Re: [RFC PATCH 0/4] Restore change_pte optimization to its former glory

[Andrea Arcangeli]

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.