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

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Date:   Sat, 21 May 2022 22:22:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Chih-En Lin <shiyn.lin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <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>,
        Andy Lutomirski <luto@...nel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        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 21.05.22 22:12, Matthew Wilcox wrote:
> On Sat, May 21, 2022 at 06:07:27PM +0200, David Hildenbrand wrote:
>> I'm missing the most important point: why do we care and why should we
>> care to make our COW/fork implementation even more complicated?
>>
>> Yes, we might save some page tables and we might reduce the fork() time,
>> however, which specific workload really benefits from this and why do we
>> really care about that workload? Without even hearing about an example
>> user in this cover letter (unless I missed it), I naturally wonder about
>> relevance in practice.
> 
> As I get older (and crankier), I get less convinced that fork() is
> really the right solution for implementing system().

Heh, I couldn't agree more. IMHO, fork() is mostly a blast from the
past. There *are* still a lot of user and there are a couple of sane use
cases.

Consequently, I am not convinced that it is something to optimize for,
especially if it adds additional complexity. For the use case of
snapshotting, we have better mechanisms nowadays (uffd-wp) that avoid
messing with copying address spaces.

Calling fork()/system() from a big, performance-sensitive process is
usually a bad idea.

Note: there is an (for me) interesting paper about this topic from 2019
("A fork() in the road"), although it might be a bit biased coming from
Microsoft research :). It comes to a similar conclusion regarding fork
and how it should or shouldn't dictate our OS design.

[1] https://www.microsoft.com/en-us/research/publication/a-fork-in-the-road/

-- 
Thanks,

David / dhildenb