Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability

["Daniel P. Berrangé" <berrange@redhat.com>]

On Thu, Mar 30, 2023 at 06:01:51PM -0400, Peter Xu wrote:
> On Thu, Mar 30, 2023 at 03:03:20PM -0300, Fabiano Rosas wrote:
> > From: Nikolay Borisov <nborisov@suse.com>
> > 
> > Implement 'fixed-ram' feature. The core of the feature is to ensure that
> > each ram page of the migration stream has a specific offset in the
> > resulting migration stream. The reason why we'd want such behavior are
> > two fold:
> > 
> >  - When doing a 'fixed-ram' migration the resulting file will have a
> >    bounded size, since pages which are dirtied multiple times will
> >    always go to a fixed location in the file, rather than constantly
> >    being added to a sequential stream. This eliminates cases where a vm
> >    with, say, 1G of ram can result in a migration file that's 10s of
> >    GBs, provided that the workload constantly redirties memory.
> > 
> >  - It paves the way to implement DIO-enabled save/restore of the
> >    migration stream as the pages are ensured to be written at aligned
> >    offsets.
> > 
> > The feature requires changing the stream format. First, a bitmap is
> > introduced which tracks which pages have been written (i.e are
> > dirtied) during migration and subsequently it's being written in the
> > resulting file, again at a fixed location for every ramblock. Zero
> > pages are ignored as they'd be zero in the destination migration as
> > well. With the changed format data would look like the following:
> > 
> > |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages|
> 
> What happens with huge pages?  Would page size matter here?
> 
> I would assume it's fine it uses a constant (small) page size, assuming
> that should match with the granule that qemu tracks dirty (which IIUC is
> the host page size not guest's).
> 
> But I didn't yet pay any further thoughts on that, maybe it would be
> worthwhile in all cases to record page sizes here to be explicit or the
> meaning of bitmap may not be clear (and then the bitmap_size will be a
> field just for sanity check too).

I think recording the page sizes is an anti-feature in this case.

The migration format / state needs to reflect the guest ABI, but
we need to be free to have different backend config behind that
either side of the save/restore.

IOW, if I start a QEMU with 2 GB of RAM, I should be free to use
small pages initially and after restore use 2 x 1 GB hugepages,
or vica-verca.

The important thing with the pages that are saved into the file
is that they are a 1:1 mapping guest RAM regions to file offsets.
IOW, the 2 GB of guest RAM is always a contiguous 2 GB region
in the file.

If the src VM used 1 GB pages, we would be writing a full 2 GB
of data assuming both pages were dirty.

If the src VM used 4k pages, we would be writing some subset of
the 2 GB of data, and the rest would be unwritten.

Either way, when reading back the data we restore it into either
1 GB pages of 4k pages, beause any places there were unwritten
orignally  will read back as zeros.

> If postcopy might be an option, we'd want the page size to be the host page
> size because then looking up the bitmap will be straightforward, deciding
> whether we should copy over page (UFFDIO_COPY) or fill in with zeros
> (UFFDIO_ZEROPAGE).

This format is only intended for the case where we are migrating to
a random-access medium, aka a file, because the fixed RAM mappings
to disk mean that we need to seek back to the original location to
re-write pages that get dirtied. It isn't suitable for a live
migration stream, and thus postcopy is inherantly out of scope.

With regards,
Daniel
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