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

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Date:   Sun, 22 May 2022 20:40:44 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Andy Lutomirski <luto@...nel.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 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.