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

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...

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

Thanks,