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=-2.0 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 3280DC3A5A4 for ; Wed, 28 Aug 2019 19:46:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0556D20856 for ; Wed, 28 Aug 2019 19:46:19 +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="Rm8Ri2tE" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726828AbfH1TqO (ORCPT ); Wed, 28 Aug 2019 15:46:14 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:60686 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726591AbfH1TqO (ORCPT ); Wed, 28 Aug 2019 15:46:14 -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-Transfer-Encoding :Content-Type:MIME-Version:References:Message-ID:Subject:Cc:To:From:Date: Sender:Reply-To: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=hgz1R5QUT/3HwdqWiBoXJEjgsTAkIAv2dRGNBsyqnxk=; b=Rm8Ri2tEMBapDNdB3tS8NGKJBM z5Rq8Hj5vRSAxcFpyrTIzaG/+MT4NsQnvWV7/mV2hJF6gRa7y6uflXN/pKocPsEW3+1uIDcqhDT/d nVn5GK/ST6ZnS4k1deCvzxjlhOzDyvPUBhbmsaCbiqSy4yWGM5TlmlSbxvkYeV7PUGjKfq11w4O6d B6K21kFHA5FEY7Y5poC9Rbup6llyf0SFLQyUZGcPNKUtyKUUKmtqXMwj9rmC4ynL+BB6is1FCaG0v 5RDdSskGb5hOH0jTQ1cq94H/Lpm3HBF4RN3Hl/sqRH3RcAUqISgadxnEhq3e2+jm9c50kgmalm08Y v6mF9MJw==; Received: from willy by bombadil.infradead.org with local (Exim 4.92 #3 (Red Hat Linux)) id 1i33tQ-0004Z6-3n; Wed, 28 Aug 2019 19:46:08 +0000 Date: Wed, 28 Aug 2019 12:46:08 -0700 From: Matthew Wilcox To: Christopher Lameter Cc: Vlastimil Babka , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Pekka Enberg , David Rientjes , Ming Lei , Dave Chinner , "Darrick J . Wong" , Christoph Hellwig , linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-block@vger.kernel.org, James Bottomley , linux-btrfs@vger.kernel.org Subject: Re: [PATCH v2 2/2] mm, sl[aou]b: guarantee natural alignment for kmalloc(power-of-two) Message-ID: <20190828194607.GB6590@bombadil.infradead.org> References: <20190826111627.7505-1-vbabka@suse.cz> <20190826111627.7505-3-vbabka@suse.cz> <0100016cd98bb2c1-a2af7539-706f-47ba-a68e-5f6a91f2f495-000000@email.amazonses.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <0100016cd98bb2c1-a2af7539-706f-47ba-a68e-5f6a91f2f495-000000@email.amazonses.com> User-Agent: Mutt/1.11.4 (2019-03-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org On Wed, Aug 28, 2019 at 06:45:07PM +0000, Christopher Lameter wrote: > > Ideally we should provide to mm users what they need without difficult > > workarounds or own reimplementations, so let's make the kmalloc() alignment to > > size explicitly guaranteed for power-of-two sizes under all configurations. > > The objection remains that this will create exceptions for the general > notion that all kmalloc caches are aligned to KMALLOC_MINALIGN which may Hmm? kmalloc caches will be aligned to both KMALLOC_MINALIGN and the natural alignment of the object. > be suprising and it limits the optimizations that slab allocators may use > for optimizing data use. The SLOB allocator was designed in such a way > that data wastage is limited. The changes here sabotage that goal and show > that future slab allocators may be similarly constrained with the > exceptional alignents implemented. Additional debugging features etc etc > must all support the exceptional alignment requirements. While I sympathise with the poor programmer who has to write the fourth implementation of the sl*b interface, it's more for the pain of picking a new letter than the pain of needing to honour the alignment of allocations. There are many places in the kernel which assume alignment. They break when it's not supplied. I believe we have a better overall system if the MM developers provide stronger guarantees than the MM consumers have to work around only weak guarantees. > > * SLOB has no implicit alignment so this patch adds it explicitly for > > kmalloc(). The potential downside is increased fragmentation. While > > pathological allocation scenarios are certainly possible, in my testing, > > after booting a x86_64 kernel+userspace with virtme, around 16MB memory > > was consumed by slab pages both before and after the patch, with difference > > in the noise. > > This change to slob will cause a significant additional use of memory. The > advertised advantage of SLOB is that *minimal* memory will be used since > it is targeted for embedded systems. Different types of slab objects of > varying sizes can be allocated in the same memory page to reduce > allocation overhead. Did you not read the part where he said the difference was in the noise? > The result of this patch is just to use more memory to be safe from > certain pathologies where one subsystem was relying on an alignment that > was not specified. That is why this approach should not be called > �natural" but "implicit alignment". The one using the slab cache is not > aware that the slab allocator provides objects aligned in a special way > (which is in general not needed. There seems to be a single pathological > case that needs to be addressed and I thought that was due to some > brokenness in the hardware?). It turns out there are lots of places which assume this, including the pmem driver, the ramdisk driver and a few other similar drivers. > It is better to ensure that subsystems that require special alignment > explicitly tell the allocator about this. But it's not the subsystems which have this limitation which do the allocation; it's the subsystems who allocate the memory that they then pass to the subsystems. So you're forcing communication of these limits up & down the stack. > I still think implicit exceptions to alignments are a bad idea. Those need > to be explicity specified and that is possible using kmem_cache_create(). I swear we covered this last time the topic came up, but XFS would need to create special slab caches for each size between 512 and PAGE_SIZE. Potentially larger, depending on whether the MM developers are willing to guarantee that kmalloc(PAGE_SIZE * 2, GFP_KERNEL) will return a PAGE_SIZE aligned block of memory indefinitely.