Re: [Qemu-devel] [RFC 00/13] Live memory snapshot based on userfaultfd

[Christian Pinto]

Hello Hailiang,

are there any updates on this patch series? Are you planning to release 
a new version?

You say there are some issues with the current snapshot-v2 version, 
which issues were you referring to? On my side the only problem I have 
seen was that the live snapshot was not working on ARMv8, but I have 
fixed that and managed to successfully snapshot and restore a QEMU ARMv8 
tcg machine on an ARMv8 host. I will gladly contribute with these fixes 
once you will release a new version of the patches.


Thanks a lot,

Christian

On 20/08/2016 08:31, Hailiang Zhang wrote:
> Hi,
>
> I updated this series, but didn't post it, because there are some 
> problems while i tested the snapshot function.
> I didn't know if it is the userfaultfd issue or not.
> I don't have time to investigate it this month. I have put them in github
> https://github.com/coloft/qemu/tree/snapshot-v2
>
> Anyone who want to test and modify it are welcomed!
>
> Besides, will you join the linuxcon or KVM forum in Canada ?
> I wish to see you there if you join the conference ;)
>
> Thanks,
> Hailiang
>
>
>
> On 2016/8/18 23:56, Andrea Arcangeli wrote:
> > Hello everyone,
> >
> > I've an aa.git tree uptodate on the master & userfault branch (master
> > includes other pending VM stuff, userfault branch only contains
> > userfault enhancements):
> >
> > https://git.kernel.org/cgit/linux/kernel/git/andrea/aa.git/log/?h=userfault 
> >
> >
> > I didn't have time to test KVM live memory snapshot on it yet as I'm
> > still working to improve it. Did anybody test it? However I'd be happy
> > to take any bugreports and quickly solve anything that isn't working
> > right with the shadow MMU.
> >
> > I got positive report already for another usage of the uffd WP support:
> >
> > https://medium.com/@MartinCracauer/generational-garbage-collection-write-barriers-write-protection-and-userfaultfd-2-8b0e796b8f7f 
> >
> >
> > The last few things I'm working on to finish the WP support are:
> >
> > 1) pte_swp_mkuffd_wp equivalent of pte_swp_mksoft_dirty to mark in a
> >     vma->vm_flags with VM_UFFD_WP set, which swap entries were
> >     generated while the pte was wrprotected.
> >
> > 2) to avoid all false positives the equivalent of pte_mksoft_dirty is
> >     needed too... and that requires spare software bits on the pte
> >     which are available on x86. I considered also taking over the
> >     soft_dirty bit but then you couldn't do checkpoint restore of a
> >     JIT/to-native compiler that uses uffd WP support so it wasn't
> >     ideal. Perhaps it would be ok as an incremental patch to make the
> >     two options mutually exclusive to defer the arch changes that
> >     pte_mkuffd_wp would require for later.
> >
> > 3) prevent UFFDIO_ZEROPAGE if registering WP|MISSING or trigger a
> >     cow in userfaultfd_writeprotect.
> >
> > 4) WP selftest
> >
> > In theory things should work ok already if the userland code is
> > tolerant against false positives through swap and after fork() and
> > KSM. For an usage like snapshotting false positives shouldn't be an
> > issue (it'll just run slower if you swap in the worst case), and point
> > 3) above also isn't an issue because it's going to register into uffd
> > with WP only.
> >
> > The current status includes:
> >
> > 1) WP support for anon (with false positives.. work in progress)
> >
> > 2) MISSING support for tmpfs and hugetlbfs
> >
> > 3) non cooperative support
> >
> > Thanks,
> > Andrea
> >
> > .