ioctl_userfaultfd.2: Add write-protect mode docs

Userfaultfd write-protect mode is supported starting from Linux 5.7.

Acked-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
[alx: ffix + srcfix]
Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/ioctl_userfaultfd.2

@@ -234,6 +234,11 @@ operation is supported.
 The
 .B UFFDIO_UNREGISTER
 operation is supported.
+.TP
+.B 1 << _UFFDIO_WRITEPROTECT
+The
+.B UFFDIO_WRITEPROTECT
+operation is supported.
 .PP
 This
 .BR ioctl (2)
@@ -322,9 +327,6 @@ Track page faults on missing pages.
 .B UFFDIO_REGISTER_MODE_WP
 Track page faults on write-protected pages.
 .PP
-Currently, the only supported mode is
-.BR UFFDIO_REGISTER_MODE_MISSING .
-.PP
 If the operation is successful, the kernel modifies the
 .I ioctls
 bit-mask field to indicate which
@@ -443,6 +445,16 @@ operation:
 .TP
 .B UFFDIO_COPY_MODE_DONTWAKE
 Do not wake up the thread that waits for page-fault resolution
+.TP
+.B UFFDIO_COPY_MODE_WP
+Copy the page with read-only permission.
+This allows the user to trap the next write to the page,
+which will block and generate another write-protect userfault message.
+This is only used when both
+.B UFFDIO_REGISTER_MODE_MISSING
+and
+.B UFFDIO_REGISTER_MODE_WP
+modes are enabled for the registered range.
 .PP
 The
 .I copy
@@ -654,6 +666,74 @@ field of the
 structure was not a multiple of the system page size; or
 .I len
 was zero; or the specified range was otherwise invalid.
+.SS UFFDIO_WRITEPROTECT (Since Linux 5.7)
+Write-protect or write-unprotect an userfaultfd registered memory range
+registered with mode
+.BR UFFDIO_REGISTER_MODE_WP .
+.PP
+The
+.I argp
+argument is a pointer to a
+.I uffdio_range
+structure as shown below:
+.PP
+.in +4n
+.EX
+struct uffdio_writeprotect {
+    struct uffdio_range range; /* Range to change write permission*/
+    __u64 mode;                /* Mode to change write permission */
+};
+.EE
+.in
+.PP
+There're two mode bits that are supported in this structure:
+.TP
+.B UFFDIO_WRITEPROTECT_MODE_WP
+When this mode bit is set,
+the ioctl will be a write-protect operation upon the memory range specified by
+.IR range .
+Otherwise it'll be a write-unprotect operation upon the specified range,
+which can be used to resolve an userfaultfd write-protect page fault.
+.TP
+.B UFFDIO_WRITEPROTECT_MODE_DONTWAKE
+When this mode bit is set,
+do not wake up any thread that waits for
+page-fault resolution after the operation.
+This could only be specified if
+.B UFFDIO_WRITEPROTECT_MODE_WP
+is not specified.
+.PP
+This
+.BR ioctl (2)
+operation returns 0 on success.
+On error, \-1 is returned and
+.I errno
+is set to indicate the error.
+Possible errors include:
+.TP
+.B EINVAL
+The
+.I start
+or the
+.I len
+field of the
+.I ufdio_range
+structure was not a multiple of the system page size; or
+.I len
+was zero; or the specified range was otherwise invalid.
+.TP
+.B EAGAIN
+The process was interrupted; retry this call.
+.TP
+.B ENOENT
+The range specified in
+.I range
+is not valid.
+For example, the virtual address does not exist,
+or not registered with userfaultfd write-protect mode.
+.TP
+.B EFAULT
+Encountered a generic fault during processing.
 .SH RETURN VALUE
 See descriptions of the individual operations, above.
 .SH ERRORS