linux-kernel - Re: [PATCH v4 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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Date:   Tue, 23 Mar 2021 19:11:04 +0100
From:   "Alejandro Colomar (man-pages)" <alx.manpages@...il.com>
To:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v4 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

Hi Peter,

Please see a few comments below.

Thanks,

Alex

On 3/22/21 11:08 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 | 84 ++++++++++++++++++++++++++++++++++++++--
>   1 file changed, 81 insertions(+), 3 deletions(-)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index d4a8375b8..5419687a6 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 +445,16 @@ 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 generate another write-protect userfault message.

s/write-protect/write-protected/
?

> +This is only used when both
> +.B UFFDIO_REGISTER_MODE_MISSING
> +and
> +.B UFFDIO_REGISTER_MODE_WP
> +modes are enabled for the registered range.
>   .PP
>   The
>   .I copy
> @@ -654,6 +666,72 @@ 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 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 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.

Maybe: "The process was interrupted; retry this call."?
I don't know what other pager say about this kind of error.

> +.TP
> +.B ENOENT
> +The range specified in
> +.I range
> +is not valid.

I'm not sure how this is different from the wording above in EINVAL.  An 
"otherwise invalid range" was already giving EINVAL?

> +For example, the virtual address does not exist,
> +or not registered with userfaultfd write-protect mode.
> +.TP
> +.B EFAULT
> +Encountered a generic fault during processing.

What is a "generic fault"?

>   .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/