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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 355BBC433F5 for ; Sun, 22 May 2022 19:43:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1344757AbiEVTmS (ORCPT ); Sun, 22 May 2022 15:42:18 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43400 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230252AbiEVTmO (ORCPT ); Sun, 22 May 2022 15:42:14 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 57C362FFC1 for ; Sun, 22 May 2022 12:42:13 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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; bh=VJOiU6BQi5P886CY3iFuhDrifu3rFeIvy3Ts/J6ugWs=; b=rQdbGHWULvJr/+YfcZL2BWrq9P oblMQR2ONoZ8H6J19kBsRKhePWr3eYFB20yLweMhTw3F/IwXNEXqmYc9+BPeM+yUyyl/QUnXNSkhU a0+0SPge7Oy2nI1lnaebibi+fSjO6GMLNeGwTncNF33YaBhDJURU8vgfKWmj+PQ7YnDsHeccvwH8T rCLiLgGsZNYiRpKCN/VuHJZxZW743LKi6358SohTqtJdBgsvihnj8N+gu2BLyLcHLqrjZ2ZM10hiB DeW/V8ci9utfz5HFPMr+zRk9RMMS/R4wnBVyYVFaFjJtQsxIB5aokIgduAGC3FpL0+5PQvQ+ngtcW sHW9DZXw==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1nsrRU-00FZ4M-P3; Sun, 22 May 2022 19:40:44 +0000 Date: Sun, 22 May 2022 20:40:44 +0100 From: Matthew Wilcox To: Andy Lutomirski Cc: David Hildenbrand , Chih-En Lin , Andrew Morton , linux-mm@kvack.org, Ingo Molnar , "Peter Zijlstra (Intel)" , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Christian Brauner , Vlastimil Babka , William Kucharski , John Hubbard , Yunsheng Lin , Arnd Bergmann , Suren Baghdasaryan , Colin Cross , Feng Tang , "Eric W. Biederman" , Mike Rapoport , Geert Uytterhoeven , Anshuman Khandual , "Aneesh Kumar K.V" , Daniel Axtens , Jonathan Marek , Christophe Leroy , Pasha Tatashin , Peter Xu , Andrea Arcangeli , Thomas Gleixner , Sebastian Andrzej Siewior , Fenghua Yu , Linux Kernel Mailing List , Kaiyang Zhao , Huichun Feng , Jim Huang Subject: Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table Message-ID: References: <20220519183127.3909598-1-shiyn.lin@gmail.com> <64d0da08-6ffd-4bce-bc66-5097913937b4@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 22, 2022 at 08:20:05AM -0700, Andy Lutomirski wrote: > On Sat, May 21, 2022, at 5:31 PM, Matthew Wilcox wrote: > > On Sat, May 21, 2022 at 03:19:24PM -0700, Andy Lutomirski wrote: > >> I can see a rather different use for this type of shared-pagetable > >> technology, though: monstrous MAP_SHARED mappings. For database and some VM > >> users, multiple processes will map the same file. If there was a way to > >> ensure appropriate alignment (or at least encourage it) and a way to handle > >> mappings that don't cover the whole file, then having multiple mappings > >> share the same page tables could be a decent efficiently gain. This doesn't > >> even need COW -- it's "just" pagetable sharing. > > > > The mshare proposal did not get a warm reception at LSFMM ;-( > > > > The conceptual model doesn't seem to work for the MM developers who were > > in the room. "Fear" was the most-used word. Not sure how we're going > > to get to a model of sharing page tables that doesn't scare people. > > FWIW, I didn’t like mshare. mshare was weird: it seemed to have > one mm own some page tables and other mms share them. I’m talking > about having a *file* own page tables and mms map them. This seems less > fear-inducing to me. Circular dependencies are impossible, mmap calls > don’t need to propagate, etc. OK, so that doesn't work for our use case. We need an object to own page tables that can be shared between different (co-operating) processes. Because we need the property that calling mprotect() changes the protection in all processes at the same time. Obviously we want that object to be referenced by a file descriptor, and it can also have a name. That object doesn't have to be an mm_struct. Maybe that would be enough of a change to remove the fear.