man2: udpate mm/userfaultfd manpages to latest

UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.

Signed-off-by Peter Xu <[email protected]>

---
 man2/userfaultfd.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

man2/userfaultfd.2 +0 -0

index e7dc9f813..555e37409 100644

man2/userfaultfd.2 +6 -0

 .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

Signature · 1 line

2.26.2

Quoted 16 lines · Peter Xu

UFFD_FEATURE_THREAD_ID is supported since Linux 4.14.

Signed-off-by Peter Xu <[email protected]>

---
 man2/userfaultfd.2 | 13 +++++++++++++
 1 file changed, 13 insertions(+)

man2/userfaultfd.2 +1 -0

index e7dc9f813..555e37409 100644
@@ -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

man2/userfaultfd.2 When the last file descriptor referring to a userfaultfd object is closed, · line 86

Quoted 6 lines

man2/userfaultfd.2 +6 -0

+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.

^ disabled :)

Write-protect mode is supported starting from Linux 5.7.

Signed-off-by Peter Xu <[email protected]>

---
  man2/userfaultfd.2 | 104 ++++++++++++++++++++++++++++++++++++++++++++-
  1 file changed, 102 insertions(+), 2 deletions(-)

man2/userfaultfd.2 +11 -0

index 555e37409..8ad4a71b5 100644
@@ -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

man2/userfaultfd.2 +5 -0

+.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

man2/userfaultfd.2 +74 -2

+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 +256,65 @@ 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 +460,12 @@ 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

Hi Peter,

Please see a few more comments below.

Thanks,

Alex

Quoted 34 lines · Peter Xu

Write-protect mode is supported starting from Linux 5.7.

Signed-off-by Peter Xu <[email protected]>

---
  man2/userfaultfd.2 | 104 ++++++++++++++++++++++++++++++++++++++++++++-
  1 file changed, 102 insertions(+), 2 deletions(-)

man2/userfaultfd.2 +19 -0

index 555e37409..8ad4a71b5 100644
@@ -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.

man2/userfaultfd.2 all memory ranges that were registered with the object are unregistered · line 101

Quoted 2 lines

man2/userfaultfd.2 +2 -0

+The faulted thread will be stopped from execution
+until the userspace un-write-protect the page using an

Here you use un-write-protect, but in the other patch you use write-unprotect. Please, use a consistent wording if it's the same thing (if there are other similar things with different wordings in different pages, please fix them too, but I didn't see more of those). If there's already a wording for that in any page, please reuse it (I ignore it).

Quoted 10 lines

man2/userfaultfd.2 +5 -0

+.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.

man2/userfaultfd.2 single threaded non-cooperative userfaultfd manager implementations. · line 174

Quoted 5 lines

man2/userfaultfd.2 +2 -0

  .\" 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

The previous paragraph uses "Siince Linux 4.14". For consistency, please use that same wording here.

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

Signed-off-by Peter Xu <[email protected]>

---
  man2/ioctl_userfaultfd.2 | 84 ++++++++++++++++++++++++++++++++++++++--
  1 file changed, 81 insertions(+), 3 deletions(-)

man2/ioctl_userfaultfd.2 +5 -3

index d4a8375b8..5419687a6 100644
@@ -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

man2/ioctl_userfaultfd.2 +51 -0

+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

man2/ioctl_userfaultfd.2 +0 -0

  .SH RETURN VALUE
  See descriptions of the individual operations, above.
  .SH ERRORS

Hi Peter,

man2/ioctl_userfaultfd.2 operation: · line 452

Quoted 5 lines

+.TP
+.B UFFDIO_COPY_MODE_WP

man2/ioctl_userfaultfd.2 +3 -0

+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.

s/write-protect/write-protected/
?

Quoted 2 lines

+.B EAGAIN
+The process was interrupted and need to retry.

Maybe: "The process was interrupted; retry this call."? I don't know what other pager say about this kind of error.

man2/ioctl_userfaultfd.2 field of the · line 729

Quoted 6 lines

+.TP
+.B ENOENT

man2/ioctl_userfaultfd.2 +3 -0

+The range specified in
+.I range
+is not valid.

I'm not sure how this is different from the wording above in EINVAL. An "otherwise invalid range" was already giving EINVAL?

man2/ioctl_userfaultfd.2 field of the · line 734

Quoted 6 lines

man2/ioctl_userfaultfd.2 +5 -0

+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.

What is a "generic fault"?

Quoted 8 lines · Alejandro Colomar (man-pages)

man2/ioctl_userfaultfd.2 operation: · line 452

Quoted 5 lines

+.TP
+.B UFFDIO_COPY_MODE_WP

man2/ioctl_userfaultfd.2 +3 -0

+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.

s/write-protect/write-protected/
?

I think here "write-protect" is the wording I wanted to use, it is the name of the type of the message in plain text.

[...]

Quoted 5 lines

Quoted 2 lines

+.B EAGAIN
+The process was interrupted and need to retry.

Maybe: "The process was interrupted; retry this call."? I don't know what other pager say about this kind of error.

Frankly I see no difference between the two.. If you prefer the latter, I can switch.

Quoted 10 lines

man2/ioctl_userfaultfd.2 field of the · line 729

Quoted 7 lines

+.TP
+.B ENOENT

man2/ioctl_userfaultfd.2 +3 -0

+The range specified in
+.I range
+is not valid.

I'm not sure how this is different from the wording above in EINVAL. An "otherwise invalid range" was already giving EINVAL?

This can be returned when vma is not found (mwriteprotect_range()):

	err = -ENOENT;
	dst_vma = find_dst_vma(dst_mm, start, len);

	if (!dst_vma)
		goto out_unlock;

I think maybe I could simply remove this entry, because from an user app developer pov I'd only be interested in specific error that I'd be able to detect and (even better) recover from. For such error I'd say there's not much to do besides failing the app.

Quoted 9 lines

man2/ioctl_userfaultfd.2 field of the · line 734

Quoted 7 lines

man2/ioctl_userfaultfd.2 +5 -0

+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.

What is a "generic fault"?

For example when the user copy failed due to some reason. See userfaultfd_writeprotect():

	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
			   sizeof(struct uffdio_writeprotect)))
		return -EFAULT;

But I didn't check other places, generally I'd return -EFAULT if I can't find a proper other replacement which has a clearer meaning.

I don't think this is really helpful to user app too because no user app would start to read this -EFAULT to do anything useful.. how about I drop it too if you think the description is confusing?

Hi Peter,

Quoted 10 lines · Peter Xu

Quoted 8 lines · Alejandro Colomar (man-pages)

man2/ioctl_userfaultfd.2 operation: · line 452

Quoted 5 lines

+.TP
+.B UFFDIO_COPY_MODE_WP

man2/ioctl_userfaultfd.2 +3 -0

+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.

s/write-protect/write-protected/
?

Quoted 2 lines

I think here "write-protect" is the wording I wanted to use, it is the name of the type of the message in plain text.

Okay.

Quoted 9 lines

[...]

Quoted 5 lines

Quoted 2 lines

+.B EAGAIN
+The process was interrupted and need to retry.

Maybe: "The process was interrupted; retry this call."? I don't know what other pager say about this kind of error.

Quoted 2 lines

Frankly I see no difference between the two.. If you prefer the latter, I can switch.

I understand yours, but technically it's a bit incorrect: The subject of the sentence changes: in "The process was interrupted" it's the process, and in "need to retry" it's [you]. By separating the sentence into two, it's more natural. :)

Quoted 20 lines

Quoted 11 lines

man2/ioctl_userfaultfd.2 field of the · line 729

Quoted 8 lines

+.TP
+.B ENOENT

man2/ioctl_userfaultfd.2 +3 -0

+The range specified in
+.I range
+is not valid.

I'm not sure how this is different from the wording above in EINVAL. An "otherwise invalid range" was already giving EINVAL?

This can be returned when vma is not found (mwriteprotect_range()):

	err = -ENOENT;
	dst_vma = find_dst_vma(dst_mm, start, len);

	if (!dst_vma)
		goto out_unlock;

Quoted 4 lines

I think maybe I could simply remove this entry, because from an user app developer pov I'd only be interested in specific error that I'd be able to detect and (even better) recover from. For such error I'd say there's not much to do besides failing the app.

If there's any possibility that the error can happen, it should be documented, even if it's to say "Fatal error; abort!". Just try to explain the causes and how to avoid causing them and/or possibly what to do when they happen (abort?).

Quoted 18 lines

Quoted 10 lines

man2/ioctl_userfaultfd.2 field of the · line 734

Quoted 8 lines

man2/ioctl_userfaultfd.2 +5 -0

+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.

What is a "generic fault"?

For example when the user copy failed due to some reason. See userfaultfd_writeprotect():

	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
			   sizeof(struct uffdio_writeprotect)))
		return -EFAULT;

Quoted 6 lines

But I didn't check other places, generally I'd return -EFAULT if I can't find a proper other replacement which has a clearer meaning.

I don't think this is really helpful to user app too because no user app would start to read this -EFAULT to do anything useful.. how about I drop it too if you think the description is confusing?

Same as above.

Quoted 22 lines · Alejandro Colomar (man-pages)

Quoted 17 lines

Quoted 8 lines

man2/ioctl_userfaultfd.2 field of the · line 729

Quoted 5 lines

+.TP
+.B ENOENT

man2/ioctl_userfaultfd.2 +3 -0

+The range specified in
+.I range
+is not valid.

I'm not sure how this is different from the wording above in EINVAL. An "otherwise invalid range" was already giving EINVAL?

This can be returned when vma is not found (mwriteprotect_range()):

	err = -ENOENT;
	dst_vma = find_dst_vma(dst_mm, start, len);

	if (!dst_vma)
		goto out_unlock;

Quoted 4 lines

I think maybe I could simply remove this entry, because from an user app developer pov I'd only be interested in specific error that I'd be able to detect and (even better) recover from. For such error I'd say there's not much to do besides failing the app.

Quoted 4 lines

If there's any possibility that the error can happen, it should be documented, even if it's to say "Fatal error; abort!". Just try to explain the causes and how to avoid causing them and/or possibly what to do when they happen (abort?).

Okay. Would you mind me keeping my original wording? Because IMHO that exactly does what you said as "trying to explain the causes" and so on:

.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.

It's indeed slightly duplicated with EINVAL, but if you don't agree with the wording meanwhile if you don't agree on overlapping of the errors, then what I need is not reworking this patchset, but proposing a kernel patch to change the error retval to make them match. I am not against proposing a kernel patch, but I just don't see it extremely necessary.

For my own experience on working with the kernel, the return value sometimes is not that strict - say, it's hard to control every single bit of the possible return code of a syscall/ioctl to reflect everything matching the document. We should always try to do it accurate but it seems not easy to me. It's also hard to write up the document that 100% matching the kernel code, because at least that'll require a full-path workthrough of every single piece of kernel code that the syscall/ioctl has called, so as to collect all the errors, then summarize their meanings. That could be a lot of work.

Quoted 15 lines

Quoted 15 lines

Quoted 7 lines

man2/ioctl_userfaultfd.2 field of the · line 734

Quoted 5 lines

man2/ioctl_userfaultfd.2 +5 -0

+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.

What is a "generic fault"?

For example when the user copy failed due to some reason. See userfaultfd_writeprotect():

	if (copy_from_user(&uffdio_wp, user_uffdio_wp,
			   sizeof(struct uffdio_writeprotect)))
		return -EFAULT;

Quoted 8 lines

Quoted 6 lines

But I didn't check other places, generally I'd return -EFAULT if I can't find a proper other replacement which has a clearer meaning.

I don't think this is really helpful to user app too because no user app would start to read this -EFAULT to do anything useful.. how about I drop it too if you think the description is confusing?

Same as above.

Above copy_from_user() is the only place that could trigger -EFAULT so far I can find. So either I can change above into:

.TP
.B EFAULT
Failure on copying ioctl parameters into the kernel.

Would you think it okay (before I repost)? I'd still prefer my original wording because I bet 90% user developer may not even know what does it mean when the kernel cannot copy the user parameter, and what he/she can do with it.. However if you think it's proper I'll use it.

v4:
- Fixed a few "subordinate clauses" (SC) cases [Alex]
- Reword in ioctl_userfaultfd.2 to use bold font for the two modes referenced, so as to be clear on what is "both" referring to [Alex]

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 |  89 ++++++++++++++++++++++++++++-
 man2/userfaultfd.2       | 117 ++++++++++++++++++++++++++++++++++++++-
 2 files changed, 201 insertions(+), 5 deletions(-)

Signature · 1 line

2.26.2