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

[Daniel P . Berrangé]

On Fri, Mar 31, 2023 at 10:39:23AM -0400, Peter Xu wrote:
> On Fri, Mar 31, 2023 at 08:56:01AM +0100, Daniel P. Berrangé wrote:
> > 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.
> 
> I think there's already the page size information, because there's a bitmap
> embeded in the format at least in the current proposal, and the bitmap can
> only be defined with a page size provided in some form.
> 
> Here I agree the backend can change before/after a migration (live or
> not).  Though the question is whether page size matters in the snapshot
> layout rather than what the loaded QEMU instance will use as backend.

IIUC, the page size information merely sets a constraint on the granularity
of unwritten (sparse) regions in the file. If we didn't want to express
page size directly in the file format we would need explicit start/end
offsets for each written block. This is less convenient that just having
a bitmap, so I think its ok to use the page size bitmap

> > > 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.
> 
> Yes, I've commented also in the cover letter, but I can expand a bit.
> 
> I mean support postcopy only when loading, but not when saving.
> 
> Saving to file definitely cannot work with postcopy because there's no dest
> qemu running.
> 
> Loading from file, OTOH, can work together with postcopy.

Ahh, I see what you mean.

> Right now AFAICT current approach is precopy loading the whole guest image
> with the supported snapshot format (if I can call it just a snapshot).
> 
> What I want to say is we can consider supporting postcopy on loading in
> that we start an "empty" QEMU dest node, when any page fault triggered we
> do it using userfault and lookup the snapshot file instead rather than
> sending a request back to the source.  I mentioned that because there'll be
> two major benefits which I mentioned in reply to the cover letter quickly,
> but I can also extend here:
> 
>   - Firstly, the snapshot format is ideally storing pages in linear
>     offsets, it means when we know some page missing we can use O(1) time
>     looking it up from the snapshot image.
> 
>   - Secondly, we don't need to let the page go through the wires, neither
>     do we need to send a request to src qemu or anyone.  What we need here
>     is simply test the bit on the snapshot bitmap, then:
> 
>     - If it is copied, do UFFDIO_COPY to resolve page faults,
>     - If it is not copied, do UFFDIO_ZEROPAGE (e.g., if not hugetlb,
>       hugetlb can use a fake UFFDIO_COPY)
> 
> So this is a perfect testing ground for using postcopy in a very efficient
> way against a file snapshot.

Yes, that's an nice unexpected benefit of this fixed ram file format.

With regards,
Daniel
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