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Tue, 09 Feb 2021 23:12:07 -0800 (PST) Received: from google.com ([2620:15c:211:201:d107:fbfb:a7c8:913e]) by smtp.gmail.com with ESMTPSA id q15sm899483pfk.181.2021.02.09.23.12.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 09 Feb 2021 23:12:06 -0800 (PST) Date: Tue, 9 Feb 2021 23:12:04 -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: <43cd6fc4-5bc5-50ec-0252-ffe09afd68ea@nvidia.com> <7cc229f4-609c-71dd-9361-063ef1bf7c73@nvidia.com> <09e60732-6a46-dd00-f9d5-4ef17ee685c8@nvidia.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 Wed, Feb 10, 2021 at 07:43:37AM +0100, Greg KH wrote: > On Tue, Feb 09, 2021 at 01:13:17PM -0800, Minchan Kim wrote: > > On Tue, Feb 09, 2021 at 12:11:20PM -0800, John Hubbard wrote: > > > On 2/9/21 9:49 AM, Greg KH wrote: > > > > > > 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. > > > > > > > > No, that is not ok. > > > > > > > > > > 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. > > > > > > > > So the array is _NEVER_ freed? If not, fine, don't provide a release > > > > function for the kobject, but ick, just make a dynamic kobject I don't > > > > see the problem for something so tiny and not very many... > > > > > > > > > > Yeah, I wasn't trying to generate so much discussion, I initially thought it > > > would be a minor comment: "just use an embedded struct and avoid some extra > > > code", at first. > > > > > > > I worry that any static kobject might be copied/pasted as someone might > > > > think this is an ok thing to do. And it's not an ok thing to do. > > > > > > > > > > Overall, then, we're seeing that there is a small design hole: in order > > > to use sysfs most naturally, you either much provide a dynamically allocated > > > item for it, or you must use the static kobject, and the second one sets a > > > bad example. > > > > > > I think we should just use a static kobject, with a cautionary comment to > > > would-be copy-pasters, that explains the design constraints above. That way, > > > we still get a nice, less-code implementation, a safe design, and it only > > > really costs us a single carefully written comment. > > > > > > thanks, > > > > Agreed. How about this for the warning part? > > > > + > > +/* > > + * note: kobj_type should provide a release function to free dynamically > > + * allocated object since kobject is responsible for controlling lifespan > > + * of the object. However, cma_area is static object so technially, it > > + * doesn't need release function. It's very exceptional case so pleaes > > + * do not follow this model. > > + */ > > static struct kobj_type cma_ktype = { > > .sysfs_ops = &kobj_sysfs_ops, > > .default_groups = cma_groups > > + .release = NULL, /* do not follow. See above */ > > }; > > > > No, please no. Just do it the correct way, what is the objection to > creating a few dynamic kobjects from the heap? How many of these are > you going to have that it will somehow be "wasteful"? > > Please do it properly. Oh, I misunderstood your word "don't provide a release function for the kobject" so thought you agreed on John. If you didn't, we are stuck again: IIUC, the objection from John was the cma_stat lifetime should be on parent object, which is reasonable and make code simple. Frankly speaking, I don't have strong opinion about either approach. John?