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

Peter Xu
2021-03-04T16:04:51Z

On Thu, Mar 04, 2021 at 08:46:59AM +0200, Mike Rapoport wrote:
> 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@redhat.com>
> > ---
> > 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"

I mentioned it below [1]. However I agree it's indeed making more sense to
mention it when listing the modes, especially knowing that the 3rd minor mode
is coming. I think I'll keep both, assuming a bit more verbose is still
acceptable in man pages, but changed to:

"Multiple modes can be enabled at the same time 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/

Sure.

>
> > +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.

[1]

Thanks,

--
Peter Xu