linux-kernel - Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table

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Message-Id: <c82ac102-7935-4b07-9d8c-82a2efe59956@www.fastmail.com>
Date:   Sun, 22 May 2022 08:20:05 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     "David Hildenbrand" <david@...hat.com>,
        "Chih-En Lin" <shiyn.lin@...il.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        "Ingo Molnar" <mingo@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Juri Lelli" <juri.lelli@...hat.com>,
        "Vincent Guittot" <vincent.guittot@...aro.org>,
        "Dietmar Eggemann" <dietmar.eggemann@....com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        "Ben Segall" <bsegall@...gle.com>, "Mel Gorman" <mgorman@...e.de>,
        "Daniel Bristot de Oliveira" <bristot@...hat.com>,
        "Christian Brauner" <brauner@...nel.org>,
        "Vlastimil Babka" <vbabka@...e.cz>,
        "William Kucharski" <william.kucharski@...cle.com>,
        "John Hubbard" <jhubbard@...dia.com>,
        "Yunsheng Lin" <linyunsheng@...wei.com>,
        "Arnd Bergmann" <arnd@...db.de>,
        "Suren Baghdasaryan" <surenb@...gle.com>,
        "Colin Cross" <ccross@...gle.com>,
        "Feng Tang" <feng.tang@...el.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Mike Rapoport" <rppt@...nel.org>,
        "Geert Uytterhoeven" <geert@...ux-m68k.org>,
        "Anshuman Khandual" <anshuman.khandual@....com>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        "Daniel Axtens" <dja@...ens.net>,
        "Jonathan Marek" <jonathan@...ek.ca>,
        "Christophe Leroy" <christophe.leroy@...roup.eu>,
        "Pasha Tatashin" <pasha.tatashin@...een.com>,
        "Peter Xu" <peterx@...hat.com>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Sebastian Andrzej Siewior" <bigeasy@...utronix.de>,
        "Fenghua Yu" <fenghua.yu@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "Kaiyang Zhao" <zhao776@...due.edu>,
        "Huichun Feng" <foxhoundsk.tw@...il.com>,
        "Jim Huang" <jserv.tw@...il.com>
Subject: Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table



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.

It would still be quite a change, though.