linux-kernel - Re: [PATCH v3 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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Date:   Fri, 19 Mar 2021 23:37:20 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        linux-kernel@...r.kernel.org,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

Hi Peter,

A few more comments below.

Thanks,

Alex

On 3/10/21 11:23 PM, Peter Xu wrote:
> Userfaultfd write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  man2/ioctl_userfaultfd.2 | 81 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 78 insertions(+), 3 deletions(-)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index d4a8375b8..d8380896a 100644
> --- a/man2/ioctl_userfaultfd.2
> +++ b/man2/ioctl_userfaultfd.2
> @@ -234,6 +234,11 @@ operation is supported.
>  The
>  .B UFFDIO_UNREGISTER
>  operation is supported.
> +.TP
> +.B 1 << _UFFDIO_WRITEPROTECT
> +The
> +.B UFFDIO_WRITEPROTECT
> +operation is supported.
>  .PP
>  This
>  .BR ioctl (2)
> @@ -322,9 +327,6 @@ Track page faults on missing pages.
>  .B UFFDIO_REGISTER_MODE_WP
>  Track page faults on write-protected pages.
>  .PP
> -Currently, the only supported mode is
> -.BR UFFDIO_REGISTER_MODE_MISSING .
> -.PP
>  If the operation is successful, the kernel modifies the
>  .I ioctls
>  bit-mask field to indicate which
> @@ -443,6 +445,13 @@ operation:
>  .TP
>  .B UFFDIO_COPY_MODE_DONTWAKE
>  Do not wake up the thread that waits for page-fault resolution
> +.TP
> +.B UFFDIO_COPY_MODE_WP
> +Copy the page with read-only permission.
> +This allows the user to trap the next write to the page, which will block and

Break at the comma instead.

> +generate another write-protect userfault message.
> +This is only used in conjunction with write-protect mode when both missing and

"when both missing"

both what?

> +write-protect modes are enabled.
>  .PP
>  The
>  .I copy
> @@ -654,6 +663,72 @@ field of the
>  structure was not a multiple of the system page size; or
>  .I len
>  was zero; or the specified range was otherwise invalid.
> +.SS UFFDIO_WRITEPROTECT (Since Linux 5.7)
> +Write-protect or write-unprotect an userfaultfd registered memory range
> +registered with mode
> +.BR UFFDIO_REGISTER_MODE_WP .
> +.PP
> +The
> +.I argp
> +argument is a pointer to a
> +.I uffdio_range
> +structure as shown below:
> +.PP
> +.in +4n
> +.EX
> +struct uffdio_writeprotect {
> +    struct uffdio_range range;  /* Range to change write permission */
> +    __u64 mode;                 /* Mode to change write permission */
> +};
> +.EE
> +.in
> +There're two mode bits that are supported in this structure:
> +.TP
> +.B UFFDIO_WRITEPROTECT_MODE_WP
> +When this mode bit is set, the ioctl will be a write-protect operation upon the
> +memory range specified by
> +.IR range .
> +Otherwise it'll be a write-unprotect operation upon the specified range, which

Break at the comma instead.

> +can be used to resolve an userfaultfd write-protect page fault.
> +.TP
> +.B UFFDIO_WRITEPROTECT_MODE_DONTWAKE
> +When this mode bit is set, do not wake up any thread that waits for page-fault

Break at the comma.

> +resolution after the operation.
> +This could only be specified if
> +.B UFFDIO_WRITEPROTECT_MODE_WP
> +is not specified.
> +.PP
> +This
> +.BR ioctl (2)
> +operation returns 0 on success.
> +On error, \-1 is returned and
> +.I errno
> +is set to indicate the error.
> +Possible errors include:
> +.TP
> +.B EINVAL
> +The
> +.I start
> +or the
> +.I len
> +field of the
> +.I ufdio_range
> +structure was not a multiple of the system page size; or
> +.I len
> +was zero; or the specified range was otherwise invalid.
> +.TP
> +.B EAGAIN
> +The process was interrupted and need to retry.
> +.TP
> +.B ENOENT
> +The range specified in
> +.I range
> +is not valid.
> +For example, the virtual address does not exist, or not registered with

Better break at the comma.

> +userfaultfd write-protect mode.
> +.TP
> +.B EFAULT
> +Encountered a generic fault during processing.
>  .SH RETURN VALUE
>  See descriptions of the individual operations, above.
>  .SH ERRORS
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/