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=-5.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 C9E22C3A5A4 for ; Fri, 30 Aug 2019 22:29:44 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7AF7F23431 for ; Fri, 30 Aug 2019 22:29:44 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=armlinux.org.uk header.i=@armlinux.org.uk header.b="folcv0CY" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7AF7F23431 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=armlinux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 287796B0006; Fri, 30 Aug 2019 18:29:44 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 237806B0008; Fri, 30 Aug 2019 18:29:44 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 127EB6B000A; Fri, 30 Aug 2019 18:29:44 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0219.hostedemail.com [216.40.44.219]) by kanga.kvack.org (Postfix) with ESMTP id E3CF26B0006 for ; Fri, 30 Aug 2019 18:29:43 -0400 (EDT) Received: from smtpin24.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with SMTP id 51203181AC9AE for ; Fri, 30 Aug 2019 22:29:43 +0000 (UTC) X-FDA: 75880537446.24.cow21_1a8a4e5fc4a17 X-HE-Tag: cow21_1a8a4e5fc4a17 X-Filterd-Recvd-Size: 6462 Received: from pandora.armlinux.org.uk (pandora.armlinux.org.uk [78.32.30.218]) by imf35.hostedemail.com (Postfix) with ESMTP for ; Fri, 30 Aug 2019 22:29:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=armlinux.org.uk; s=pandora-2019; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=BWWRszBDY8GYUUZj497FcR9Z0dm6vMXF/gAZdr1ItkU=; b=folcv0CYTG/fwfjtFT9od8gVs LmEkdhrber88LNPUMfbnmY4rZ5fcd6bEa4DdOxmDoMu8xDExn/Ah5NsTeoI67i87J4KX+zvMgMHIb Ws9JXOfrWIwDKAxjHZQtmgeB4RYPXtOCted2vh317YUbPEILBPMVpm3trknP9hAJMv6/Y2wgPA+WP wNCmXpqm4dfbnsqB6hRtF87ZAG3KuEF6wLqrmulnjvI4Q/59G99z2HxGzlO9ADkFfPthetXTJumhD 23Mnjr5oNCzxiwqoWuV2HZnjQQPp2jL64KL9cYbdk5DXvRG6sWjdB3OTqaVm3UT3YlCBvws4YFgZW XWmelzPCA==; Received: from shell.armlinux.org.uk ([fd8f:7570:feb6:1:5054:ff:fe00:4ec]:39464) by pandora.armlinux.org.uk with esmtpsa (TLSv1.2:ECDHE-RSA-AES256-GCM-SHA384:256) (Exim 4.90_1) (envelope-from ) id 1i3pOJ-00013m-HY; Fri, 30 Aug 2019 23:29:11 +0100 Received: from linux by shell.armlinux.org.uk with local (Exim 4.92) (envelope-from ) id 1i3pOE-0000Tw-MP; Fri, 30 Aug 2019 23:29:06 +0100 Date: Fri, 30 Aug 2019 23:29:06 +0100 From: Russell King - ARM Linux admin To: "Eric W. Biederman" Cc: Jing Xiangfeng , kstewart@linuxfoundation.org, gregkh@linuxfoundation.org, gustavo@embeddedor.com, bhelgaas@google.com, tglx@linutronix.de, sakari.ailus@linux.intel.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] arm: fix page faults in do_alignment Message-ID: <20190830222906.GH13294@shell.armlinux.org.uk> References: <1567171877-101949-1-git-send-email-jingxiangfeng@huawei.com> <20190830133522.GZ13294@shell.armlinux.org.uk> <87d0gmwi73.fsf@x220.int.ebiederm.org> <20190830203052.GG13294@shell.armlinux.org.uk> <87y2zav01z.fsf@x220.int.ebiederm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87y2zav01z.fsf@x220.int.ebiederm.org> User-Agent: Mutt/1.10.1 (2018-07-13) 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 Fri, Aug 30, 2019 at 04:02:48PM -0500, Eric W. Biederman wrote: > Russell King - ARM Linux admin writes: > > > On Fri, Aug 30, 2019 at 02:45:36PM -0500, Eric W. Biederman wrote: > >> Russell King - ARM Linux admin writes: > >> > >> > On Fri, Aug 30, 2019 at 09:31:17PM +0800, Jing Xiangfeng wrote: > >> >> The function do_alignment can handle misaligned address for user and > >> >> kernel space. If it is a userspace access, do_alignment may fail on > >> >> a low-memory situation, because page faults are disabled in > >> >> probe_kernel_address. > >> >> > >> >> Fix this by using __copy_from_user stead of probe_kernel_address. > >> >> > >> >> Fixes: b255188 ("ARM: fix scheduling while atomic warning in alignment handling code") > >> >> Signed-off-by: Jing Xiangfeng > >> > > >> > NAK. > >> > > >> > The "scheduling while atomic warning in alignment handling code" is > >> > caused by fixing up the page fault while trying to handle the > >> > mis-alignment fault generated from an instruction in atomic context. > >> > > >> > Your patch re-introduces that bug. > >> > >> And the patch that fixed scheduling while atomic apparently introduced a > >> regression. Admittedly a regression that took 6 years to track down but > >> still. > > > > Right, and given the number of years, we are trading one regression for > > a different regression. If we revert to the original code where we > > fix up, we will end up with people complaining about a "new" regression > > caused by reverting the previous fix. Follow this policy and we just > > end up constantly reverting the previous revert. > > > > The window is very small - the page in question will have had to have > > instructions read from it immediately prior to the handler being entered, > > and would have had to be made "old" before subsequently being unmapped. > > > Rather than excessively complicating the code and making it even more > > inefficient (as in your patch), we could instead retry executing the > > instruction when we discover that the page is unavailable, which should > > cause the page to be paged back in. > > My patch does not introduce any inefficiencies. It onlys moves the > check for user_mode up a bit. My patch did duplicate the code. > > > If the page really is unavailable, the prefetch abort should cause a > > SEGV to be raised, otherwise the re-execution should replace the page. > > > > The danger to that approach is we page it back in, and it gets paged > > back out before we're able to read the instruction indefinitely. > > I would think either a little code duplication or a function that looks > at user_mode(regs) and picks the appropriate kind of copy to do would be > the best way to go. Because what needs to happen in the two cases for > reading the instruction are almost completely different. That is what I mean. I'd prefer to avoid that with the large chunk of code. How about instead adding a local replacement for probe_kernel_address() that just sorts out the reading, rather than duplicating all the code to deal with thumb fixup. > > However, as it's impossible for me to contact the submitter, anything > > I do will be poking about in the dark and without any way to validate > > that it does fix the problem, so I think apart from reviewing of any > > patches, there's not much I can do. > > I didn't realize your emails to him were bouncing. That is odd. Mine > don't appear to be. Hmm, so the fact I posted publically in reply to my reply with the MTA bounce message didn't give you a clue? -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up According to speedtest.net: 11.9Mbps down 500kbps up