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

Peter Xu
2021-03-04T02:02:06Z

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
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
+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.
+When there is only
+.BR 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
+.BR UFFDIO_REGISTER
+ioctl completed with
+.BR UFFDIO_REGISTER_MODE_WP
+mode set, one can write-protect any existing memory within the range using the
+ioctl
+.BR 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
+.BR UFFD_PAGEFAULT_FLAG_WP
+flag set. Note: since only writes can trigger such kind of fault,
+write-protect messages will always be with
+.BR UFFD_PAGEFAULT_FLAG_WRITE
+bit set too along with
+.BR UFFD_PAGEFAULT_FLAG_WP .
+.PP
+Currently, write-protect mode only supports private anonymous memory.
.SS Reading from the userfaultfd structure
Each
.BR read (2)
@@ -363,8 +443,12 @@ 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
--
2.26.2