Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754170AbaK0KHK (ORCPT ); Thu, 27 Nov 2014 05:07:10 -0500 Received: from [133.145.228.5] ([133.145.228.5]:33068 "EHLO mail4.hitachi.co.jp" rhost-flags-FAIL-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1752887AbaK0KHG (ORCPT ); Thu, 27 Nov 2014 05:07:06 -0500 Message-ID: <5476F7AD.3020601@hitachi.com> Date: Thu, 27 Nov 2014 19:06:37 +0900 From: Masami Hiramatsu Organization: Hitachi, Ltd., Japan User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:13.0) Gecko/20120614 Thunderbird/13.0.1 MIME-Version: 1.0 To: Josh Poimboeuf Cc: Jiri Kosina , Seth Jennings , Vojtech Pavlik , Steven Rostedt , Petr Mladek , Miroslav Benes , Christoph Hellwig , Greg KH , Andy Lutomirski , live-patching@vger.kernel.org, x86@kernel.org, kpatch@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCHv4 0/3] Kernel Live Patching References: <1416935709-404-1-git-send-email-sjenning@redhat.com> <547596C6.2050303@hitachi.com> <20141126152759.GA29079@treble.hsd1.ky.comcast.net> In-Reply-To: <20141126152759.GA29079@treble.hsd1.ky.comcast.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org (2014/11/27 0:27), Josh Poimboeuf wrote: > On Wed, Nov 26, 2014 at 10:18:24AM +0100, Jiri Kosina wrote: >> On Wed, 26 Nov 2014, Masami Hiramatsu wrote: >> >>>> Note to Steve: >>>> Masami's IPMODIFY patch is heading for -next via your tree. Once it arrives, >>>> I'll rebase and make the change to set IPMODIFY. Do not pull this for -next >>>> yet. This version (v4) is for review and gathering acks. >>> >>> BTW, as we discussed IPMODIFY is an exclusive flag. So if we allocate >>> ftrace_ops for each function in each patch, it could be conflict each >>> other. >> >> Yup, this corresponds to what Petr brought up yesterday. There are cases >> where all solutions (kpatch, kgraft, klp) would allocate multiple >> ftrace_ops for a single function entry (think of patching one function >> multiple times in a row). >> >> So it's not as easy as just setting the flag. >> >>> Maybe we need to have another ops hashtable to find such conflict and >>> new handler to handle it. >> >> If I understand your proposal correctly, that would sound like a hackish >> workaround, trying to basically trick the IPMODIFY flag semantics you just >> implemented :) > > I think Masami may be proposing something similar to what we do in > kpatch today. We have a single ftrace_ops and handler which is used for > all functions. The handler accesses a global hash of kpatch_func > structs which is indexed by the original function's IP address. Hmm, I think both is OK. kpatch method is less memory consuming and will have a bigger overhead. However, as Steven talked at Plumbers Conf., he will introduce a direct code modifying interface for ftrace. After that is introduced, we don't need to care about performance degradation by patching :) > It actually works out pretty well because it nicely encapsulates the > knowledge about which functions are patched in a single place. And it > makes it easy to track function versions (for incremental patching and > rollback). > >> What I'd propose instead is to make sure that we always have >> just a ftrace_ops per function entry, and only update the pointers there >> as necessary. Fortunately we can do the switch atomically, by making use >> of ->private. > > But how would you update multiple functions atomically, to enforce > per-thread consistency? At this point, both can do it atomically. We just need an atomic flag for applying patches. Thank you, -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@hitachi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/