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Date:   Sun, 22 May 2022 01:31:02 +0100
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 <peterz@infradead.org>,
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        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>,
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        Feng Tang <feng.tang@intel.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Mike Rapoport <rppt@kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
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        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
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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.