linux-kernel - Re: [PATCH v2] Userfaultfd: Add description for UFFD_FEATURE

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Date:   Wed, 18 Oct 2017 10:42:43 +0200
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Prakash Sangappa <prakash.sangappa@...cle.com>
Cc:     mtk.manpages@...il.com, linux-man@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, aarcange@...hat.com, rppt@...ux.vnet.ibm.com,
        mhocko@...e.com
Subject: Re: [PATCH v2] Userfaultfd: Add description for UFFD_FEATURE_SIGBUS

Hello Prakash,

On 10/10/2017 12:45 AM, 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@...cle.com>
> Date: Wed Sep 6 16:23:39 2017 -0700
> 
>      mm: userfaultfd: add feature to request for a signal delivery

Thanks for the patch. I've applied it, and polished it a little.
The results are already visible in Git.

Thanks, Mike and Andrea, for the reviews!

Cheers,

Michael


> Signed-off-by: Prakash Sangappa <prakash.sangappa@...cle.com>
> ---
> 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
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/