Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots

[Andrey Gruzdev]

On 24.02.2021 20:01, David Hildenbrand wrote:
> On 24.02.21 17:56, Andrey Gruzdev wrote:
> > On 22.02.2021 21:11, David Hildenbrand wrote:
> > > On 22.02.21 18:54, Peter Xu wrote:
> > > > On Mon, Feb 22, 2021 at 06:33:27PM +0100, David Hildenbrand wrote:
> > > > > On 22.02.21 18:29, Peter Xu wrote:
> > > > > > On Sat, Feb 20, 2021 at 02:59:42AM -0500, David Hildenbrand wrote:
> > > > > > > Live snapshotting ends up reading all guest memory (dirty bitmap
> > > > > > > starts with all 1s), which is not what we want for virtio-mem - we
> > > > > > > don’t want to read and migrate memory that has been discarded and
> > > > > > > has no stable content.
> > > > > > >
> > > > > > > For ordinary migration we use the guest page hint API to clear
> > > > > > > bits in the dirty bitmap after dirty bitmap sync. Well, if we
> > > > > > > don‘t do bitmap syncs we‘ll never clear any dirty bits. That‘s the
> > > > > > > problem.
> > > > > >
> > > > > > Using dirty bitmap for that information is less efficient, becase 
> > > > > > it's
> > > > > > definitely a larger granularity information than PAGE_SIZE. If the
> > > > > > disgarded
> > > > > > ranges are always continuous and at the end of a memory region, we
> > > > > > should have
> > > > > > some parameter in the ramblock showing that where we got shrinked
> > > > > > then we don't
> > > > > > check dirty bitmap at all, rather than always assuming used_length
> > > > > > is the one.
> > > > >
> > > > > They are randomly scattered across the whole RAMBlock.
> > > > > Shrinking/growing
> > > > > will be done to some degree in the future (but it won't get rid of the
> > > > > general sparse layout we can produce).
> > > >
> > > > OK. Btw I think currently live snapshot should still be reading dirty
> > > > bitmap,
> > > > so maybe it's still fine.  It's just that it's still not very clear
> > > > to hide
> > > > virtio-mem information into dirty bitmap, imho, since that's not how we
> > > > interpret dirty bitmap - which is only for the sake of tracking page
> > > > changes.
> > >
> > > Well, currently it is "what do we have to migrate".
> > Using dirty bitmap can prohibit unexpected (overwritten) content of
> > pre-discarded pages, but they'll appear as zeroed on destination.
>
> To be precise, they'll usually appear as discarded (no pages in the 
> migration stream).

I just mean the retained content of the page after population - zeroes.

> > What about other virtio-baloon features like HW_POISON when poisoned
> > pages should also be retained on population filled with certain pattern?
>
> In case of free page reporting, we skip discarding if poisoning is set 
> to != 0 because of that reason:
>
> if (virtio_balloon_inhibited() || dev->poison_val) {
>     goto skip_element;
> }
Got it, thanks.

--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                virtuzzo.com