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

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Date:   Thu, 4 Mar 2021 11:02:17 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-man@...r.kernel.org, Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        linux-kernel@...r.kernel.org,
        Linux MM Mailing List <linux-mm@...ck.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Alejandro Colomar <alx.manpages@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [PATCH 2/4] userfaultfd.2: Add write-protect mode

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@...hat.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