Re: [RFC] virtio_pmem: enable live migration support

[Pankaj Gupta <pankaj.gupta@ionos.com>]

> >>>> I mean, that would be fundamentally broken, because the fsync() would
> >>>> corrupt the file. So I assume in a sane environment, the dst could only
> >>>> have stale clean pagecache pages. And we'd have to get rid of these to
> >>>> re-read everything from file.
> >>>
> >>> In case of write back cache mode, we could still have stale dirty
> >>> pages at the destination
> >>> host and destination fsync is not the right thing to do. We need to
> >>> invalidate these pages
> >>> (Can we invalidate dirty pages resident in page cache with
> >>> POSIX_FADV_DONTNEED as
> >>> well?) man pages say, we cannot (unless i misunderstood it).
> >>>
> >>
> >> I think you'd have to fsync + POSIX_FADV_DONTNEED. But I am still
> >> confused how we could end up with dirty pagecache pages on the
> >> destination. In my opinion, there should only be clean pagecache pages
> >> -- can someone enlighten me? :)
> >
> > because of activity on the page cache pages corresponding to mmap region
> > in the past which is not synced yet or not reclaimed yet. Maybe this
> > is hypothetical
> > or not possible, happy to learn?
>
> Right, but assume the following *sane*
>
> #1 H0 starts and runs VM.
> #2 H0 migrates VM to H1.
> #3 H1 runs VM.
> #4 H1 migrates VM to H0.
> #5 H0 runs VM.
>
> We'd expect a proper fsync during #2, writing back any dirty pages to
> the memory backend. Otherwise, #3 would already be broken. Similarly,
> we'd expect a proper fsync during #4.
>
> I assume during #4 we could find clean pagecache pages that are actually
> invalid, because the underlying file was changed by H1. So we have to
> make sure to invalidate all pagecache pages (all clean).

Yes, you mean fsync on source host before migration starts. My point
is something
like another process mmap same backend file on destination host and/or
guest/qemu
crashing abruptly.