linux-kernel - [PATCH v2 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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Message-Id: <20210304163140.543171-5-peterx@redhat.com>
Date:   Thu,  4 Mar 2021 11:31:40 -0500
From:   Peter Xu <peterx@...hat.com>
To:     linux-man@...r.kernel.org
Cc:     Alejandro Colomar <alx.manpages@...il.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-kernel@...r.kernel.org,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM Mailing List <linux-mm@...ck.org>,
        peterx@...hat.com
Subject: [PATCH v2 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

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

Signed-off-by: Peter Xu <peterx@...hat.com>
---
 man2/ioctl_userfaultfd.2 | 76 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 73 insertions(+), 3 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index b58983fe7..7497e63c4 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 +445,11 @@ 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
+Do not set write permission when copying the page.  The next write to the page
+will trigger a write fault.  This can be used in conjunction with write-protect
+mode so that there will be another message generated when the page is written again.
 .PP
 The
 .I copy
@@ -654,6 +661,69 @@ 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) Do write-protect or write-unprotect for an userfaultfd
+registered memory range 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 modes 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.
+.TP
+.B UFFDIO_WRITEPROTECT_MODE_DONTWAKE
+Do not wake up the thread that waits for page-fault resolution after the
+operation.  This could only be specified if
+.B UFFDIO_WRITEPROTECT_MODE_WP
+is not specified (in a resolving stage, not protecting stage).
+.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 and need to retry.
+.TP
+.B ENOENT
+The range specified in
+.I range
+is not valid.  E.g., 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