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Fri, 30 Sep 2022 21:34:27 +0000 (UTC) X-FDA: 79970055774.05.0889258 Received: from eu-smtp-delivery-151.mimecast.com (eu-smtp-delivery-151.mimecast.com [185.58.85.151]) by imf14.hostedemail.com (Postfix) with ESMTP id 30BEE100004 for ; Fri, 30 Sep 2022 21:34:26 +0000 (UTC) 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; 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dkim=none; spf=pass (imf14.hostedemail.com: domain of david.laight@aculab.com designates 185.58.85.151 as permitted sender) smtp.mailfrom=david.laight@aculab.com; dmarc=pass (policy=none) header.from=aculab.com X-Rspamd-Server: rspam01 X-HE-Tag: 1664573666-269493 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: From: Hugh Dickins > Sent: 30 September 2022 17:22 >=20 > 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. >=20 > 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. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)