Re: [PATCH v2 1/2] UFFDIO_API.2const: Update userfaultfd handshake and feature probe

[Alejandro Colomar <alx@kernel.org>]

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Hi Peter,

On Wed, May 14, 2025 at 01:26:29PM -0400, Peter Xu 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.
> 
> Fixes: a252b3345 ("ioctl_userfaultfd.2: Describe two-step feature handshake")
> Reviewed-by: Kyle Huey <khuey@kylehuey.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Thanks!  I've applied the patch.


Have a lovely day!
Alex

> ---
>  man/man2const/UFFDIO_API.2const | 44 +++++++++++++++++++--------------
>  1 file changed, 25 insertions(+), 19 deletions(-)
> 
> diff --git a/man/man2const/UFFDIO_API.2const b/man/man2const/UFFDIO_API.2const
> index 54b34a1bc..aca27dc5c 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 +51,31 @@ 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 zero.
> +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
> 

-- 
<https://www.alejandro-colomar.es/>

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