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=-3.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 95DB8C4363A for ; Mon, 26 Oct 2020 13:29:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5DBEB22281 for ; Mon, 26 Oct 2020 13:29:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ibm1wh4V" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1774855AbgJZN3P (ORCPT ); Mon, 26 Oct 2020 09:29:15 -0400 Received: from casper.infradead.org ([90.155.50.34]:39728 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1774827AbgJZN3P (ORCPT ); Mon, 26 Oct 2020 09:29:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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; bh=34Y6r4GAweG7Mt/E2/mBFuI95NWQikkATWq3rx/SY8E=; b=ibm1wh4V//Ao40finn9v+c/DR0 WKib4xCrTtJVXIcaYUSJ1zU9hxgEEylo/MOwv+q5ZiA8y+g2DZLWIzl8wDZgPrphU363e/eovSkbe wa8+Q/VSuBNCwj2sFg7XcOvhFS/EE0Svh1QiUwmtbID0haANDZN3FPixUgMNfP2TKAwyB7du+BqX3 57Zm9kGjWCqnlE1u6dMQSnpHdc1Q3QeMZ4MsfZ5NFLADi86YvqhE8PZsiwcstdJ9d6eQLLgTALS2D qTsGz1vLwcBqGa+R08Uhn6VTC1hgBXjQz19Ryi4baKdQnioISiyXvnUwX4F1y7aH0k8+ydR8NXeyq T3YFjdTQ==; Received: from willy by casper.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1kX2Y2-000704-4d; Mon, 26 Oct 2020 13:28:30 +0000 Date: Mon, 26 Oct 2020 13:28:30 +0000 From: Matthew Wilcox To: John Hubbard Cc: "Kirill A. Shutemov" , Dave Hansen , Andy Lutomirski , Peter Zijlstra , Paolo Bonzini , Sean Christopherson , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , David Rientjes , Andrea Arcangeli , Kees Cook , Will Drewry , "Edgecombe, Rick P" , "Kleen, Andi" , Liran Alon , Mike Rapoport , x86@kernel.org, kvm@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" Subject: Re: [RFCv2 08/16] KVM: Use GUP instead of copy_from/to_user() to access guest memory Message-ID: <20201026132830.GQ20115@casper.infradead.org> References: <20201020061859.18385-1-kirill.shutemov@linux.intel.com> <20201020061859.18385-9-kirill.shutemov@linux.intel.com> <20201022114946.GR20115@casper.infradead.org> <30ce6691-fd70-76a2-8b61-86d207c88713@nvidia.com> <20201026042158.GN20115@casper.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Sun, Oct 25, 2020 at 09:44:07PM -0700, John Hubbard wrote: > On 10/25/20 9:21 PM, Matthew Wilcox wrote: > > I don't think the page pinning approach is ever valid. For file > > Could you qualify that? Surely you don't mean that the entire pin_user_pages > story is a waste of time--I would have expected you to make more noise > earlier if you thought that, yes? I do think page pinning is the wrong approach for everything. I did say so at the time, and I continue to say so when the opportunity presents itself. But shouting about it constantly only annoys people, so I don't generally bother. I have other things to work on, and they're productive, so I don't need to spend my time arguing. > > It's been five years since DAX was merged, and page pinning still > > doesn't work. How much longer before the people who are pushing it > > realise that it's fundamentally flawed? > > Is this a separate rant about *only* DAX, or is general RDMA in your sights > too? :) This is a case where it's not RDMA's _fault_ that there's no good API for it to do what it needs to do. There's a lot of work needed to wean Linux device drivers off their assumption that there's a struct page for every byte of memory.