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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 F2B24C433E0 for ; Wed, 20 Jan 2021 02:00:00 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 6C1B822241 for ; Wed, 20 Jan 2021 02:00:00 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6C1B822241 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=xmission.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id A2E566B0005; Tue, 19 Jan 2021 20:59:59 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 9B96C6B0006; Tue, 19 Jan 2021 20:59:59 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 880BD6B0007; Tue, 19 Jan 2021 20:59:59 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0175.hostedemail.com [216.40.44.175]) by kanga.kvack.org (Postfix) with ESMTP id 6FCD16B0005 for ; Tue, 19 Jan 2021 20:59:59 -0500 (EST) Received: from smtpin05.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 38D26181AEF30 for ; Wed, 20 Jan 2021 01:59:59 +0000 (UTC) X-FDA: 77724497718.05.fly84_410dd3627556 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin05.hostedemail.com (Postfix) with ESMTP id 1FB2B1802BE0E for ; Wed, 20 Jan 2021 01:59:59 +0000 (UTC) X-HE-Tag: fly84_410dd3627556 X-Filterd-Recvd-Size: 6704 Received: from out01.mta.xmission.com (out01.mta.xmission.com [166.70.13.231]) by imf09.hostedemail.com (Postfix) with ESMTP for ; Wed, 20 Jan 2021 01:59:58 +0000 (UTC) Received: from in02.mta.xmission.com ([166.70.13.52]) by out01.mta.xmission.com with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1l22mp-0092wV-9L; Tue, 19 Jan 2021 18:59:55 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in02.mta.xmission.com with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.93) (envelope-from ) id 1l22mo-00B0pL-8x; Tue, 19 Jan 2021 18:59:54 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Alexey Gladkov Cc: Linus Torvalds , LKML , io-uring , Kernel Hardening , Linux Containers , Linux-MM , Andrew Morton , Christian Brauner , Jann Horn , Jens Axboe , Kees Cook , Oleg Nesterov References: <116c7669744404364651e3b380db2d82bb23f983.1610722473.git.gladkov.alexey@gmail.com> <20210118194551.h2hrwof7b3q5vgoi@example.org> <20210118205629.zro2qkd3ut42bpyq@example.org> <87eeig74kv.fsf@x220.int.ebiederm.org> Date: Tue, 19 Jan 2021 19:58:44 -0600 In-Reply-To: <87eeig74kv.fsf@x220.int.ebiederm.org> (Eric W. Biederman's message of "Tue, 19 Jan 2021 19:57:36 -0600") Message-ID: <878s8o74iz.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1l22mo-00B0pL-8x;;;mid=<878s8o74iz.fsf@x220.int.ebiederm.org>;;;hst=in02.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/T2B95TMqxtikO/3YyPpBrhK2ILrwugMw= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [RFC PATCH v3 1/8] Use refcount_t for ucounts reference counting X-SA-Exim-Version: 4.2.1 (built Sat, 08 Feb 2020 21:53:50 +0000) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) 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: ebiederm@xmission.com (Eric W. Biederman) writes: > Alexey Gladkov writes: > >> On Mon, Jan 18, 2021 at 12:34:29PM -0800, Linus Torvalds wrote: >>> On Mon, Jan 18, 2021 at 11:46 AM Alexey Gladkov >>> wrote: >>> > >>> > Sorry about that. I thought that this code is not needed when switching >>> > from int to refcount_t. I was wrong. >>> >>> Well, you _may_ be right. I personally didn't check how the return >>> value is used. >>> >>> I only reacted to "it certainly _may_ be used, and there is absolutely >>> no comment anywhere about why it wouldn't matter". >> >> I have not found examples where checked the overflow after calling >> refcount_inc/refcount_add. >> >> For example in kernel/fork.c:2298 : >> >> current->signal->nr_threads++; >> atomic_inc(¤t->signal->live); >> refcount_inc(¤t->signal->sigcnt); >> >> $ semind search signal_struct.sigcnt >> def include/linux/sched/signal.h:83 refcount_t sigcnt; >> m-- kernel/fork.c:723 put_signal_struct if (refcount_dec_and_test(&sig->sigcnt)) >> m-- kernel/fork.c:1571 copy_signal refcount_set(&sig->sigcnt, 1); >> m-- kernel/fork.c:2298 copy_process refcount_inc(¤t->signal->sigcnt); >> >> It seems to me that the only way is to use __refcount_inc and then compare >> the old value with REFCOUNT_MAX >> >> Since I have not seen examples of such checks, I thought that this is >> acceptable. Sorry once again. I have not tried to hide these changes. > > The current ucount code does check for overflow and fails the increment > in every case. > > So arguably it will be a regression and inferior error handling behavior > if the code switches to the ``better'' refcount_t data structure. > > I originally didn't use refcount_t because silently saturating and not > bothering to handle the error makes me uncomfortable. > > Not having to acquire the ucounts_lock every time seems nice. Perhaps > the path forward would be to start with stupid/correct code that always > takes the ucounts_lock for every increment of ucounts->count, that is > later replaced with something more optimal. > > Not impacting performance in the non-namespace cases and having good > performance in the other cases is a fundamental requirement of merging > code like this. So starting with something easy to comprehend and simple, may make it easier to figure out how to optimize the code. Eric