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

From: Dr. David Alan Gilbert
Subject: Re: [PATCH RFC] memory: Don't allow to resize RAM while migrating
Date: Fri, 14 Feb 2020 11:02:26 +0000
User-agent: Mutt/1.13.3 (2020-01-12)

* David Hildenbrand (address@hidden) wrote:
> On 14.02.20 11:42, Dr. David Alan Gilbert wrote:
> > * David Hildenbrand (address@hidden) wrote:
> >> On 14.02.20 11:25, Dr. David Alan Gilbert wrote:
> >>> * David Hildenbrand (address@hidden) 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.
> >>>
> >>> Interesting; we need to do something about this - but banning resets
> >>> during migration is a bit harsh; and aborting the source VM is really
> >>> nasty - for a precopy especially we shouldn't kill the source VM,
> >>> we should just abort the migration.
> >>
> >> Any alternative, easy solutions to handle this? I do wonder how often
> >> this will actually trigger in real life.
> > 
> > Well it's not that hard to abort a migration (I'm not sure we've got a
> > convenient wrapper to do it - but it shouldn't be hard to add).
> > 
> We do have qmp_migrate_cancel(). I hope that can be called under BQL.

Well it's a monitor command so I think so; although it's not really
designed for an error - it's a user action.  Doing a
migrate_set_error(..) followed by a qemu_file_shutdown is probably a
good bet.

> Can that be called in both, precopy and postcopy case? I assume in the
> precopy, it's easy.

The problem is during postcopy you're toast when that happens because
you can't restart; however, can this happen once we're actually in
postcopy?  It's a little different - if it happens before the transition
to postcopy then it's the same as precopy; if it happens afterwards..
well it's going to happen ont he destination side and that's quite


> -- 
> Thanks,
> David / dhildenb
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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