[PATCH v3 0/4] man2: udpate mm/userfaultfd manpages to latest

[PATCH v3 0/4] man2: udpate mm/userfaultfd manpages to latest

[Peter Xu]

v3:
- Don't use "Currently", instead add "(since x.y)" mark where proper [Alex]
- Always use semantic newlines across the whole patchset [Alex]
- Use quote when possible, rather than escapes [Alex]
- Fix one missing replacement of ".BR" -> ".B" [Alex]
- Some other trivial rephrases here and there when fixing up above

v2 changes:
- Fix wordings as suggested [MikeR]
- convert ".BR" to ".B" where proper for the patchset [Alex]
- rearrange a few lines in the last two patches where they got messed up
- document more things, e.g. UFFDIO_COPY_MODE_WP; and also on how to resolve a
  wr-protect page fault.

There're two features missing in current manpage, namely:

  (1) Userfaultfd Thread-ID feature
  (2) Userfaultfd write protect mode

There's also a 3rd one which was just contributed from Axel - Axel, I think it
would be great if you can add that part too, probably after the whole
hugetlbfs/shmem minor mode reaches the linux master branch.

Please review, thanks.

Peter Xu (4):
  userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs
  userfaultfd.2: Add write-protect mode
  ioctl_userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs
  ioctl_userfaultfd.2: Add write-protect mode docs

 man2/ioctl_userfaultfd.2 |  86 ++++++++++++++++++++++++++++-
 man2/userfaultfd.2       | 116 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 197 insertions(+), 5 deletions(-)

-- 
2.26.2

[PATCH v3 1/4] userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs

[Peter Xu]

UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 man2/userfaultfd.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index e7dc9f813..555e37409 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -77,6 +77,13 @@ When the last file descriptor referring to a userfaultfd object is closed,
 all memory ranges that were registered with the object are unregistered
 and unread events are flushed.
 .\"
+.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
+.BR UFFD_FEATURE_THREAD_ID
+feature bit when initializing the userfaultfd context.
+By default, thread ID reporting is diabled.
 .SS Usage
 The userfaultfd mechanism is designed to allow a thread in a multithreaded
 program to perform user-space paging for the other threads in the process.
@@ -229,6 +236,9 @@ struct uffd_msg {
         struct {
             __u64 flags;    /* Flags describing fault */
             __u64 address;  /* Faulting address */
+            union {
+                __u32 ptid; /* Thread ID of the fault */
+            } feat;
         } pagefault;
 
         struct {            /* Since Linux 4.11 */
@@ -358,6 +368,9 @@ otherwise it is a read fault.
 .\" UFFD_PAGEFAULT_FLAG_WP is not yet supported.
 .RE
 .TP
+.I pagefault.feat.pid
+The thread ID that triggered the page fault.
+.TP
 .I fork.ufd
 The file descriptor associated with the userfault object
 created for the child created by
-- 
2.26.2

[PATCH v3 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@redhat.com>
---
 man2/userfaultfd.2 | 103 ++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 101 insertions(+), 2 deletions(-)

diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 555e37409..d1f9aad24 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -78,6 +78,32 @@ all memory ranges that were registered with the object are unregistered
 and unread events are flushed.
 .\"
 .PP
+Userfaultfd supports two modes of registration:
+.TP
+.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)"
+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
+.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)"
+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 +170,17 @@ 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 refer to "Userfaultfd write-protect mode" section.
 .SS Userfaultfd operation
 After the userfaultfd object is created with
 .BR userfaultfd (),
@@ -219,6 +256,64 @@ 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 5.7)
+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
+.B UFFD_FEATURE_PAGEFAULT_FLAG_WP
+before using this feature.
+.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 bit
+.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
+.B UFFDIO_WRITEPROTECT_MODE_WP
+cleared upon the faulted page or range.
+.PP
+Write-protect mode only supports private anonymous memory.
 .SS Reading from the userfaultfd structure
 Each
 .BR read (2)
@@ -364,8 +459,12 @@ 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

Re: [PATCH v3 2/4] userfaultfd.2: Add write-protect mode

[Alejandro Colomar (man-pages)]

Hi Peter,

Please, see a few minor comments below.

Thanks,

Alex

On 3/10/21 11:22 PM, Peter Xu wrote:
> Write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  man2/userfaultfd.2 | 103 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 101 insertions(+), 2 deletions(-)
> 
> diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
> index 555e37409..d1f9aad24 100644
> --- a/man2/userfaultfd.2
> +++ b/man2/userfaultfd.2
> @@ -78,6 +78,32 @@ all memory ranges that were registered with the object are unregistered
>  and unread events are flushed.
>  .\"
>  .PP
> +Userfaultfd supports two modes of registration:
> +.TP
> +.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)"
> +When registered with
> +.B UFFDIO_REGISTER_MODE_MISSING
> +mode, the userspace will receive a page fault message when a missing page is

SC (see below)

> +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
> +.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)"
> +When registered with
> +.B UFFDIO_REGISTER_MODE_WP
> +mode, the userspace will receive a page fault message when a write-protected
> +page is written.

Prefer breaking sentences at subordinate clauses rather than at random
points:

[
mode, the userspace will receive a page fault message
when a write-protected page is written.
]

There are other similar cases around the patch, marked with SC.

> +The faulted thread will be stopped from execution until the userspace

SC

> +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 +170,17 @@ 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

SC

> +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 refer to "Userfaultfd write-protect mode" section.
>  .SS Userfaultfd operation
>  After the userfaultfd object is created with
>  .BR userfaultfd (),
> @@ -219,6 +256,64 @@ 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 5.7)
> +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
> +.B UFFD_FEATURE_PAGEFAULT_FLAG_WP
> +before using this feature.
> +.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

SC (break at the comma better)

> +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

SC (break at the comma better)

> +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,

Break at the point above too.

> +write-protect messages will always be with
> +.B UFFD_PAGEFAULT_FLAG_WRITE
> +bit set too along with bit
> +.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
> +.B UFFDIO_WRITEPROTECT_MODE_WP
> +cleared upon the faulted page or range.
> +.PP
> +Write-protect mode only supports private anonymous memory.
>  .SS Reading from the userfaultfd structure
>  Each
>  .BR read (2)
> @@ -364,8 +459,12 @@ 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.

Break at the point.

>  .RE
>  .TP
>  .I pagefault.feat.pid
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/

[PATCH v3 3/4] ioctl_userfaultfd.2: Add UFFD_FEATURE_THREAD_ID docs

[Peter Xu]

UFFD_FEATURE_THREAD_ID is supported in Linux 4.14.

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 man2/ioctl_userfaultfd.2 | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 47ae5f473..d4a8375b8 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -208,6 +208,11 @@ signal will be sent to the faulting process.
 Applications using this
 feature will not require the use of a userfaultfd monitor for processing
 memory accesses to the regions registered with userfaultfd.
+.TP
+.BR UFFD_FEATURE_THREAD_ID " (since Linux 4.14)"
+If this feature bit is set,
+.I uffd_msg.pagefault.feat.ptid
+will be set to the faulted thread ID for each page fault message.
 .PP
 The returned
 .I ioctls
-- 
2.26.2

[PATCH v3 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

[Peter Xu]

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

Signed-off-by: Peter Xu <peterx@redhat.com>
---
 man2/ioctl_userfaultfd.2 | 81 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 78 insertions(+), 3 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index d4a8375b8..d8380896a 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 +445,13 @@ 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 in conjunction with write-protect mode when both missing and
+write-protect modes are enabled.
 .PP
 The
 .I copy
@@ -654,6 +663,72 @@ 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 and need to retry.
+.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

Re: [PATCH v3 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

[Alejandro Colomar (man-pages)]

Hi Peter,

A few more comments below.

Thanks,

Alex

On 3/10/21 11:23 PM, Peter Xu wrote:
> Userfaultfd write-protect mode is supported starting from Linux 5.7.
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  man2/ioctl_userfaultfd.2 | 81 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 78 insertions(+), 3 deletions(-)
> 
> diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> index d4a8375b8..d8380896a 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 +445,13 @@ 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

Break at the comma instead.

> +generate another write-protect userfault message.
> +This is only used in conjunction with write-protect mode when both missing and

"when both missing"

both what?

> +write-protect modes are enabled.
>  .PP
>  The
>  .I copy
> @@ -654,6 +663,72 @@ 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

Break at the comma instead.

> +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

Break at the comma.

> +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 and need to retry.
> +.TP
> +.B ENOENT
> +The range specified in
> +.I range
> +is not valid.
> +For example, the virtual address does not exist, or not registered with

Better break at the comma.

> +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
> 

-- 
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/

Re: [PATCH v3 4/4] ioctl_userfaultfd.2: Add write-protect mode docs

[Peter Xu]

On Fri, Mar 19, 2021 at 11:37:20PM +0100, Alejandro Colomar (man-pages) wrote:
> Hi Peter,

Hi, Alex,

> > +generate another write-protect userfault message.
> > +This is only used in conjunction with write-protect mode when both missing and
> 
> "when both missing"
> 
> both what?

I modified it to:

        This is only used when both
        .B UFFDIO_REGISTER_MODE_MISSING
        and
        .B UFFDIO_REGISTER_MODE_WP
        modes are enabled for the registered range.

And I fixed all the rest, including all the comments in patch 3.

Thanks for looking, I'll repost shortly.

-- 
Peter Xu