linux-kernel - Re: [PATCH 03/10] userfaultfd.2: comment on feature detection in the example program

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Message-ID: <fgavhpky2ba3tfv6abkln7kzzayfxl5mnu5vzzzop5ct5mtmdl@dbdf2w4d4fva>
Date:   Tue, 26 Sep 2023 01:32:15 +0200
From:   Alejandro Colomar <alx@...nel.org>
To:     Axel Rasmussen <axelrasmussen@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/10] userfaultfd.2: comment on feature detection in the
 example program

Hi Axel,

On Tue, Sep 19, 2023 at 12:01:59PM -0700, Axel Rasmussen wrote:
> The example program doesn't depend on any extra features, so it does not
> make use of the two-step feature handshake process. This is fine, but it
> might set a bad example for programs which *do* depend on specific
> features (e.g. they may conclude they don't need to do anything to
> enable / detect them).
> 
> No need to make the example program more complicated: let's just add a
> comment indicating why we do it the way we do it in the example, and
> describing briefly what a more complicated program would need to do
> instead.
> 
> The comment is kept rather brief; a full description of this feature
> will be included in ioctl_userfaultfd.2 instead.
> 
> Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>

Patch applied.

Thanks,
Alex

> ---
>  man2/userfaultfd.2 | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 00d94e514..b2b79f61d 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -881,6 +881,13 @@ main(int argc, char *argv[])
>      if (uffd == \-1)
>          err(EXIT_FAILURE, "userfaultfd");
>  \&
> +    /* NOTE: Two-step feature handshake is not needed here, since this
> +       example doesn't require any specific features.
> +
> +       Programs that *do* should call UFFDIO_API twice: once with
> +       `features = 0` to detect features supported by this kernel, and
> +       again with the subset of features the program actually wants to
> +       enable. */
>      uffdio_api.api = UFFD_API;
>      uffdio_api.features = 0;
>      if (ioctl(uffd, UFFDIO_API, &uffdio_api) == \-1)
> -- 
> 2.42.0.459.ge4e396fd5e-goog
> 

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