linux-kernel - [PATCH v5 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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Date:   Mon, 29 Mar 2021 18:18:33 -0400
From:   Peter Xu <peterx@...hat.com>
To:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-man@...r.kernel.org
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>, peterx@...hat.com,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Alejandro Colomar <alx.manpages@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v5 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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

Acked-by: Mike Rapoport <rppt@...ux.vnet.ibm.com>
Signed-off-by: Peter Xu <peterx@...hat.com>
---
 man2/ioctl_userfaultfd.2 | 84 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 81 insertions(+), 3 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index d4a8375b8..ca533a383 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/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 +666,72 @@ 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
+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
-- 
2.26.2