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

[Peter Xu <peterx@redhat.com>]

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?

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.

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.

Thanks,

-- 
Peter Xu