Re: [PATCH 1/3] mm: userfaultfd: add new UFFDIO_SIGBUS ioctl

[Jiaqi Yan <jiaqiyan@google.com>]

On Wed, May 17, 2023 at 3:29 PM Axel Rasmussen <axelrasmussen@google.com> wrote:
>
> On Wed, May 17, 2023 at 3:20 PM Peter Xu <peterx@redhat.com> wrote:
> >
> > On Wed, May 17, 2023 at 06:12:33PM -0400, Peter Xu wrote:
> > > On Thu, May 11, 2023 at 03:00:09PM -0700, James Houghton wrote:
> > > > On Thu, May 11, 2023 at 11:24 AM Axel Rasmussen
> > > > <axelrasmussen@google.com> wrote:
> > > > >
> > > > > So the basic way to use this new feature is:
> > > > >
> > > > > - On the new host, the guest's memory is registered with userfaultfd, in
> > > > >   either MISSING or MINOR mode (doesn't really matter for this purpose).
> > > > > - On any first access, we get a userfaultfd event. At this point we can
> > > > >   communicate with the old host to find out if the page was poisoned.
> > > > > - If so, we can respond with a UFFDIO_SIGBUS - this places a swap marker
> > > > >   so any future accesses will SIGBUS. Because the pte is now "present",
> > > > >   future accesses won't generate more userfaultfd events, they'll just
> > > > >   SIGBUS directly.
> > > >
> > > > I want to clarify the SIGBUS mechanism here when KVM is involved,
> > > > keeping in mind that we need to be able to inject an MCE into the
> > > > guest for this to be useful.
> > > >
> > > > 1. vCPU gets an EPT violation --> KVM attempts GUP.
> > > > 2. GUP finds a PTE_MARKER_UFFD_SIGBUS and returns VM_FAULT_SIGBUS.
> > > > 3. KVM finds that GUP failed and returns -EFAULT.
> > > >
> > > > This is different than if GUP found poison, in which case KVM will
> > > > actually queue up a SIGBUS *containing the address of the fault*, and
> > > > userspace can use it to inject an appropriate MCE into the guest. With
> > > > UFFDIO_SIGBUS, we are missing the address!
> > > >
> > > > I see three options:
> > > > 1. Make KVM_RUN queue up a signal for any VM_FAULT_SIGBUS. I think
> > > > this is pointless.
> > > > 2. Don't have UFFDIO_SIGBUS install a PTE entry, but instead have a
> > > > UFFDIO_WAKE_MODE_SIGBUS, where upon waking, we return VM_FAULT_SIGBUS
> > > > instead of VM_FAULT_RETRY. We will keep getting userfaults on repeated
> > > > accesses, just like how we get repeated signals for real poison.
> > > > 3. Use this in conjunction with the additional KVM EFAULT info that
> > > > Anish proposed (the first part of [1]).
> > > >
> > > > I think option 3 is fine. :)
> > >
> > > Or... option 4) just to use either MADV_HWPOISON or hwpoison-inject? :)
> >
> > I just remember Axel mentioned this in the commit message, and just in case
> > this is why option 4) was ruled out:
> >
> >         They expect that once poisoned, pages can never become
> >         "un-poisoned". So, when we live migrate the VM, we need to preserve
> >         the poisoned status of these pages.
> >
> > Just to supplement on this point: we do have unpoison (echoing to
> > "debug/hwpoison/hwpoison_unpoison"), or am I wrong?

If I read unpoison_memory() correctly, once there is a real hardware
memory corruption (hw_memory_failure will be set), unpoison will stop
working and return EOPNOTSUPP.

I know some cloud providers evacuating VMs once a single memory error
happens, so not supporting unpoison is probably not a big deal for
them. BUT others do keep VM running until more errors show up later,
which could be long after the 1st error.

> >
> > >
> > > Besides what James mentioned on "missing addr", I didn't quickly see what's
> > > the major difference comparing to the old hwpoison injection methods even
> > > without the addr requirement. If we want the addr for MCE then it's more of
> > > a question to ask.
> > >
> > > I also didn't quickly see why for whatever new way to inject a pte error we
> > > need to have it registered with uffd.  Could it be something like
> > > MADV_PGERR (even if MADV_HWPOISON won't suffice) so you can inject even
> > > without an userfault context (but still usable when uffd registered)?
> > >
> > > And it'll be alawys nice to have a cover letter too (if there'll be a new
> > > version) explaining the bits.
>
> I do plan a v2, if for no other reason than to update the
> documentation. Happy to add a cover letter with it as well.
>
> +Jiaqi back to CC, this is one piece of a larger memory poisoning /
> recovery design Jiaqi is working on, so he may have some ideas why
> MADV_HWPOISON or MADV_PGER will or won't work.

Per https://man7.org/linux/man-pages/man2/madvise.2.html,
MADV_HWPOISON "is available only for privileged (CAP_SYS_ADMIN)
processes." So for a non-root VMM, MADV_HWPOISON is out of option.

Another issue with MADV_HWPOISON is, it requires to first successfully
get_user_pages_fast(). I don't think it will work if memory is not
mapped yet.

With the UFFDIO_SIGBUS feature introduced in this patchset, it may
even be possible to free the emulated-hwpoison page back to the kernel
so we don't lose a 4K page.

I didn't find any ref/doc for MADV_PGERR. Is it something you suggest
to build, Peter?


>
> One idea is, at least for our use case, we have to have the range be
> userfaultfd registered, because we need to intercept the first access
> and check at that point whether or not it should be poisoned. But, I
> think in principle a scheme like this could work:
>
> 1. Intercept first access with UFFD
> 2. Issue MADV_HWPOISON or MADV_PGERR or etc to put a pte denoting the
> poisoned page in place
> 3. UFFDIO_WAKE to have the faulting thread retry, see the new entry, and SIGBUS
>
> It's arguably slightly weird, since normally UFFD events are resolved
> with UFFDIO_* operations, but I don't see why it *couldn't* work.
>
> Then again I am not super familiar with MADV_HWPOISON, I will have to
> do a bit of reading to understand if its semantics are the same
> (future accesses to this address get SIGBUS).
>
>
> > >
> > > Thanks,
> > >
> > > --
> > > Peter Xu
> >
> > --
> > Peter Xu
> >