Re: [PATCH v3 2/4] userfaultfd.2: Add write-protect mode

[Alejandro Colomar (man-pages)]

Hi Peter,

Please, see a few minor comments below.

Thanks,

Alex

On 3/10/21 11:22 PM, Peter Xu wrote:
> Write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  man2/userfaultfd.2 | 103 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 101 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 555e37409..d1f9aad24 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -78,6 +78,32 @@ all memory ranges that were registered with the object are unregistered
>  and unread events are flushed.
>  .\"
>  .PP
> +Userfaultfd supports two modes of registration:
> +.TP
> +.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)"
> +When registered with
> +.B UFFDIO_REGISTER_MODE_MISSING
> +mode, the userspace will receive a page fault message when a missing page is

SC (see below)

> +accessed.
> +The faulted thread will be stopped from execution until the page fault is
> +resolved from the userspace by either an
> +.B UFFDIO_COPY
> +or an
> +.B UFFDIO_ZEROPAGE
> +ioctl.
> +.TP
> +.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)"
> +When registered with
> +.B UFFDIO_REGISTER_MODE_WP
> +mode, the userspace will receive a page fault message when a write-protected
> +page is written.

Prefer breaking sentences at subordinate clauses rather than at random
points:

[
mode, the userspace will receive a page fault message
when a write-protected page is written.
]

There are other similar cases around the patch, marked with SC.

> +The faulted thread will be stopped from execution until the userspace

SC

> +un-write-protect the page using an> +.B UFFDIO_WRITEPROTECT
> +ioctl.
> +.PP
> +Multiple modes can be enabled at the same time for the same memory range.
> +.PP
>  Since Linux 4.14, userfaultfd page fault message can selectively embed faulting
>  thread ID information into the fault message.
>  One needs to enable this feature explicitly using the
> @@ -144,6 +170,17 @@ 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

SC

> +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 refer to "Userfaultfd write-protect mode" section.
>  .SS Userfaultfd operation
>  After the userfaultfd object is created with
>  .BR userfaultfd (),
> @@ -219,6 +256,64 @@ 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 5.7)
> +Since Linux 5.7, userfaultfd supports write-protect mode.
> +The user needs to first check availability of this feature using
> +.B UFFDIO_API
> +ioctl against the feature bit
> +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP
> +before using this feature.
> +.PP
> +To register with userfaultfd write-protect mode, the user needs to initiate the
> +.B UFFDIO_REGISTER
> +ioctl with mode
> +.B 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
> +.B UFFDIO_REGISTER
> +with the mode set to
> +.BR "UFFDIO_REGISTER_MODE_MISSING | UFFDIO_REGISTER_MODE_WP" .
> +When there is only
> +.B 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
> +.B UFFDIO_REGISTER
> +ioctl completed with
> +.B UFFDIO_REGISTER_MODE_WP
> +mode set, the user can write-protect any existing memory within the range using

SC (break at the comma better)

> +the ioctl
> +.B 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

SC (break at the comma better)

> +message whose
> +.I uffd_msg.pagefault.flags
> +will be with
> +.B UFFD_PAGEFAULT_FLAG_WP
> +flag set.  Note: since only writes can trigger such kind of fault,

Break at the point above too.

> +write-protect messages will always be with
> +.B UFFD_PAGEFAULT_FLAG_WRITE
> +bit set too along with bit
> +.BR UFFD_PAGEFAULT_FLAG_WP .
> +.PP
> +To resolve a write-protection page fault, the user should initiate another
> +.B UFFDIO_WRITEPROTECT
> +ioctl, whose
> +.I uffd_msg.pagefault.flags
> +should have the flag
> +.B UFFDIO_WRITEPROTECT_MODE_WP
> +cleared upon the faulted page or range.
> +.PP
> +Write-protect mode only supports private anonymous memory.
>  .SS Reading from the userfaultfd structure
>  Each
>  .BR read (2)
> @@ -364,8 +459,12 @@ 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.

Break at the point.

>  .RE
>  .TP
>  .I pagefault.feat.pid
> 

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