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

From: David Hildenbrand <david@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Chih-En Lin <shiyn.lin@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Christian Brauner <brauner@kernel.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	William Kucharski <william.kucharski@oracle.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Yunsheng Lin <linyunsheng@huawei.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Suren Baghdasaryan <surenb@google.com>,
	Colin Cross <ccross@google.com>, Feng Tang <feng.tang@intel.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Mike Rapoport <rppt@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
	Daniel Axtens <dja@axtens.net>,
	Jonathan Marek <jonathan@marek.ca>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Pasha Tatashin <pasha.tatashin@soleen.com>,
	Peter Xu <peterx@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Lutomirski <luto@kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Fenghua Yu <fenghua.yu@intel.com>,
	linux-kernel@vger.kernel.org, Kaiyang Zhao <zhao776@purdue.edu>,
	Huichun Feng <foxhoundsk.tw@gmail.com>,
	Jim Huang <jserv.tw@gmail.com>
Subject: Re: [RFC PATCH 0/6] Introduce Copy-On-Write to Page Table
Date: Sat, 21 May 2022 22:22:03 +0200	[thread overview]
Message-ID: <d4716615-fbac-9d93-c386-e47385e85bef@redhat.com> (raw)
In-Reply-To: <YolHr1GwfA++i9jj@casper.infradead.org>

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