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 CEC70C433E0 for ; Sat, 13 Jun 2020 16:01:20 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 66A0B20789 for ; Sat, 13 Jun 2020 16:01:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 66A0B20789 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=zeniv.linux.org.uk Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id E4FD26B0005; Sat, 13 Jun 2020 12:01:19 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id E264F6B0006; Sat, 13 Jun 2020 12:01:19 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id D61736B0007; Sat, 13 Jun 2020 12:01:19 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0068.hostedemail.com [216.40.44.68]) by kanga.kvack.org (Postfix) with ESMTP id BD3C26B0005 for ; Sat, 13 Jun 2020 12:01:19 -0400 (EDT) Received: from smtpin27.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 82BAD1EE6 for ; Sat, 13 Jun 2020 16:01:19 +0000 (UTC) X-FDA: 76924653078.27.push86_1f17faa26de6 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin27.hostedemail.com (Postfix) with ESMTP id 55B743D668 for ; Sat, 13 Jun 2020 16:01:19 +0000 (UTC) X-HE-Tag: push86_1f17faa26de6 X-Filterd-Recvd-Size: 3576 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [195.92.253.2]) by imf42.hostedemail.com (Postfix) with ESMTP for ; Sat, 13 Jun 2020 16:01:18 +0000 (UTC) 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 Cc: Arnd Bergmann , Russell King - ARM Linux admin , Linus Walleij , "linux-kernel@vger.kernel.org" , Linux-MM , Linux ARM , Nicolas Pitre , Catalin Marinas , Will Deacon , "Paul E. McKenney" 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200613154118.GH23230@ZenIV.linux.org.uk> X-Rspamd-Queue-Id: 55B743D668 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam03 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 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...