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=-0.8 required=3.0 tests=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 01DE3C352A4 for ; Wed, 12 Feb 2020 19:18:59 +0000 (UTC) Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.kernel.org (Postfix) with SMTP id 615FA2082F for ; Wed, 12 Feb 2020 19:18:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 615FA2082F Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=xmission.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=kernel-hardening-return-17799-kernel-hardening=archiver.kernel.org@lists.openwall.com Received: (qmail 9810 invoked by uid 550); 12 Feb 2020 19:18:50 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Received: (qmail 9770 invoked from network); 12 Feb 2020 19:18:49 -0000 From: ebiederm@xmission.com (Eric W. Biederman) To: Linus Torvalds Cc: LKML , Kernel Hardening , Linux API , Linux FS Devel , Linux Security Module , Akinobu Mita , Alexander Viro , Alexey Dobriyan , Andrew Morton , Andy Lutomirski , Daniel Micay , Djalal Harouni , "Dmitry V . Levin" , Greg Kroah-Hartman , Ingo Molnar , "J . Bruce Fields" , Jeff Layton , Jonathan Corbet , Kees Cook , Oleg Nesterov , Solar Designer References: <20200210150519.538333-1-gladkov.alexey@gmail.com> <20200210150519.538333-8-gladkov.alexey@gmail.com> <87v9odlxbr.fsf@x220.int.ebiederm.org> <20200212144921.sykucj4mekcziicz@comp-core-i7-2640m-0182e6> <87tv3vkg1a.fsf@x220.int.ebiederm.org> Date: Wed, 12 Feb 2020 13:16:38 -0600 In-Reply-To: (Linus Torvalds's message of "Wed, 12 Feb 2020 10:45:06 -0800") Message-ID: <87v9obipk9.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=1j1xWq-0000bg-Ky;;;mid=<87v9obipk9.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+FpT+28WqNfECAgpxZXu0sEMmAMmxCfO0= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in02.mta.xmission.com) Linus Torvalds writes: > On Wed, Feb 12, 2020 at 7:01 AM Eric W. Biederman wrote: >> >> Fundamentally proc_flush_task is an optimization. Just getting rid of >> dentries earlier. At least at one point it was an important >> optimization because the old process dentries would just sit around >> doing nothing for anyone. > > I'm pretty sure it's still important. It's very easy to generate a > _ton_ of dentries with /proc. > >> I wonder if instead of invalidating specific dentries we could instead >> fire wake up a shrinker and point it at one or more instances of proc. > > It shouldn't be the dentries themselves that are a freeing problem. > They're being RCU-free'd anyway because of lookup. It's the > proc_mounts list that is the problem, isn't it? > > So it's just fs_info that needs to be rcu-delayed because it contains > that list. Or is there something else? The fundamental dcache thing we are playing with is: dentry = d_hash_and_lookup(proc_root, &name); if (dentry) { d_invalidate(dentry); dput(dentry); } As Al pointed out upthread dput and d_invalidate can both sleep. The dput can potentially go away if we use __d_lookup_rcu instead of d_lookup. The challenge is d_invalidate. It has the fundamentally sleeping detach_mounts loop. Even shrink_dcache_parent has a cond_sched() in there to ensure it doesn't live lock the system. We could and arguabley should set DCACHE_CANT_MOUNT on the proc pid dentries. Which will prevent having to deal with mounts. But I don't see an easy way of getting shrink_dcache_parent to run without sleeping. Ideas? Eric