .stat as a static member variable, withou= t > > > a kobject in it, and done? > > >=20 > >=20 > > Sorry, I think I get it now: this is in order to allow a separate lif= etime > > for the .stat member. I was sort of implicitly assuming that the "rig= ht" > > way to do it is just have the whole object use one lifetime managemen= t, > > but as you say, there is no kobject being added to the parent. > >=20 > > I still feel odd about the allocation and freeing of something that s= eems > > to be logically the same lifetime (other than perhaps a few, briefly = pending > > sysfs reads, at the end of life). So I'd still think that the kobject= should > > be added to the parent... sruct cma_stat { spinlock_t lock; unsigned long pages_attemtp; unsigned long pages_fail; }; struct cma { .. .. struct kobject kobj; struct cma_stat stat; }; I guess this is what Johan suggested. I agree with it. >=20 > That's fine if you want to add it to the parent. If so, then the > kobject controls the lifetime of the structure, nothing else can. The problem was parent object(i.e., struct cma cma_areas) is static arrary so kobj->release function will be NULL or just dummy. Is it okay? I thought it was one of the what you wanted to avoid it. >=20 > Either is fine with me, what is "forbidden" is having a kobject and > somehow thinking that it does not control the lifetime of the structure= . Since parent object is static arrary, there is no need to control the lifetime so I am curious if parent object approach is okay from kobject handling point of view. If it's no problem, I am happy to change it.