Re: [PATCH 1/3] userfaultfd/selftests: fix feature support detection

From: Peter Xu
Date: Fri Sep 24 2021 - 16:09:47 EST

On Wed, Sep 22, 2021 at 10:43:40PM -0700, Jue Wang wrote:


> > > Could I know what's the workaround? Normally if the workaround works solidly,
> > > then there's less need to introduce a kernel interface for that. Otherwise I'm
> > > glad to look into such a formal proposal.
> >
> > The workaround is, for the region that you want to zap, run through
> > this sequence of syscalls: mumap, mmap, and re-register with
> > userfaultfd if it was registered before. If we're using tmpfs, we can
> > use madvise(DONTNEED) instead, but this is kind of an abuse of the
> > API. I don't think there's a guarantee that the PTEs will get zapped,
> > but currently they will always get zapped if we're using tmpfs. I
> > really like the idea of adding a new madvise() mode that is guaranteed
> > to zap the PTEs.

I see.

> >
> > >
> > > > It's also useful for memory poisoning, I think, if the host
> > > > decides some page(s) are "bad" and wants to intercept any future guest
> > > > accesses to those page(s).
> > >
> > > Curious: isn't hwpoison information come from MCEs; or say, host kernel side?
> > > Then I thought the host kernel will have full control of it already.
> > >
> > > Or there's other way that the host can try to detect some pages are going to be
> > > rotten? So the userspace can do something before the kernel handles those
> > > exceptions?
> >
> > Here's a general idea of how we would like to use userfaultfd to support MPR:
> >
> > If a guest accesses a poisoned page for the first time, we will get an
> > MCE through the host kernel and send an MCE to the guest. The guest
> > will now no longer be able to access this page, and we have to enforce
> > this. After a live migration, the pages that were poisoned before
> > probably won't still be poisoned (from the host's perspective), so we
> > can't rely on the host kernel's MCE handling path. This is where
> > userfaultfd and this new madvise mode come in: we can just
> > madvise(MADV_ZAP) the poisoned page(s) on the target during a
> > migration. Now all accesses will be routed to the VMM and we can
> > inject an MCE. We don't *need* the new madvise mode, as we can also
> > use fallocate(PUNCH_HOLE) (works for tmpfs and hugetlbfs), but it
> > would be more convenient if we didn't have to use fallocate.
> >
> > Jue Wang can provide more context here, so I've cc'd him. There may be
> > some things I'm wrong about, so Jue feel free to correct me.
> >
> James is right.
> The page is marked PG_HWPoison in the source VM host's kernel. The need
> of intercepting guest accesses to it exist on the target VM host, where
> the same physical page is no longer poisoned.
> On the target host, the hypervisor needs to intercept all guest accesses
> to pages poisoned from the source VM host.

Thanks for these information, James, Jue, Axel. I'm not familiar with memory
failures yet, so please bare with me with a few naive questions.

So now I can undertand that hw-poisonsed pages on src host do not mean these
pages will be hw-poisoned on dest host too, but I may have missed the reason on
why dest host needs to trap it with pgtable removed.

AFAIU after pages got hw-poisoned on src, and after vmm injects MCEs into the
guest, the guest shouldn't be accessing these pages any more, am I right? Then
after migration completes, IIUC the guest shouldn't be accessing these pages
too. My current understanding is, instead of trapping these pages on dest, we
should just (somehow, which I have no real idea...) un-hw-poison these pages
after migration because these pages are very possibly normal pages there. When
there's real hw-poisoned pages reported on dst host, we should re-inject MCE
errors to guest with another set of pages.

Could you tell me where did I miss?


Peter Xu