Re: [Qemu-devel] [PATCH v0 0/7] Background snapshots

[Mike Rapoport]

On Mon, Aug 13, 2018 at 08:00:19PM +0100, Dr. David Alan Gilbert wrote:
> cc'ing in Mike*2
> * Denis Plotnikov (address@hidden) wrote:
> > 
> > 
> > On 26.07.2018 12:23, Peter Xu wrote:
> > > On Thu, Jul 26, 2018 at 10:51:33AM +0200, Paolo Bonzini wrote:
> > > > On 25/07/2018 22:04, Andrea Arcangeli wrote:
> > > > > 
> > > > > It may look like the uffd-wp model is wish-feature similar to an
> > > > > optimization, but without the uffd-wp model when the WP fault is
> > > > > triggered by kernel code, the sigsegv model falls apart and requires
> > > > > all kind of ad-hoc changes just for this single feature. Plus uffd-wp
> > > > > has other benefits: it makes it all reliable in terms of not
> > > > > increasing the number of vmas in use during the snapshot. Finally it
> > > > > makes it faster too with no mmap_sem for reading and no sigsegv
> > > > > signals.
> > > > > 
> > > > > The non cooperative features got merged first because there was much
> > > > > activity on the kernel side on that front, but this is just an ideal
> > > > > time to nail down the remaining issues in uffd-wp I think. That I
> > > > > believe is time better spent than trying to emulate it with sigsegv
> > > > > and changing all drivers to send new events down to qemu specific to
> > > > > the sigsegv handling. We considered this before doing uffd for
> > > > > postcopy too but overall it's unreliable and more work (no single
> > > > > change was then needed to KVM code with uffd to handle postcopy and
> > > > > here it should be the same).
> > > > 
> > > > I totally agree.  The hard part in userfaultfd was the changes to the
> > > > kernel get_user_pages API, but the payback was huge because _all_ kernel
> > > > uses (KVM, vhost-net, syscalls, etc.) just work with userfaultfd.  Going
> > > > back to mprotect would be a huge mistake.
> > > 
> > > Thanks for explaining the bits.  I'd say I wasn't aware of the
> > > difference before I started the investigation (and only until now I
> > > noticed that major difference between mprotect and userfaultfd).  I'm
> > > really glad that it's much clear (at least for me) on which way we
> > > should choose.
> > > 
> > > Now I'm thinking whether we can move the userfault write protect work
> > > forward.  The latest discussion I saw so far is in 2016, when someone
> > > from Huawei tried to use the write protect feature for that old
> > > version of live snapshot but reported issue:
> > > 
> > >    https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg01127.html
> > > 
> > > Is that the latest status for userfaultfd wr-protect?
> > > 
> > > If so, I'm thinking whether I can try to re-verify the work (I tried
> > > his QEMU repository but I failed to compile somehow, so I plan to
> > > write some even simpler code to try) to see whether I can get the same
> > > KVM error he encountered.
> > > 
> > > Thoughts?
> > 
> > Just to sum up all being said before.
> > 
> > Using mprotect is a bad idea because VM's memory can be accessed from the
> > number of places (KVM, vhost, ...) which need their own special care
> > of tracking memory accesses and notifying QEMU which makes the mprotect
> > using unacceptable.
> > 
> > Protected memory accesses tracking can be done via userfaultfd's WP mode
> > which isn't available right now.
> > 
> > So, the reasonable conclusion is to wait until the WP mode is available and
> > build the background snapshot on top of userfaultfd-wp.
> > But, works on adding the WP-mode is pending for a quite a long time already.
> > 
> > Is there any way to estimate when it could be available?
> 
> I think a question is whether anyone is actively working on it; I
> suspect really it's on a TODO list rather than moving at the moment.

I thought Andrea was working on it :)
 
> What I don't really understand is what stage the last version got upto.
> 
> Dave
> 
> > > 
> > > Regards,
> > > 
> > 
> > -- 
> > Best,
> > Denis
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 

-- 
Sincerely yours,
Mike.