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 0BE63C43466 for ; Mon, 21 Sep 2020 15:44:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id CE44D20888 for ; Mon, 21 Sep 2020 15:44:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727790AbgIUPoM convert rfc822-to-8bit (ORCPT ); Mon, 21 Sep 2020 11:44:12 -0400 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:27783 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727386AbgIUPoH (ORCPT ); Mon, 21 Sep 2020 11:44:07 -0400 Received: from AcuMS.aculab.com (156.67.243.126 [156.67.243.126]) (Using TLS) by relay.mimecast.com with ESMTP id uk-mta-29--2oWre79P1qw86_f0GzqkQ-1; Mon, 21 Sep 2020 16:44:02 +0100 X-MC-Unique: -2oWre79P1qw86_f0GzqkQ-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) by AcuMS.aculab.com (fd9f:af1c:a25b:0:43c:695e:880f:8750) with Microsoft SMTP Server (TLS) id 15.0.1347.2; Mon, 21 Sep 2020 16:44:00 +0100 Received: from AcuMS.Aculab.com ([fe80::43c:695e:880f:8750]) by AcuMS.aculab.com ([fe80::43c:695e:880f:8750%12]) with mapi id 15.00.1347.000; Mon, 21 Sep 2020 16:44:00 +0100 From: David Laight To: 'Al Viro' 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() Thread-Topic: [PATCH 02/11] mm: call import_iovec() instead of rw_copy_check_uvector() in process_vm_rw() Thread-Index: AQHWkCg9ha58Xpw3RkmKZfc82fYDTKlzMiEw///00oCAABILoA== Date: Mon, 21 Sep 2020 15:44:00 +0000 Message-ID: <226e03bf941844eba4d64af31633c177@AcuMS.aculab.com> 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> In-Reply-To: <20200921152937.GX3421308@ZenIV.linux.org.uk> Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=C51A453 smtp.mailfrom=david.laight@aculab.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org 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). 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. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)