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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 3E45BC433F5 for ; Fri, 30 Sep 2022 21:34:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231772AbiI3Ver convert rfc822-to-8bit (ORCPT ); Fri, 30 Sep 2022 17:34:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56852 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231654AbiI3Ve1 (ORCPT ); Fri, 30 Sep 2022 17:34:27 -0400 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B339B1D1A7A for ; Fri, 30 Sep 2022 14:34:26 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-92-6L0_Y9URMfO4eHTHzlMxwg-1; Fri, 30 Sep 2022 22:34:24 +0100 X-MC-Unique: 6L0_Y9URMfO4eHTHzlMxwg-1 Received: from AcuMS.Aculab.com (10.202.163.4) by AcuMS.aculab.com (10.202.163.4) with Microsoft SMTP Server (TLS) id 15.0.1497.38; Fri, 30 Sep 2022 22:34:23 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.040; Fri, 30 Sep 2022 22:34:23 +0100 From: David Laight To: 'Hugh Dickins' , Kees Cook CC: Vlastimil Babka , Joel Fernandes , Hyeonggon Yoo <42.hyeyoo@gmail.com>, Matthew Wilcox , Andrew Morton , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" Subject: RE: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE Thread-Topic: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE Thread-Index: AQHY1LmyNvo5HN+EEECxPXiP8YKQXa33zoaAgABIuACAAGaCkA== Date: Fri, 30 Sep 2022 21:34:23 +0000 Message-ID: References: <35502bdd-1a78-dea1-6ac3-6ff1bcc073fa@suse.cz> <16fe443a-dabe-aa46-6bc7-dad03f29e0dc@google.com> <06329f-e498-ccc-6223-32f87716436d@google.com> In-Reply-To: <06329f-e498-ccc-6223-32f87716436d@google.com> 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 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Hugh Dickins > Sent: 30 September 2022 17:22 > > On Fri, 30 Sep 2022, David Laight wrote: > > > > > Regarding "conflicting" alignment requests: yes, I agree with you, > > > > > it would have to be a toolchain bug if when asked to align 2 and to > > > > > align 4, it chose not to align 4. > > > > See https://godbolt.org/z/3nGsTaf5e > > the align() directive takes precedence. > > > > Here you only want to ensure the alignment is at least 4. > > Sorry, I don't understand the point you are making, > nor how to deduce it from the link which you give: > I'll leave it to those who understand better. (I've copied Kees Cook - I think it is his patches that are increasing the function alignment. He'll need to find the thread history...) IIRC -malign_functions is being used to put functions onto 16 byte boundaries to get aligned pad space before the function. Adding __align(4) overrides this and (I think) will remove the pad space. I could only see the alignment directives in gcc ouput. clang didn't seem to generate them. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)