Re: [PATCH v2] Userfaultfd: Add description for UFFD_FEATURE_SIGBUS

From: Mike Rapoport
Date: Tue Oct 10 2017 - 00:26:04 EST


On Mon, Oct 09, 2017 at 03:45:51PM -0700, Prakash Sangappa wrote:
> Userfaultfd feature UFFD_FEATURE_SIGBUS was merged recently and should
> be available in Linux 4.14 release. This patch is for the manpage
> changes documenting this API.
>
> Documents the following commit:
>
> commit 2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e
> Author: Prakash Sangappa <prakash.sangappa@xxxxxxxxxx>
> Date: Wed Sep 6 16:23:39 2017 -0700
>
> mm: userfaultfd: add feature to request for a signal delivery
>
> Signed-off-by: Prakash Sangappa <prakash.sangappa@xxxxxxxxxx>

Reviewed-by: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>

> ---
> v2: Incorporated review feedback changes.
> ---
> man2/ioctl_userfaultfd.2 | 9 +++++++++
> man2/userfaultfd.2 | 23 +++++++++++++++++++++++
> 2 files changed, 32 insertions(+)
>
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index 60fd29b..32f0744 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -196,6 +196,15 @@ with the
> flag set,
> .BR memfd_create (2),
> and so on.
> +.TP
> +.B UFFD_FEATURE_SIGBUS
> +Since Linux 4.14, If this feature bit is set, no page-fault events
> +.B (UFFD_EVENT_PAGEFAULT)
> +will be delivered, instead a
> +.B SIGBUS
> +signal will be sent to the faulting process. Applications using this
> +feature will not require the use of a userfaultfd monitor for processing
> +memory accesses to the regions registered with userfaultfd.
> .IP
> The returned
> .I ioctls
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 1741ee3..3c5b9c0 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -172,6 +172,29 @@ or
> .BR ioctl (2)
> operations to resolve the page fault.
> .PP
> +Starting from Linux 4.14, if application sets
> +.B UFFD_FEATURE_SIGBUS
> +feature bit using
> +.B UFFDIO_API
> +.BR ioctl (2),
> +no page fault notification will be forwarded to
> +the user-space, instead a
> +.B SIGBUS
> +signal is delivered to the faulting process. With this feature,
> +userfaultfd can be used for robustness purpose to simply catch
> +any access to areas within the registered address range that do not
> +have pages allocated, without having to listen to userfaultfd events.
> +No userfaultfd monitor will be required for dealing with such memory
> +accesses. For example, this feature can be useful for applications that
> +want to prevent the kernel from automatically allocating pages and filling
> +holes in sparse files when the hole is accessed thru mapped address.
> +.PP
> +The
> +.B UFFD_FEATURE_SIGBUS
> +feature is implicitly inherited through fork() if used in combination with
> +.BR UFFD_FEATURE_FORK .
> +
> +.PP
> Details of the various
> .BR ioctl (2)
> operations can be found in
> --
> 2.7.4
>

--
Sincerely yours,
Mike.