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 AC7EDC7113C for ; Fri, 18 Aug 2023 12:17:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1376813AbjHRMQs convert rfc822-to-8bit (ORCPT ); Fri, 18 Aug 2023 08:16:48 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56122 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1376864AbjHRMQf (ORCPT ); Fri, 18 Aug 2023 08:16:35 -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 D7B79421B for ; Fri, 18 Aug 2023 05:16:30 -0700 (PDT) Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-202-98C0T_vJOJK6bOwFbwR9Yw-1; Fri, 18 Aug 2023 13:16:27 +0100 X-MC-Unique: 98C0T_vJOJK6bOwFbwR9Yw-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Fri, 18 Aug 2023 13:16:23 +0100 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Fri, 18 Aug 2023 13:16:23 +0100 From: David Laight To: 'David Howells' , Linus Torvalds CC: Al Viro , Jens Axboe , "Christoph Hellwig" , Christian Brauner , "Matthew Wilcox" , Jeff Layton , "linux-fsdevel@vger.kernel.org" , "linux-block@vger.kernel.org" , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" Subject: RE: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc() Thread-Topic: [PATCH v3 2/2] iov_iter: Don't deal with iter->copy_mc in memcpy_from_iter_mc() Thread-Index: AQHZ0DpP/l59sWTPXU+UuQ9VGbJikq/s16Kg///6foCAACRpIIAC+zJggAAE0WA= Date: Fri, 18 Aug 2023 12:16:23 +0000 Message-ID: References: <03730b50cebb4a349ad8667373bb8127@AcuMS.aculab.com> <20230816120741.534415-1-dhowells@redhat.com> <20230816120741.534415-3-dhowells@redhat.com> <608853.1692190847@warthog.procyon.org.uk> <3dabec5643b24534a1c1c51894798047@AcuMS.aculab.com> <665724.1692218114@warthog.procyon.org.uk> <1748360.1692358952@warthog.procyon.org.uk> In-Reply-To: <1748360.1692358952@warthog.procyon.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 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-block@vger.kernel.org From: David Howells > Sent: Friday, August 18, 2023 12:43 PM > > Linus Torvalds wrote: > > > Well, that part is trivially fixable, and we should do that anyway for > > other reasons. > > .. > > enum iter_type { > > /* iter types */ > > - ITER_IOVEC, > > - ITER_KVEC, > > - ITER_BVEC, > > - ITER_XARRAY, > > - ITER_DISCARD, > > - ITER_UBUF, > > + ITER_IOVEC = 1, > > + ITER_UBUF = 2, > > + ITER_KVEC = 4, > > + ITER_BVEC = 8, > > + ITER_XARRAY = 16, > > + ITER_DISCARD = 32, That could be zero - no bits and default. > > }; > > It used to be this way, but Al switched it: > > 8cd54c1c848031a87820e58d772166ffdf8c08c0 > iov_iter: separate direction from flavour Except it also had the direction flag inside the enum. That caused its own piles of grief. IIRC Linus had type:6 - that doesn't leave any headroom for additional types (even though they shouldn't proliferate). It may be best to avoid bits 15+ (in a bitfield) due to issues with large constants and sign extension. On x86 (I think) 'and immediate' and 'bit test' are the same size for bit 0 to 7, BIT wins for higher bits. gcc generates strange code for some initialisers (see yesterday's thread) and you definitely mustn't leave unused bits in a bitfield. Might be better is the fields are assigned later! (I also saw clang carefully preserving %eax on stabck!) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)