Re: [PATCH 00/17] RFC: userfault v2

[zhanghailiang]

On 2014/10/30 20:49, Dr. David Alan Gilbert wrote:
> * zhanghailiang (zhang.zhanghailiang@xxxxxxxxxx) wrote:
> > On 2014/10/30 1:46, Andrea Arcangeli wrote:
> > > Hi Zhanghailiang,
> > >
> > > On Mon, Oct 27, 2014 at 05:32:51PM +0800, zhanghailiang wrote:
> > > > Hi Andrea,
> > > >
> > > > Thanks for your hard work on userfault;)
> > > >
> > > > This is really a useful API.
> > > >
> > > > I want to confirm a question:
> > > > Can we support distinguishing between writing and reading memory for userfault?
> > > > That is, we can decide whether writing a page, reading a page or both trigger userfault.
> > > >
> > > > I think this will help supporting vhost-scsi,ivshmem for migration,
> > > > we can trace dirty page in userspace.
> > > >
> > > > Actually, i'm trying to relize live memory snapshot based on pre-copy and userfault,
> > > > but reading memory from migration thread will also trigger userfault.
> > > > It will be easy to implement live memory snapshot, if we support configuring
> > > > userfault for writing memory only.
> > >
> > > Mail is going to be long enough already so I'll just assume tracking
> > > dirty memory in userland (instead of doing it in kernel) is worthy
> > > feature to have here.
> > >
> > > After some chat during the KVMForum I've been already thinking it
> > > could be beneficial for some usage to give userland the information
> > > about the fault being read or write, combined with the ability of
> > > mapping pages wrprotected to mcopy_atomic (that would work without
> > > false positives only with MADV_DONTFORK also set, but it's already set
> > > in qemu). That will require "vma->vm_flags & VM_USERFAULT" to be
> > > checked also in the wrprotect faults, not just in the not present
> > > faults, but it's not a massive change. Returning the read/write
> > > information is also a not massive change. This will then payoff mostly
> > > if there's also a way to remove the memory atomically (kind of
> > > remap_anon_pages).
> > >
> > > Would that be enough? I mean are you still ok if non present read
> > > fault traps too (you'd be notified it's a read) and you get
> > > notification for both wrprotect and non present faults?
> > Hi Andrea,
> >
> > Thanks for your reply, and your patience;)
> >
> > Er, maybe i didn't describe clearly. What i really need for live memory snapshot
> > is only wrprotect fault, like kvm's dirty tracing mechanism, *only tracing write action*.
> >
> > My initial solution scheme for live memory snapshot is:
> > (1) pause VM
> > (2) using userfaultfd to mark all memory of VM is wrprotect (readonly)
> > (3) save deivce state to snapshot file
> > (4) resume VM
> > (5) snapshot thread begin to save page of memory to snapshot file
> > (6) VM is going to run, and it is OK for VM or other thread to read ram (no fault trap),
> >      but if VM try to write page (dirty the page), there will be
> >      a userfault trap notification.
> > (7) a fault-handle-thread reads the page request from userfaultfd,
> >      it will copy content of the page to some buffers, and then remove the page's
> >      wrprotect limit(still using the userfaultfd to tell kernel).
> > (8) after step (7), VM can continue to write the page which is now can be write.
> > (9) snapshot thread save the page cached in step (7)
> > (10) repeat step (5)~(9) until all VM's memory is saved to snapshot file.
>
> Hmm, I can see the same process being useful for the fault-tolerance schemes
> like COLO, it needs a memory state snapshot.
>
> > So, what i need for userfault is supporting only wrprotect fault. i don't
> > want to get notification for non present reading faults, it will influence
> > VM's performance and the efficiency of doing snapshot.
>
> What pages would be non-present at this point - just balloon?

Er, sorry, it should be 'no-present page faults';)

> Dave
>
> > Also, i think this feature will benefit for migration of ivshmem and vhost-scsi
> > which have no dirty-page-tracing now.
> >
> > > The question then is how you mark the memory readonly to let the
> > > wrprotect faults trap if the memory already existed and you didn't map
> > > it yourself in the guest with mcopy_atomic with a readonly flag.
> > >
> > > My current plan would be:
> > >
> > > - keep MADV_USERFAULT|NOUSERFAULT just to set VM_USERFAULT for the
> > >    fast path check in the not-present and wrprotect page fault
> > >
> > > - if VM_USERFAULT is set, find if there's a userfaultfd registered
> > >    into that vma too
> > >
> > >      if yes engage userfaultfd protocol
> > >
> > >      otherwise raise SIGBUS (single threaded apps should be fine with
> > >      SIGBUS and it'll avoid them to spawn a thread in order to talk the
> > >      userfaultfd protocol)
> > >
> > > - if userfaultfd protocol is engaged, return read|write fault + fault
> > >    address to read(ufd) syscalls
> > >
> > > - leave the "userfault" resolution mechanism independent of the
> > >    userfaultfd protocol so we keep the two problems separated and we
> > >    don't mix them in the same API which makes it even harder to
> > >    finalize it.
> > >
> > >      add mcopy_atomic (with a flag to map the page readonly too)
> > >
> > >      The alternative would be to hide mcopy_atomic (and even
> > >      remap_anon_pages in order to "remove" the memory atomically for
> > >      the externalization into the cloud) as userfaultfd commands to
> > >      write into the fd. But then there would be no much point to keep
> > >      MADV_USERFAULT around if I do so and I could just remove it
> > >      too or it doesn't look clean having to open the userfaultfd just
> > >      to issue an hidden mcopy_atomic.
> > >
> > >      So it becomes a decision if the basic SIGBUS mode for single
> > >      threaded apps should be supported or not. As long as we support
> > >      SIGBUS too and we don't force to use userfaultfd as the only
> > >      mechanism to be notified about userfaults, having a separate
> > >      mcopy_atomic syscall sounds cleaner.
> > >
> > >      Perhaps mcopy_atomic could be used in other cases that may arise
> > >      later that may not be connected with the userfault.
> > >
> > > Questions to double check the above plan is ok:
> > >
> > > 1) should I drop the SIGBUS behavior and MADV_USERFAULT?
> > >
> > > 2) should I hide mcopy_atomic as a write into the userfaultfd?
> > >
> > >     NOTE: even if I hide mcopy_atomic as a userfaultfd command to write
> > >     into the fd, the buffer pointer passed to write() syscall would
> > >     still _not_ be pointing to the data like a regular write, but it
> > >     would be a pointer to a command structure that points to the source
> > >     and destination data of the "hidden" mcopy_atomic, the only
> > >     advantage is that perhaps I could wakeup the blocked page faults
> > >     without requiring an additional syscall.
> > >
> > >     The standalone mcopy_atomic would still require a write into the
> > >     userfaultfd as it happens now after remap_anon_pages returns, in
> > >     order to wakeup the stopped page faults.
> > >
> > > 3) should I add a registration command to trap only write faults?
> >
> > Sure, that is what i really need;)
> >
> >
> > Best Regards???
> > zhanghailiang
> >
> > >     The protocol can always be extended later anyway in a backwards
> > >     compatible way but it's better if we get it fully featured from the
> > >     start.
> > >
> > > For completeness, some answers for other questions I've seen floating
> > > around but that weren't posted on the list yet (you can skip reading
> > > the below part if not interested):
> > >
> > > - open("/dev/userfault") instead of sys_userfaultfd(), I don't see the
> > >    benefit: userfaultfd is just like eventfd in terms of kernel API and
> > >    registering a /dev/ device actually sounds trickier. userfault is a
> > >    core VM feature and generally we prefer syscalls for core VM
> > >    features instead of running ioctl on some chardev that may or may
> > >    not exist. (like we did with /dev/ksm -> MADV_MERGEABLE)
> > >
> > > - there was a suggestion during KVMForum about allowing an external
> > >    program to attach to any MM. Like ptrace. So you could have a single
> > >    process managing all userfaults for different processes. However
> > >    because I cannot allow multiple userfaultfd to register into the
> > >    same range, this doesn't look very reliable (ptrace is kind of an
> > >    optional/debug feature while if userfault goes wrong and returns
> > >    -EBUSY things go bad) and there may be other complications. If I'd
> > >    allow multiple userfaultfd to register into the same range, I
> > >    wouldn't even know who to deliver the userfault to. It is an erratic
> > >    behavior. Currently it'd return -EBUSY if the app has a bug and does
> > >    that, but maybe later this can be relaxed to allow higher
> > >    scalability with a flag (userfaultfd gets flags as parameters), but
> > >    it still would need to be the same logic that manages userfaults and
> > >    the only point of allowing multiple ufd to map the same range would
> > >    be SMP scalability. So I tend to see the userfaultfd as a MM local
> > >    thing. The thread managing the userfaults can still talk with
> > >    another process in the local machine using pipes or sockets if it
> > >    needs to.
> > >
> > > - the userfaultfd protocol version handshake was done this way because
> > >    it looked more reliable.
> > >
> > >    Of course we could pass the version of the protocol as parameter to
> > >    userfaultfd too, but running the syscall multiple times until
> > >    -EPROTO didn't return anymore doesn't seem any better than writing
> > >    into the fd the wanted protocol until you read it back instead of
> > >    -1ULL. It just looked more reliable not having to run the syscall
> > >    again and again while depending on -EPROTO or some other
> > >    -Esomething.
> > >
> > > Thanks,
> > > Andrea
> > >
> > > .
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
>
> .

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