From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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 Date: Thu, 11 Apr 2019 06:28:19 -0700 From: Matthew Wilcox 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> Sender: linux-xfs-owner@vger.kernel.org List-ID: List-Id: xfs 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" 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.