Re: [PATCH 2/4] userfaultfd.2: Add write-protect mode

[Mike Rapoport]

On Wed, Mar 03, 2021 at 08:59:45PM -0500, Peter Xu wrote:
> Write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  man2/userfaultfd.2 | 88 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 86 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 2d14effc6..8e1602d62 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -78,6 +78,28 @@ all memory ranges that were registered with the object are unregistered
>  and unread events are flushed.
>  .\"
>  .PP
> +Currently, userfaultfd supports two modes of registration:
> +.TP
> +.BR UFFDIO_REGISTER_MODE_MISSING
> +When registered with
> +.BR UFFDIO_REGISTER_MODE_MISSING
> +mode, the userspace will receive a page fault message when a missing page is
> +accessed.  The faulted thread will be stopped from execution until the page
> +fault is resolved from the userspace by either an
> +.BR UFFDIO_COPY
> +or an
> +.BR UFFDIO_ZEROPAGE
> +ioctl.
> +.TP
> +.BR UFFDIO_REGISTER_MODE_WP
> +When registered with
> +.BR UFFDIO_REGISTER_MODE_WP
> +mode, the userspace will receive a page fault message when a write-protected
> +page is written.  The faulted thread will be stopped from execution until the
> +userspace un-write-protect the page using an
> +.BR UFFDIO_WRITEPROTECT
> +ioctl.
> +.PP

I'd add a sentence about combining the modes together. Something like

"Both modes can be enabled together for the same memory range"

>  Since Linux 4.14, userfaultfd page fault message can selectively embed fault
>  thread ID information into the fault message.  One needs to enable this feature
>  explicitly using the
> @@ -143,6 +165,16 @@ single threaded non-cooperative userfaultfd manager implementations.
>  .\" and limitations remaining in 4.11
>  .\" Maybe it's worth adding a dedicated sub-section...
>  .\"
> +.PP
> +Starting from Linux 5.7, userfaultfd is able to do synchronous page dirty
> +tracking using the new write-protection register mode.  One should check
> +against the feature bit
> +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP
> +before using this feature.  Similar to the original userfaultfd missing mode,
> +the write-protect mode will generate an userfaultfd message when the protected
> +page is written.  The user needs to resolve the page fault by unprotecting the
> +faulted page and kick the faulted thread to continue.  For more information,
> +please read the "Userfaultfd write-protect mode" section below.
>  .SS Userfaultfd operation
>  After the userfaultfd object is created with
>  .BR userfaultfd (),
> @@ -218,6 +250,54 @@ userfaultfd can be used only with anonymous private memory mappings.
>  Since Linux 4.11,
>  userfaultfd can be also used with hugetlbfs and shared memory mappings.
>  .\"
> +.SS Userfaultfd write-protect mode
> +Since Linux 5.7, userfaultfd started to support write-protect mode.  The user

Maybe s/started to support/supports/

> +needs to first check availability of this feature using
> +.BR UFFDIO_API
> +ioctl against the feature bit
> +.BR UFFD_FEATURE_PAGEFAULT_FLAG_WP .
> +.PP
> +To register with userfaultfd write-protect mode, the user needs to send the
> +.BR UFFDIO_REGISTER
> +ioctl with mode
> +.BR UFFDIO_REGISTER_MODE_WP
> +set.  Note that it's legal to monitor the same memory range with multiple
> +modes.  For example, the user can do
> +.BR UFFDIO_REGISTER
> +with the mode set to
> +.BR UFFDIO_REGISTER_MODE_MISSING\ |\ UFFDIO_REGISTER_MODE_WP.
> +When there is only
> +.BR UFFDIO_REGISTER_MODE_WP
> +registered, the userspace will
> +.I not
> +receive any message when a missing page is written.  Instead, the userspace
> +will only receive a write-protect page fault message when an existing but
> +write-protected page got written.
> +.PP
> +After the
> +.BR UFFDIO_REGISTER
> +ioctl completed with
> +.BR UFFDIO_REGISTER_MODE_WP
> +mode set, one can write-protect any existing memory within the range using the
> +ioctl
> +.BR UFFDIO_WRITEPROTECT
> +where
> +.I uffdio_writeprotect.mode
> +should be set to
> +.BR UFFDIO_WRITEPROTECT_MODE_WP .
> +.PP
> +When a write-protect event happens, the userspace will receive a page fault
> +message whose
> +.I uffd_msg.pagefault.flags
> +will be with
> +.BR UFFD_PAGEFAULT_FLAG_WP
> +flag set.  Note: since only writes can trigger such kind of fault,
> +write-protect messages will always be with
> +.BR UFFD_PAGEFAULT_FLAG_WRITE
> +bit set too along with
> +.BR UFFD_PAGEFAULT_FLAG_WP .
> +.PP
> +Currently, write-protect mode only supports private anonymous memory.
>  .SS Reading from the userfaultfd structure
>  Each
>  .BR read (2)
> @@ -363,8 +443,12 @@ flag (see
>  .BR ioctl_userfaultfd (2))
>  and this flag is set, this a write fault;
>  otherwise it is a read fault.
> -.\"
> -.\" UFFD_PAGEFAULT_FLAG_WP is not yet supported.
> +.TP
> +.B UFFD_PAGEFAULT_FLAG_WP
> +If the address is in a range that was registered with the
> +.B UFFDIO_REGISTER_MODE_WP
> +flag, when this bit is set it means it's a write-protect fault.  Otherwise it's
> +a page missing fault.
>  .RE
>  .TP
>  .I pagefault.feat.pid
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.