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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 792E8C433F5 for ; Sun, 22 May 2022 22:09:50 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 21BC88D0003; Sun, 22 May 2022 18:09:50 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 1CBCC8D0001; Sun, 22 May 2022 18:09:50 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 093C08D0003; Sun, 22 May 2022 18:09:50 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0015.hostedemail.com [216.40.44.15]) by kanga.kvack.org (Postfix) with ESMTP id EF9FE8D0001 for ; Sun, 22 May 2022 18:09:49 -0400 (EDT) Received: from smtpin06.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay01.hostedemail.com (Postfix) with ESMTP id CD69760D76 for ; Sun, 22 May 2022 19:41:22 +0000 (UTC) X-FDA: 79494398004.06.746D0A4 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) by imf23.hostedemail.com (Postfix) with ESMTP id CB7CE14001D for ; Sun, 22 May 2022 19:41:01 +0000 (UTC) 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: X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: CB7CE14001D X-Stat-Signature: rwtjnu6hqp6wxi1nrdjs7myinueayh4w Authentication-Results: imf23.hostedemail.com; dkim=pass header.d=infradead.org header.s=casper.20170209 header.b=rQdbGHWU; dmarc=none; spf=none (imf23.hostedemail.com: domain of willy@infradead.org has no SPF policy when checking 90.155.50.34) smtp.mailfrom=willy@infradead.org X-Rspam-User: X-HE-Tag: 1653248461-418804 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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.