Re: [PATCH] mm/userfaultfd: provide unmasked address on page-fault

[David Hildenbrand]

On 09.10.21 00:02, Nadav Amit wrote:
> > On Oct 8, 2021, at 1:05 AM, David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 08.10.21 01:50, Nadav Amit wrote:
> > > From: Nadav Amit <namit@xxxxxxxxxx>
> > > Userfaultfd is supposed to provide the full address (i.e., unmasked) of
> > > the faulting access back to userspace. However, that is not the case for
> > > quite some time.
> > > Even running "userfaultfd_demo" from the userfaultfd man page provides
> > > the wrong output (and contradicts the man page). Notice that
> > > "UFFD_EVENT_PAGEFAULT event" shows the masked address.
> > > 	Address returned by mmap() = 0x7fc5e30b3000
> > > 	fault_handler_thread():
> > > 	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
> > > 	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
> > > 		(uffdio_copy.copy returned 4096)
> > > 	Read address 0x7fc5e30b300f in main(): A
> > > 	Read address 0x7fc5e30b340f in main(): A
> > > 	Read address 0x7fc5e30b380f in main(): A
> > > 	Read address 0x7fc5e30b3c0f in main(): A
> > > Add a new "real_address" field to vmf to hold the unmasked address. It
> > > is possible to keep the unmasked address in the existing address field
> > > (and mask whenever necessary) instead, but this is likely to cause
> > > backporting problems of this patch.
> >
> > Can we be sure that no existing users will rely on this behavior that has been the case since end of 2016 IIRC, one year after UFFD was upstreamed?
>
> Let me to blow off your mind: how do you be sure that the current behavior does not make applications to misbehave? It might cause performance issues as it did for me or hidden correctness issues.

Fair point, but now we can speculate what's more likely:

Having an app rely on >4 year old kernel behavior just after the feature 
was released or having and app rely on kernel behavior that was the case 
for the last 4 years?

<offtopic>
Someone once told me about the unwritten way to remove things from the 
kernel. 1) Silently break it upstream 2) Wait 2 kernel releases 3) 
Propose removal of the feature because it's broken and nobody complained.
<\offtopic>

You might ask "why does David even care?", here is why:

For the records, I *do* have a prototype from last year that breaks with 
this new behavior as far as I can tell: using uffd in the context of 
virtio-balloon in QEMU. I just pushed the latest state to a !private 
github tree:
  https://github.com/davidhildenbrand/qemu/tree/virtio-balloon-uffd


In that code, I made sure that I'm only dealing with 4k pages (because 
that's the only thing virtio-balloon really can deal with), and during 
the debugging I figured that the kernel always returns 4k aligned page 
fault addresses, so I didn't care about masking. I'll reuse the 
unmodified fault address for UFFDIO_ZEROPAGE()/UFFDIO_COPY()/... which 
should then fail because:

"
EINVAL The start or the len field of the ufdio_range structure
              was not a multiple of the system page size; or len was
              zero; or the specified range was otherwise invalid.
"


If I'm too lazy to read all documentation, I'm quite sure that there are 
other people that don't. I don't care to much if this patch breaks that 
prototype, it's just a prototype after all, but I am concerned that we 
might break other users in a similar way.

> > I do wonder what the official ABI nowadays is, because man pages aren't necessarily the source of truth.
>
> Documentation/admin-guide/mm/userfaultfd.rst says: "You get the address of the access that triggered the missing page
> event”.
>
> So it is a bug.

The least thing I would expect in the patch description is a better 
motivation ("who cares and why" -- I know you have a better motivation 
that making the doc correct :) ) and a discussion on the chances of this 
actually breaking other apps (see my example).

I'd sleep better if we'd glue the changed behavior to a new feature 
flag, but that's just my 2 cents.

--
Thanks,

David / dhildenb