Re: PROBLEM: userfaultfd REGISTER minor mode on MAP_PRIVATE range fails

["David P. Reed" <dpreed@deepplum.com>]

On Tuesday, September 16, 2025 10:48, "Peter Xu" <peterx@redhat.com> said:

> On Mon, Sep 15, 2025 at 05:31:51PM -0700, James Houghton wrote:
>> I still don't have a solid grasp of what your use case is.
> 
> David, are you trying to provide a synchronous trap for an anon swapin
> event?  Say, you want to be able to stop a thread from swapping in anything
> from disk (or swap cache), do something, and UFFDIO_CONTINUE to kick it off
> again?

synchronous would be better. But what I want to do is at least get notifications of swapin events (including the case when the page is in swap cache). Also, using UFFDIO_COPY can be useful for the swap in case might make sense (but rarely, because there's no way to access the data that was swapped out).

> 
> That might make some sense when trying to match what MINOR mode means
> v.s. the mm's minor faults, but some explanation of why you wanted to do
> that would be helpful.  I agree with James that it was at least not the
> intention when userfaultfd MINOR trap was introduced.

I suspected that - however, notice that the rejection of registering minor mode is what I was reporting. It's oddly coded as if ordinary (4K) pages that are MAP_PRIVATE are the problem - no comment in the line of code I quoted explains why.

> 
> The other thing to mention is, AFAIU userfaultfd's major use case is not
> through a fork(), even though it should work..  In many cases, userfaultfd
> is used within a process, with a dedicated thread resolving faults. When
> it's used across processes, fork() should work but UFFD_FEATURE_EVENT_FORK
> is required, or otherwise via SCM_RIGHTS.  For the latter, the tracee needs
> to not only share the uffd object, but tell the tracer explicitly about the
> memory layout, because those addresses in the events will be reported in
> tracee's mm address space.

Yeah, the documentation tends to suggest that the file descriptor should be shared via a Linux Socket. But the case of a fork() should work. (the examples use O_CLOEXEC, but of course that isn't "close on fork").



> 
> Thanks,
> 
> --
> Peter Xu
> 
>