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 A0264C433F5 for ; Tue, 30 Nov 2021 09:23:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S240128AbhK3J0y convert rfc822-to-8bit (ORCPT ); Tue, 30 Nov 2021 04:26:54 -0500 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]:25956 "EHLO eu-smtp-delivery-151.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S240105AbhK3J0x (ORCPT ); Tue, 30 Nov 2021 04:26:53 -0500 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-13-Ag_JXOyjMBuSXQrxBjFzkA-1; Tue, 30 Nov 2021 09:23:32 +0000 X-MC-Unique: Ag_JXOyjMBuSXQrxBjFzkA-1 Received: from AcuMS.Aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) by AcuMS.aculab.com (fd9f:af1c:a25b:0:994c:f5c2:35d6:9b65) with Microsoft SMTP Server (TLS) id 15.0.1497.26; Tue, 30 Nov 2021 09:23:30 +0000 Received: from AcuMS.Aculab.com ([fe80::994c:f5c2:35d6:9b65]) by AcuMS.aculab.com ([fe80::994c:f5c2:35d6:9b65%12]) with mapi id 15.00.1497.026; Tue, 30 Nov 2021 09:23:30 +0000 From: David Laight To: 'Rustam Kovhaev' CC: 'Vlastimil Babka' , Christoph Lameter , "penberg@kernel.org" , "rientjes@google.com" , "iamjoonsoo.kim@lge.com" , "akpm@linux-foundation.org" , "corbet@lwn.net" , "djwong@kernel.org" , "david@fromorbit.com" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-doc@vger.kernel.org" , "gregkh@linuxfoundation.org" , "viro@zeniv.linux.org.uk" , "dvyukov@google.com" Subject: RE: [PATCH v4] slob: add size header to all allocations Thread-Topic: [PATCH v4] slob: add size header to all allocations Thread-Index: AQHX344YYrlQE8gw8k+1+rn2N9Qq+awQ435QgArNSACAACX/IA== Date: Tue, 30 Nov 2021 09:23:30 +0000 Message-ID: References: <037227db-c869-7d9c-65e8-8f5f8682171d@suse.cz> <20211122013026.909933-1-rkovhaev@gmail.com> <3c996e22-034f-1013-3978-1f786aae38fb@suse.cz> <148d2774-77b9-bb25-c132-80b00e16ea06@suse.cz> <69fc0cead9774dfdba816a8e25f30a53@AcuMS.aculab.com> In-Reply-To: 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-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: Rustam Kovhaev > Sent: 30 November 2021 07:00 > > On Tue, Nov 23, 2021 at 10:18:27AM +0000, David Laight wrote: > > From: Vlastimil Babka > > > Sent: 22 November 2021 10:46 > > > > > > On 11/22/21 11:36, Christoph Lameter wrote: > > > > On Mon, 22 Nov 2021, Vlastimil Babka wrote: > > > > > > > >> But it seems there's no reason we couldn't do better? I.e. use the value of > > > >> SLOB_HDR_SIZE only to align the beginning of actual object (and name the > > > >> define different than SLOB_HDR_SIZE). But the size of the header, where we > > > >> store the object lenght could be just a native word - 4 bytes on 32bit, 8 on > > > >> 64bit. The address of the header shouldn't have a reason to be also aligned > > > >> to ARCH_KMALLOC_MINALIGN / ARCH_SLAB_MINALIGN as only SLOB itself processes > > > >> it and not the slab consumers which rely on those alignments? > > > > > > > > Well the best way would be to put it at the end of the object in order to > > > > avoid the alignment problem. This is a particular issue with SLOB because > > > > it allows multiple types of objects in a single page frame. > > > > ... > > > > > > So I guess placement at the beginning cannot be avoided. That in turn runs > > > > into trouble with the DMA requirements on some platforms where the > > > > beginning of the object has to be cache line aligned. > > > > > > It's no problem to have the real beginning of the object aligned, and the > > > prepended header not. > > > > I'm not sure that helps. > > The header can't share a cache line with the previous item (because it > > might be mapped for DMA) so will always take a full cache line. > > I thought that DMA API allocates buffers that are larger than page size. > DMA pool seems to be able to give out smaller buffers, but underneath it > seems to be calling page allocator. > The SLOB objects that have this header are all less than page size, and > they cannot end up in DMA code paths, or can they? The problem isn't dma_alloc_coherent() it is when memory allocated elsewhere is used for DMA. On systems with non-coherent DMA accesses the data cache has to be flushed before all and invalidated after read DMA transfers. The cpu must not dirty any of the cache lines associated with a read DMA. This is on top of any requirements for the alignment of the returned address. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)