From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AA56EC433E0 for ; Wed, 10 Feb 2021 06:43:45 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 272FF64E6C for ; Wed, 10 Feb 2021 06:43:45 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 272FF64E6C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=linuxfoundation.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 71A956B0006; Wed, 10 Feb 2021 01:43:44 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 6CC366B006C; Wed, 10 Feb 2021 01:43:44 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5BB816B006E; Wed, 10 Feb 2021 01:43:44 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0170.hostedemail.com [216.40.44.170]) by kanga.kvack.org (Postfix) with ESMTP id 447E86B0006 for ; Wed, 10 Feb 2021 01:43:44 -0500 (EST) Received: from smtpin06.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay02.hostedemail.com (Postfix) with ESMTP id 12B137599 for ; Wed, 10 Feb 2021 06:43:44 +0000 (UTC) X-FDA: 77801417568.06.F196CE8 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf07.hostedemail.com (Postfix) with ESMTP id 9736AA0000FB for ; Wed, 10 Feb 2021 06:43:42 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id 8A4B064E4B; Wed, 10 Feb 2021 06:43:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1612939422; bh=YOrVXXQiKglQu2Rdq4Iu7x2LG7tjwgOjO/EOjr05vk4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=SSfPY5dIxTjNZWnwyJpcfmY5K8q+NP69afgqi+e8SGP37IZpMF3MOzp4B+wgHZqDv 9O/+Nd2HHo0d2V/Zd1cd4UGlQ7bONjIynkZb0or0jX8AKmFGpknIxMIJoM6gqzfIX0 ofmRrUUvrf4FXkpKMa4DWL8MrB0mZmR+MX91W53o= Date: Wed, 10 Feb 2021 07:43:37 +0100 From: Greg KH To: Minchan Kim 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-Stat-Signature: cyptw71z16pwt4j8ddbej914ggqcugjo X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 9736AA0000FB Received-SPF: none (kernel.org>: No applicable sender policy available) receiver=imf07; identity=mailfrom; envelope-from=""; helo=mail.kernel.org; client-ip=198.145.29.99 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1612939422-890548 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 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. thanks, greg k-h