From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1033853AbeCASQw (ORCPT ); Thu, 1 Mar 2018 13:16:52 -0500 Received: from mail-pl0-f46.google.com ([209.85.160.46]:44879 "EHLO mail-pl0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1033321AbeCASQv (ORCPT ); Thu, 1 Mar 2018 13:16:51 -0500 X-Google-Smtp-Source: AG47ELu9+7Hzr7J3IjfDHD7u72pTvaGEh+hlQ7WMKWD2Ph9AQfLGKRgLnQlQilXEJ1epC/L6bnOJVA== Message-ID: <1519928208.11375.3.camel@gmail.com> Subject: Re: Use higher-order pages in vmalloc From: Eric Dumazet To: Michal Hocko , Andy Lutomirski Cc: Matthew Wilcox , Dave Hansen , Konstantin Khlebnikov , LKML , Christoph Hellwig , Linux-MM , Andrew Morton , "Kirill A. Shutemov" Date: Thu, 01 Mar 2018 10:16:48 -0800 In-Reply-To: <20180223121300.GU30681@dhcp22.suse.cz> References: <151670492223.658225.4605377710524021456.stgit@buzz> <151670493255.658225.2881484505285363395.stgit@buzz> <20180221154214.GA4167@bombadil.infradead.org> <20180221170129.GB27687@bombadil.infradead.org> <20180222065943.GA30681@dhcp22.suse.cz> <20180222122254.GA22703@bombadil.infradead.org> <20180222133643.GJ30681@dhcp22.suse.cz> <20180223121300.GU30681@dhcp22.suse.cz> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.22.6-1+deb9u1 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 2018-02-23 at 13:13 +0100, Michal Hocko wrote: > On Thu 22-02-18 19:01:35, Andy Lutomirski wrote: > > On Thu, Feb 22, 2018 at 1:36 PM, Michal Hocko wrote: > > > On Thu 22-02-18 04:22:54, Matthew Wilcox wrote: > > > > On Thu, Feb 22, 2018 at 07:59:43AM +0100, Michal Hocko wrote: > > > > > On Wed 21-02-18 09:01:29, Matthew Wilcox wrote: > > > > > > Right. It helps with fragmentation if we can keep higher-order > > > > > > allocations together. > > > > > > > > > > Hmm, wouldn't it help if we made vmalloc pages migrateable instead? That > > > > > would help the compaction and get us to a lower fragmentation longterm > > > > > without playing tricks in the allocation path. > > > > > > > > I was wondering about that possibility. If we want to migrate a page > > > > then we have to shoot down the PTE across all CPUs, copy the data to the > > > > new page, and insert the new PTE. Copying 4kB doesn't take long; if you > > > > have 12GB/s (current example on Wikipedia: dual-channel memory and one > > > > DDR2-800 module per channel gives a theoretical bandwidth of 12.8GB/s) > > > > then we should be able to copy a page in 666ns). So there's no problem > > > > holding a spinlock for it. > > > > > > > > But we can't handle a fault in vmalloc space today. It's handled in > > > > arch-specific code, see vmalloc_fault() in arch/x86/mm/fault.c > > > > If we're going to do this, it'll have to be something arches opt into > > > > because I'm not taking on the job of fixing every architecture! > > > > > > yes. > > > > On x86, if you shoot down the PTE for the current stack, you're dead. > > vmalloc_fault() might not even be called. Instead we hit > > do_double_fault(), and the manual warns extremely strongly against > > trying to recover, and, in this case, I agree with the SDM. If you > > actually want this to work, there needs to be a special IPI broadcast > > to the task in question (with appropriate synchronization) that calls > > magic arch code that does the switcheroo. > > Why cannot we use the pte swap entry trick also for vmalloc migration. > I haven't explored this path at all, to be honest. > > > Didn't someone (Christoph?) have a patch to teach the page allocator > > to give high-order allocations if available and otherwise fall back to > > low order? > > Do you mean kvmalloc? I sent something last year but had not finished the patch series :/ https://marc.info/?l=linux-kernel&m=148233423610544&w=2