On Tue, Sep 16, 2025 at 12:11 PM James Houghton <jthoughton@google.com>
wrote:

> On Tue, Sep 16, 2025 at 11:35 AM Axel Rasmussen
> <axelrasmussen@google.com> wrote:
> >
> >
> >
> > On Tue, Sep 16, 2025 at 10:27 AM David P. Reed <dpreed@deepplum.com>
> wrote:
> >>
> >> Than -
> >>
> >> Just to clarify -
> >> Looking at the man page for UFFDIO_API, there are two "feature bits"
> that indicate cases where "minor" handling is now supported, and can be
> enabled.
> >> UFFD_FEATURE_MINOR_HUGETLBFS and UFFD_FEATURE_MINOR_SHMEM
> >> In my reading of the documents, these seem to imply that before they
> were added as new features, that MAP_PRIVATE|MAP_ANONYMOUS mappings were
> supported, and that the "new" additions to the MINOR mode were just for
> HUGETLBFS and MAP_SHARED cases.
> >
> >
> > Actually minor fault support didn't exist at all before those two
> features were added. :)
> >
> > You are right that userfaultfd's use of "minor fault" is (unfortunately)
> slightly different from the meaning in other contexts. I think the more
> normal meaning is, faults which do not incur I/O (i.e., swap faults and
> file faults [i.e., faults on non-swap-backed pages] are major, other faults
> are minor).
> >
> > For userfaultfd, a minor fault is a fault where the page already exists
> in the page cache, but the page table entry wasn't setup. I don't think
> that scenario can ever happen for anonymous, private mappings, so it
> doesn't really make sense to be able to register such mappings in this
> mode. If you create a mapping with mmap(MAP_ANON|MAP_PRIVATE) and then
> access it (read or write), that fault requires allocation of a new page, so
> userfaultfd does not consider that a "minor fault". My recollection though
> is if you make a file on tmpfs or hugetlbfs, fallocate() it or whatever,
> and you MAP_PRIVATE that file, *that* registration will work.
>
> Ah! You're right... MAP_PRIVATE *is* supported (for tmpfs and
> hugetlbfs only), and UFFDIO_CONTINUE will, upon finding the page in
> the page cache, install a RO PTE for it.
>

Why does it have to be RO? I think it depends on the PROT_ flag you
specified when you created the private mapping.


>
> But what happens when the write comes after installing the RO PTE? My
> reading of the code today makes me think that we'd get a minor
> userfault and then be unable to continue...! (The only reasonable
> behavior is that CoW is done without triggering a userfault... I
> assumed/thought this was the behavior today. I wish I had time to test
> this -- I hope I'm misreading it.)
>

It's possible my memory is wrong, but I don't think UFFD minor fault
handling really interacts with CoW faults. IOW, I think you get a UFFD
minor fault when the PTE is missing, not when it's RO resulting in CoW. I
think there we just CoW the page as per normal and no fault is reported via
UFFD?


>
> :( Here I was thinking I understood how userfaultfd minor faults worked.
>