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=-3.8 required=3.0 tests=BAYES_00, 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 9C242C4346B for ; Mon, 21 Sep 2020 16:28:08 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 092AA2396D for ; Mon, 21 Sep 2020 16:28:07 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 092AA2396D 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 6CBCD900095; Mon, 21 Sep 2020 12:28:07 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 67B63900093; Mon, 21 Sep 2020 12:28:07 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 5907C900094; Mon, 21 Sep 2020 12:28:07 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0231.hostedemail.com [216.40.44.231]) by kanga.kvack.org (Postfix) with ESMTP id 3D42D900093 for ; Mon, 21 Sep 2020 12:28:07 -0400 (EDT) Received: from smtpin20.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 070FB824999B for ; Mon, 21 Sep 2020 16:28:07 +0000 (UTC) X-FDA: 77287600614.20.vase72_2f16a1527146 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin20.hostedemail.com (Postfix) with ESMTP id CC37D180C07A3; Mon, 21 Sep 2020 16:28:06 +0000 (UTC) X-HE-Tag: vase72_2f16a1527146 X-Filterd-Recvd-Size: 3664 Received: from ZenIV.linux.org.uk (zeniv.linux.org.uk [195.92.253.2]) by imf04.hostedemail.com (Postfix) with ESMTP; Mon, 21 Sep 2020 16:28:06 +0000 (UTC) Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kKOfR-003EDr-FT; Mon, 21 Sep 2020 16:27:53 +0000 Date: Mon, 21 Sep 2020 17:27:53 +0100 From: Al Viro To: David Laight Cc: Christoph Hellwig , Andrew Morton , Jens Axboe , Arnd Bergmann , David Howells , "linux-arm-kernel@lists.infradead.org" , "linux-kernel@vger.kernel.org" , "linux-mips@vger.kernel.org" , "linux-parisc@vger.kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "linux-s390@vger.kernel.org" , "sparclinux@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-scsi@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-aio@kvack.org" , "io-uring@vger.kernel.org" , "linux-arch@vger.kernel.org" , "linux-mm@kvack.org" , "netdev@vger.kernel.org" , "keyrings@vger.kernel.org" , "linux-security-module@vger.kernel.org" Subject: Re: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw() Message-ID: <20200921162753.GY3421308@ZenIV.linux.org.uk> References: <20200921143434.707844-1-hch@lst.de> <20200921143434.707844-3-hch@lst.de> <20200921150211.GS3421308@ZenIV.linux.org.uk> <20200921152937.GX3421308@ZenIV.linux.org.uk> <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com> 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 Mon, Sep 21, 2020 at 03:44:00PM +0000, David Laight wrote: > From: Al Viro > > Sent: 21 September 2020 16:30 > > > > On Mon, Sep 21, 2020 at 03:21:35PM +0000, David Laight wrote: > > > > > You really don't want to be looping through the array twice. > > > > Profiles, please. > > I did some profiling of send() v sendmsg() much earlier in the year. > I can't remember the exact details but the extra cost of sendmsg() > is far more than you might expect. > (I was timing sending fully built IPv4 UDP packets using a raw socket.) > > About half the difference does away if you change the > copy_from_user() to __copy_from_user() when reading the struct msghdr > and iov[] from userspace (user copy hardening is expensive). Wha...? So the difference is within 4 times the overhead of the hardening checks done for one call of copy_from_user()? > The rest is just code path, my gut feeling is that a lot of that > is in import_iovec(). > > Remember semdmsg() is likely to be called with an iov count of 1. ... which makes that loop unlikely to be noticable in the entire mess, whether you pass it once or twice. IOW, unless you can show profiles where that loop is sufficiently hot or if you can show the timings change from splitting it in two, I'll remain very sceptical about that assertion.