Re: [PATCH 1/2] UFFDIO_API.2const: Update userfaultfd handshake and feature probe

[Kyle Huey <me@kylehuey.com>]

On Mon, May 12, 2025 at 10:19 AM Peter Xu <peterx@redhat.com> wrote:
>
> There's a confusing paragraph in the man page on two-steps handshake for
> userfaultfd UFFDIO_API ioctl.  In reality, after a successful UFFDIO_API
> ioctl, the userfaultfd will be locked up on the features and any further
> UFFDIO_API on top of an initialized userfaultfd would fail.
>
> Modify the UFFDIO_API(2const) man page to reflect the reality.  Instead,
> add a paragraph explaining the right way to probe userfaultfd features.
> Add that only after the "Before Linux 4.11" paragraph, as the old kernel
> doesn't support any feature anyway.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  man/man2const/UFFDIO_API.2const | 43 ++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 19 deletions(-)
>
> diff --git a/man/man2const/UFFDIO_API.2const b/man/man2const/UFFDIO_API.2const
> index 54b34a1bc..1c554107a 100644
> --- a/man/man2const/UFFDIO_API.2const
> +++ b/man/man2const/UFFDIO_API.2const
> @@ -42,25 +42,6 @@ fields to bit masks representing all the available features and the generic
>  .BR ioctl (2)
>  operations available.
>  .P
> -Since Linux 4.11,
> -applications should use the
> -.I features
> -field to perform a two-step handshake.
> -First,
> -.B UFFDIO_API
> -is called with the
> -.I features
> -field set to zero.
> -The kernel responds by setting all supported feature bits.
> -.P
> -Applications which do not require any specific features
> -can begin using the userfaultfd immediately.
> -Applications which do need specific features
> -should call
> -.B UFFDIO_API
> -again with a subset of the reported feature bits set
> -to enable those features.
> -.P
>  Before Linux 4.11, the
>  .I features
>  field must be initialized to zero before the call to
> @@ -70,6 +51,30 @@ and zero (i.e., no feature bits) is placed in the
>  field by the kernel upon return from
>  .BR ioctl (2).
>  .P
> +Since Linux 4.11,
> +userfaultfd supports features that need to be enabled explicitly.
> +To enable any of the features,
> +one needs to set the corresponding feature bits in
> +.I features
> +when issuing the
> +.B UFFDIO_API
> +ioctl.
> +.P
> +For historical reasons,
> +a temporary userfaultfd is needed to probe
> +what userfaultfd features the kernel supports.
> +The application needs to create a temporary userfaultfd,
> +issue an
> +.B UFFDIO_API
> +ioctl with
> +.I features
> +set to 0. After the
> +.B UFFDIO_API
> +ioctl returns successfully,
> +.I features
> +should contain all the userfaultfd features that the kernel supports.
> +The temporary userfaultfd can be safely closed after the probe.
> +.P
>  If the application sets unsupported feature bits,
>  the kernel will zero out the returned
>  .I uffdio_api
> --
> 2.49.0
>

lgtm

Reviewed-by: Kyle Huey <khuey@kylehuey.com>

Thanks,

- Kyle