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=-2.3 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 50EDDC10F14 for ; Thu, 11 Apr 2019 13:28:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 204A3217F4 for ; Thu, 11 Apr 2019 13:28:23 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="VuRmDIon" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726646AbfDKN2W (ORCPT ); Thu, 11 Apr 2019 09:28:22 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:34362 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726264AbfDKN2W (ORCPT ); Thu, 11 Apr 2019 09:28:22 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=6VFuhNCf+gH+llXbEHsi41/TZwlA6SKkFNV6Ppyi0Os=; b=VuRmDIoncI2k8GTzl20u00d+I eBhCx9kBdWBraFVeGej9WAi4QaDRhmbyH/CzYXmIptJtzET/2Ef9e5tiQjpr+gNDEyvF6o6rA6a+B L1PNZQuCwUsgEDdCXc0zlu/agnCQZcjLOhjgWiQyijKVVlVUqAQ5hDpJ9uHAy0ZuX7KUNhSWP1WA8 wmoiN/M18X7Xv831vE/Mllly0QDoMn+hoOHwmNFn4CdifhpWJ+GJxr0IBUOdFdRJo1fR+sIJ4pSvL SvAfzuuSCLahhHVKDCR5q2suIN4ZfSpC0ze5sJTxpbt/wT3b5E0+b+TThbtMOLxMq8a/PJA4vqv7y yYvVTtgsA==; Received: from willy by bombadil.infradead.org with local (Exim 4.90_1 #2 (Red Hat Linux)) id 1hEZkZ-0007O7-Cw; Thu, 11 Apr 2019 13:28:19 +0000 Date: Thu, 11 Apr 2019 06:28:19 -0700 From: Matthew Wilcox To: Vlastimil Babka Cc: lsf-pc@lists.linux-foundation.org, Linux-FSDevel , linux-mm , linux-block@vger.kernel.org, Michal Hocko , Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim , Ming Lei , linux-xfs@vger.kernel.org, Christoph Hellwig , Dave Chinner , "Darrick J . Wong" Subject: Re: [LSF/MM TOPIC] guarantee natural alignment for kmalloc()? Message-ID: <20190411132819.GB22763@bombadil.infradead.org> References: <790b68b7-3689-0ff6-08ae-936728bc6458@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <790b68b7-3689-0ff6-08ae-936728bc6458@suse.cz> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Thu, Apr 11, 2019 at 02:52:08PM +0200, Vlastimil Babka wrote: > In the session I hope to resolve the question whether this is indeed the > right thing to do for all kmalloc() users, without an explicit alignment > requests, and if it's worth the potentially worse > performance/fragmentation it would impose on a hypothetical new slab > implementation for which it wouldn't be optimal to split power-of-two > sized pages into power-of-two-sized objects (or whether there are any > other downsides). I think this is exactly the kind of discussion that LSFMM is for! It's really a whole-system question; is Linux better-off having the flexibility for allocators to return non-power-of-two aligned memory, or allowing consumers of the kmalloc API to assume that "sufficiently large" memory is naturally aligned. Another possibility that should be considered is introducing a kmalloc() variant like posix_memalign() that allows for specifying the alignment, or just kmalloc_naturally_aligned(). And we probably need to reiterate for the benefit of those not following the discussion that creating a slab cache (which does allow for alignment to be specified) is impractical for this use case because the actual allocations are of variable size, but always need to be 512-byte aligned.