Re: [PATCH v2 2/4] userfaultfd.2: Add write-protect mode

[Peter Xu <peterx@redhat.com>]

On Wed, Mar 10, 2021 at 08:16:24PM +0100, Alejandro Colomar (man-pages) wrote:
> Hi Peter,
> 
> Please see a few comments below.
> 
> Thanks,
> 
> Alex
> 
> On 3/4/21 5:31 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 | 98 +++++++++++++++++++++++++++++++++++++++++++++-
> >   1 file changed, 96 insertions(+), 2 deletions(-)
> > 
> > diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> > index 0cd426a8a..426307bcf 100644
> > --- a/man2/userfaultfd.2
> > +++ b/man2/userfaultfd.2
> > @@ -78,6 +78,30 @@ all memory ranges that were registered with the object are unregistered
> >   and unread events are flushed.
> >   .\"
> >   .PP
> > +Currently, userfaultfd supports two modes of registration:
> 
> "Currently"
> 
> Than word is quite unstable and unprecise.
> I think it would be better to use an absolute reference, such as "Since
> Linux x.y, ...".

I decided to remove the "Currently" and put the "(since x.y)" into each mode:

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 426307bcf..1132f52a3 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -78,9 +78,9 @@ all memory ranges that were registered with the object are unregistered
 and unread events are flushed.
 .\"
 .PP
-Currently, userfaultfd supports two modes of registration:
+Userfaultfd supports two modes of registration:
 .TP
-.B UFFDIO_REGISTER_MODE_MISSING
+.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
@@ -91,7 +91,7 @@ or an
 .B UFFDIO_ZEROPAGE
 ioctl.
 .TP
-.B UFFDIO_REGISTER_MODE_WP
+.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

> 
> > +.TP
> > +.B UFFDIO_REGISTER_MODE_MISSING
> > +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
> > +.B UFFDIO_REGISTER_MODE_WP
> > +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
> 
> Please, use "semantic newlines".
> 
> $ man 7 man-pages |sed -n '/semantic newlines/,/^$/p'
>    Use semantic newlines
>        In the source of a manual page,  new  sentences  should  be
>        started  on new lines, and long sentences should split into
>        lines at clause breaks (commas, semicolons, colons, and  so
>        on).   This  convention,  sometimes known as "semantic new-
>        lines", makes it easier to see the effect of patches, which
>        often  operate at the level of individual sentences or sen-
>        tence clauses.

Will do.

> 
> 
> 
> > +userspace 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 +168,16 @@ 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 (),
> > @@ -219,6 +253,62 @@ 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 supports write-protect mode.  The user needs to
> > +first check availability of this feature using
> > +.B 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 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 .
> 
> Please use quotes when possible:
> 
> .BR "asdasd asdsadf dfgsdfg dsf" .

Fixed.

> 
> > +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
> > +.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
> > +.BR UFFDIO_WRITEPROTECT_MODE_WP
> 
> .B

Fixed.

Thanks,

-- 
Peter Xu