Re: [PATCH v4 2/4] userfaultfd.2: Add write-protect mode

["Alejandro Colomar (man-pages)" <alx.manpages@gmail.com>]

Hi Peter,

Please see a few more comments below.

Thanks,

Alex

On 3/22/21 11:08 PM, Peter Xu wrote:
> Write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>   man2/userfaultfd.2 | 104 ++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 102 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 555e37409..8ad4a71b5 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 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.
> +The faulted thread will be stopped from execution
> +until the userspace un-write-protect the page using an

Here you use un-write-protect, but in the other patch you use 
write-unprotect.  Please, use a consistent wording if it's the same 
thing (if there are other similar things with different wordings in 
different pages, please fix them too, but I didn't see more of those). 
If there's already a wording for that in any page, please reuse it (I 
ignore it).

> +.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

The previous paragraph uses "Siince Linux 4.14".  For consistency, 
please use that same wording here.

> +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 refer to "Userfaultfd write-protect mode" section.
>   .SS Userfaultfd operation
>   After the userfaultfd object is created with
>   .BR userfaultfd (),
> @@ -219,6 +256,65 @@ 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 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 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,
> +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 +460,12 @@ 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
> 


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