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

[Peter Xu]

Write-protect mode is supported starting from Linux 5.7.

Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
---
 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:
+.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
+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 .
+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
+cleared upon the faulted page or range.
+.PP
+Currently, write-protect mode only supports private anonymous memory.
 .SS Reading from the userfaultfd structure
 Each
 .BR read (2)
@@ -364,8 +454,12 @@ 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