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

From: Matthew Wilcox <willy@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: David Hildenbrand <david@redhat.com>,
	Chih-En Lin <shiyn.lin@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Ingo Molnar <mingo@redhat.com>,
	"Peter Zijlstra (Intel)" <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>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Linux Kernel Mailing List <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: Sun, 22 May 2022 20:40:44 +0100	[thread overview]
Message-ID: <YoqRvDn2P+9i0SrF@casper.infradead.org> (raw)
In-Reply-To: <c82ac102-7935-4b07-9d8c-82a2efe59956@www.fastmail.com>

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.