From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [RFC PATCH 0/4] Restore change_pte optimization to its former
 glory
Date: Mon, 11 Feb 2019 15:02:00 -0500
Message-ID: <20190211200200.GA30128@redhat.com>
References: <20190131183706.20980-1-jglisse@redhat.com>
 <20190201235738.GA12463@redhat.com>
 <20190211190931.GA3908@redhat.com>
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Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Peter Xu <peterx@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Michal Hocko <mhocko@kernel.org>, kvm@vger.kernel.org
To: Jerome Glisse <jglisse@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
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In-Reply-To: <20190211190931.GA3908@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
List-Id: kvm.vger.kernel.org

On Mon, Feb 11, 2019 at 02:09:31PM -0500, Jerome Glisse wrote:
> Yeah, between do you have any good workload for me to test this ? I
> was thinking of running few same VM and having KSM work on them. Is
> there some way to trigger KVM to fork ? As the other case is breaking
> COW after fork.

KVM can fork on guest pci-hotplug events or network init to run host
scripts and re-init the signals before doing the exec, but it won't
move the needle because all guest memory registered in the MMU
notifier is set as MADV_DONTFORK... so fork() is a noop unless qemu is
also modified not to call MADV_DONTFORK.

Calling if (!fork()) exit(0) from a timer at regular intervals during
qemu runtime after turning off MADV_DONTFORK in qemu would allow to
exercise fork against the KVM MMU Notifier methods.

The optimized change_pte code in copy-on-write code is the same
post-fork or post-KSM merge and fork() itself doesn't use change_pte
while KSM does, so with regard to change_pte it should already provide
a good test coverage to test with only KSM without fork(). It'll cover
the read-write -> readonly transition with same PFN
(write_protect_page), the read-only to read-only changing PFN
(replace_page) as well as the readonly -> read-write transition
changing PFN (wp_page_copy) all three optimized with change_pte. Fork
would not leverage change_pte for the first two cases.