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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,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 2DB48C433E0 for ; Sat, 13 Jun 2020 16:01:30 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 00B072074D for ; Sat, 13 Jun 2020 16:01:29 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="X6tjxtLv" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 00B072074D Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=DjQvz8v/mTnPde1EDC0GnPI7odXYRnh1J7EUeXYSw5A=; b=X6tjxtLveOyOPG PpWF0sCJndpHi16V4LLS7S5xPsYkmeNCjhN1bX3SFt0+sF4Na0XtA7KKs4RPhxj3+r09XCvsn0sp8 b9G+IcHPTaKjP1nAB31ACcd674RMizKIafC1t7w8Y5LsGhiZ1Um6QE/xJVi7xvxWeM6cxlkYRepB+ m7mumQL+hWbGlk8rLkZjrIBUnWflChMmAZhwh2JvMACDNUWWpP1PAAfEJhagvURKtYU08PrKTbkSY AmZHVq/yelQi+2U6bD7DmmY/RsVITqqU5ACo8x68eL9mD6UMrtJ0TSceqETgEMF5k9N3qTUj9djHF 3IdII8PtgA4yNMQnlCWw==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1jk8aw-0007TF-UC; Sat, 13 Jun 2020 16:01:22 +0000 Received: from [2002:c35c:fd02::1] (helo=ZenIV.linux.org.uk) by bombadil.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1jk8au-0007SZ-2I for linux-arm-kernel@lists.infradead.org; Sat, 13 Jun 2020 16:01:21 +0000 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.93 #3 (Red Hat Linux)) id 1jk8aG-008Em7-6w; Sat, 13 Jun 2020 16:00:40 +0000 Date: Sat, 13 Jun 2020 17:00:40 +0100 From: Al Viro To: afzal mohammed Subject: Re: [RFC 1/3] lib: copy_{from,to}_user using gup & kmap_atomic() Message-ID: <20200613160040.GI23230@ZenIV.linux.org.uk> References: <9e1de19f35e2d5e1d115c9ec3b7c3284b4a4e077.1591885760.git.afzal.mohd.ma@gmail.com> <20200612135538.GA13399@afzalpc> <20200613120432.GA5319@afzalpc> <20200613125126.GE23230@ZenIV.linux.org.uk> <20200613125615.GF23230@ZenIV.linux.org.uk> <20200613134236.GA4086@afzalpc> <20200613153102.GG23230@ZenIV.linux.org.uk> <20200613154118.GH23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200613154118.GH23230@ZenIV.linux.org.uk> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20200613_090120_111753_36A1108E X-CRM114-Status: GOOD ( 16.51 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: "Paul E. McKenney" , Arnd Bergmann , Nicolas Pitre , Catalin Marinas , Linus Walleij , Russell King - ARM Linux admin , "linux-kernel@vger.kernel.org" , Linux-MM , Will Deacon , Linux ARM Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, Jun 13, 2020 at 04:41:18PM +0100, Al Viro wrote: > On Sat, Jun 13, 2020 at 04:31:02PM +0100, Al Viro wrote: > > On Sat, Jun 13, 2020 at 07:12:36PM +0530, afzal mohammed wrote: > > > Hi, > > > > > > On Sat, Jun 13, 2020 at 01:56:15PM +0100, Al Viro wrote: > > > > > > > Incidentally, what about get_user()/put_user()? _That_ is where it's > > > > going to really hurt... > > > > > > All other uaccess routines are also planned to be added, posting only > > > copy_{from,to}_user() was to get early feedback (mentioned in the > > > cover letter) > > > > Sure, but what I mean is that I'd expect the performance loss to be > > dominated by that, not by copy_from_user/copy_to_user on large amounts > > of data. Especially on the loads like kernel builds - a lot of stat() > > and getdents() calls there. > > To clarify: stat() means small copy_to_user(), getdents() - a mix of > put_user() and small copy_to_user(). I would be very surprised if it > does not hurt a lot. PS: there's another fun issue here: fill a file with zeroes mmap that file in two areas, MAP_SHARED thread 1: munmap() the first area fill the second one with 'X' thread 2: write() from the first area into pipe One could expect that nothing by zeroes gets written into pipe - it might be a short write() (or -EFAULT), but finding any 'X' there would be a bug. Your patches allow for a possibility of write() doing get_user_pages_fast(), getting the first page just as munmap() is about to remove it from page tables and bugger off. Then thread 1 proceeds with the store (via the second area). And then thread 2 does memcpy() from that thing via a kmap_atomic()-created alias, observing the effect of the store. That might or might not be a POSIX violation, but it does look like a QoI issue... _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel