Re: [PATCH RESEND v6 00/16] mm: Page fault enhancements

[Peter Xu]

On Sun, Mar 08, 2020 at 01:12:34PM +0100, David Hildenbrand wrote:
> [...]
> 
> > Yes, IIUC the race can happen like this in your below test:
> > 
> >      main thread          uffd thread             disgard thread
> >      ===========          ===========             ==============
> >      access page
> >        uffd page fault
> >          wait for page
> >                           UFFDIO_ZEROCOPY
> >                             put a page P there
> >                                                   MADV_DONTNEED on P
> >                             wakeup main thread
> >          return from fault
> >        page still missing
> >        uffd page fault again
> >        (without ALLOW_RETRY)
> >        --> SIGBUS.
> 
> Exactly!
> 
> >> Can we please have a way to identify that this "feature" is available?
> >> I'd appreciate a new read-only UFFD_FEAT_ , so we can detect this from
> >> user space easily and use concurrent discards without crashing our applications.
> > 
> > I'm not sure how others think about it, but to me this still fells
> > into the bucket of "solving an existing problem" rather than a
> > feature.  Also note that this should change the behavior for the page
> > fault logic in general, rather than an uffd-only change. So I'm also
> > not sure whether UFFD_FEAT_* suites here even if we want it.
> 
> So, are we planning on backporting this to stable kernels?

I don't have a plan so far.  I'm still at the phase to only worry
about whether it can be at least merged in master.. :)

I would think it won't worth it to backport this to stables though,
considering that it could potentially change quite a bit for faulting
procedures, and after all the issues we're fixing shouldn't be common
to general users.

> 
> Imagine using this in QEMU/KVM to allow discards (e.g., balloon
> inflation) while postcopy is active . You certainly don't want random
> guest crashes. So either, we treat this as a fix (and backport) or as a
> change in behavior/feature.

I think we don't need to worry on that - QEMU will prohibit ballooning
during postcopy starting from the first day.  Feel free to see QEMU
commit 371ff5a3f04cd7 ("Inhibit ballooning during postcopy").

> 
> [...]
> 
> >>
> >> 2. What will happen if I don't place a page on a pagefault, but only do a UFFDIO_WAKE?
> >>    For now we were able to trigger a signal this way.
> > 
> > If I'm not mistaken the UFFDIO_WAKE will directly trigger the sigbus
> > even without the help of the MADV_DONTNEED race.
> 
> Yes, that's the current way of injecting a SIGBUS instead of resolving
> the pagefault. And AFAIKs, you're changing that behavior. (I am not
> aware of a user, there could be use cases, but somehow it's strange to
> get a signal when accessing memory that is mapped READ|WRITE and also
> represented like this in e.g., /proc/$PID/maps). So IMHO, only the new
> behavior makes really sense.

I agree, I'm not sure how other people think on ABI stability, but...
for my own preference I don't worry much on ABI breakage for a problem
like this.

Thanks,

-- 
Peter Xu