Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating

[Juan Quintela <quintela@redhat.com>]

David Hildenbrand <david@redhat.com> wrote:
> Resizing while migrating is dangerous and does not work as expected.
> The whole migration code works on the usable_length of ram blocks and does
> not expect this to change at random points in time.
>
> Precopy: The ram block size must not change on the source, after
> ram_save_setup(), so as long as the guest is still running on the source.
>
> Postcopy: The ram block size must not change on the target, after
> synchronizing the RAM block list (ram_load_precopy()).
>
> AFAIKS, resizing can be trigger *after* (but not during) a reset in
> ACPI code by the guest
> - hw/arm/virt-acpi-build.c:acpi_ram_update()
> - hw/i386/acpi-build.c:acpi_ram_update()
>
> I see no easy way to work around this. Fail hard instead of failing
> somewhere in migration code due to strange other reasons. AFAIKs, the
> rebuilts will be triggered during reboot, so this should not affect
> running guests, but only guests that reboot at a very bad time and
> actually require size changes.
>
> Let's further limit the impact by checking if an actual resize of the
> RAM (in number of pages) is required.
>
> Don't perform the checks in qemu_ram_resize(), as that's called during
> migration when syncing the used_length. Update documentation.
>
> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Cc: Eduardo Habkost <ehabkost@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Igor Mammedov <imammedo@redhat.com>
> Cc: "Michael S. Tsirkin" <mst@redhat.com>
> Cc: Richard Henderson <richard.henderson@linaro.org>
> Cc: Shannon Zhao <shannon.zhao@linaro.org>
> Cc: Alex Bennée <alex.bennee@linaro.org>
> Cc: Shameerali Kolothum Thodi <shameerali.kolothum.thodi@huawei.com>
> Cc: Juan Quintela <quintela@redhat.com>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---


>
> Any idea how to avoid killing the guest? Anything obvious I am missing?

If you avoid the resize, it should be ok for both precopy & postcopy.

But, as you point, if acpi guest is the one changing sizes, we are in
trouble.  But really, it makes exactly zero sense to reset during
migrate.  if we _could_ catch the reset, the "intelligent" thing to do
is:

- detect reset
- launch guest on destination from zero.

I.e. not migration at all.  This would be my "better" idea, but I have
no clue how to catch that kind of things in a sane way that works in
every architecture.

You get the:

Reviewed-by: Juan Quintela <quintela@redhat.com>

because:
- your code change makes sense
- the documentation update is good.

Thanks, Juan.