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

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

On Tuesday, September 16, 2025 14:35, "Axel Rasmussen" <axelrasmussen@google.com> said:

> 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. :)

Thanks for commenting. I'm not sure that's exactly true. Why is SNMEM (MAP_SHARED) supported, but not ordinary pages? I wasn't party to the evolution here, but so far no one has explained why there's a special difference between SHMEM and ordinary VMAs.

> 
> 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.
> 
> 
>>
>> It seems odd that anonymous page faults and COW would not be handled,
>> given that context.
>>
>> Anyway, that's unclear in any of the documentation. This just adds to my
>> last response where I explain my use case.
>>
>>
>>
>