Re: [PATCH v2] selftests/mm: Fix UFFDIO_API usage with proper two-step feature negotiation

[Peter Xu <peterx@redhat.com>]

On Tue, Jun 24, 2025 at 01:48:50PM +0200, David Hildenbrand wrote:
> The man page was updated with
> 
> commit db3d5cc1a17b0ace008ebe1eaf0ac4d96b4b519a
> Author: Axel Rasmussen <axelrasmussen@google.com>
> Date:   Tue Oct 3 12:45:44 2023 -0700
> 
>     ioctl_userfaultfd.2: Correct and update UFFDIO_API ioctl error codes
>     First, it is not correct that repeated UFFDIO_API calls result in
>     EINVAL.  This is true *if both calls enable features*, but in the case
>     where we're doing a two-step feature detection handshake, the kernel
>     explicitly expects 2 calls (one with no features set).  So, correct this
>     description.
>     Then, some new error cases have been added to the kernel recently, and
>     the man page wasn't updated to note these.  So, add in descriptions of
>     these new error cases.
> 
> @Axel, did you ignore the automatically-set UFFD_FEATURE_INITIALIZED and the
> repeated calls never worked, or was there actually a time where repeated
> UFFDIO_API calls would not result in EINVAL?

The man-pages was inaccurate before. It got updated recently after Kyle
asking similar questions, see:

http://www.alejandro-colomar.es/src/alx/linux/man-pages/man-pages.git/commit/?id=6a949e9b08fd1ad4e77584276b9566e45fc07a93

Li's v2 change on using the temp fd looks correct.

Thanks,

-- 
Peter Xu