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[209.132.180.67]) by mx.google.com with ESMTP id w4si1639968oih.231.2020.02.13.14.25.17; Thu, 13 Feb 2020 14:25:32 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727720AbgBMWYA (ORCPT + 99 others); Thu, 13 Feb 2020 17:24:00 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:35034 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727347AbgBMWYA (ORCPT ); Thu, 13 Feb 2020 17:24:00 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j2Mti-00CGBO-G2; Thu, 13 Feb 2020 22:23:50 +0000 Date: Thu, 13 Feb 2020 22:23:50 +0000 From: Al Viro To: Linus Torvalds Cc: "Eric W. Biederman" , LKML , Kernel Hardening , Linux API , Linux FS Devel , Linux Security Module , Akinobu Mita , 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 Subject: Re: [PATCH v8 07/11] proc: flush task dcache entries from all procfs instances Message-ID: <20200213222350.GU23230@ZenIV.linux.org.uk> References: <20200212200335.GO23230@ZenIV.linux.org.uk> <20200212203833.GQ23230@ZenIV.linux.org.uk> <20200212204124.GR23230@ZenIV.linux.org.uk> <87lfp7h422.fsf@x220.int.ebiederm.org> <87pnejf6fz.fsf@x220.int.ebiederm.org> <20200213055527.GS23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 13, 2020 at 01:30:11PM -0800, Linus Torvalds wrote: > On Wed, Feb 12, 2020 at 9:55 PM Al Viro wrote: > > > > What I don't understand is the insistence on getting those dentries > > via dcache lookups. > > I don't think that's an "insistence", it's more of a "historical > behavior" together with "several changes over the years to deal with > dentry-level cleanups and updates". > > > _IF_ we are willing to live with cacheline > > contention (on ->d_lock of root dentry, if nothing else), why not > > do the following: > > * put all dentries of such directories ([0-9]* and [0-9]*/task/*) > > into a list anchored in task_struct; have non-counting reference to > > task_struct stored in them (might simplify part of get_proc_task() users, > > Hmm. > > Right now I don't think we actually create any dentries at all for the > short-lived process case. > > Wouldn't your suggestion make fork/exit rather worse? > > Or would you create the dentries dynamically still at lookup time, and > then attach them to the process at that point? > > What list would you use for the dentry chaining? Would you play games > with the dentry hashing, and "hash" them off the process, and never > hit in the lookup cache? I'd been thinking of ->d_fsdata pointing to a structure with list_head and a (non-counting) task_struct pointer for those guys. Allocated on lookup, of course (as well as readdir ;-/) and put on the list at the same time. IOW, for short-lived process we simply have an empty (h)list anchored in task_struct and that's it.