Re: [PATCH v2] ioctl_userfaultfd.2, userfaultfd.2: add minor fault mode

From: Alejandro Colomar (man-pages)
Date: Mon Aug 02 2021 - 08:22:02 EST

Hi Mike, Axel,

On 8/2/21 1:21 PM, Mike Rapoport wrote:
(added man-pages maintainers)

Thanks! If I'm not CCed, I may not notice the email, depending on the traffic of the lists, and the amount of time I have ;)


On Tue, Jul 27, 2021 at 09:32:34AM -0700, Axel Rasmussen wrote:
Any remaining issues with this patch? I just realized today it was
never merged. 5.13 (which contains this new feature) was released some
weeks ago.

Please see some minor formatting issues I commented below.
Other than that, it looks good to me.

Thanks,

Alex


On Fri, Jun 4, 2021 at 12:56 PM Axel Rasmussen <axelrasmussen@xxxxxxxxxx> wrote:

Userfaultfd minor fault mode is supported starting from Linux 5.13.

This commit adds a description of the new mode, as well as the new ioctl
used to resolve such faults. The two go hand-in-hand: one can't resolve
a minor fault without continue, and continue can't be used to resolve
any other kind of fault.

This patch covers just the hugetlbfs implementation (in 5.13). Support
for shmem is forthcoming, but as it has not yet made it into a kernel
release candidate, it will be added in a future commit.

Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
---
man2/ioctl_userfaultfd.2 | 125 ++++++++++++++++++++++++++++++++++++---
man2/userfaultfd.2 | 79 ++++++++++++++++++++-----
2 files changed, 182 insertions(+), 22 deletions(-)

diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
index 504f61d4b..7b990c24a 100644
--- a/man2/ioctl_userfaultfd.2
+++ b/man2/ioctl_userfaultfd.2
@@ -214,6 +214,10 @@ memory accesses to the regions registered with userfaultfd.
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.
+.TP
+.BR UFFD_FEATURE_MINOR_HUGETLBFS " (since Linux 5.13)"
+If this feature bit is set, the kernel supports registering userfaultfd ranges
+in minor mode on hugetlbfs-backed memory areas.

See the folowing extract from man-pages(7):

Use semantic newlines
In the source of a manual page, new sentences should be
started on new lines, and long sentences should be split
into lines at clause breaks (commas, semicolons, colons,
and so on). This convention, sometimes known as "semantic
newlines", makes it easier to see the effect of patches,
which often operate at the level of individual sentences or
sentence clauses.

A trick to check if some text is correct at first glance, is that the following regex should rarely match:
[,;:.] \+\w

Multi-sentence parenthetical expressions should also go on separate lines normally.

I'd for example break the above text into:

[
If this feature bit is set,
the kernel supports registering userfaultfd ranges
in minor mode on hugetlbfs-backed memory areas
]

Note the break after the comma, and another break at a sensible point (you already did that one correctly in this example, but some below don't).


.PP
The returned
.I ioctls
@@ -240,6 +244,11 @@ operation is supported.
The
.B UFFDIO_WRITEPROTECT
operation is supported.
+.TP
+.B 1 << _UFFDIO_CONTINUE
+The
+.B UFFDIO_CONTINUE
+operation is supported.
.PP
This
.BR ioctl (2)
@@ -278,14 +287,8 @@ by the current kernel version.
(Since Linux 4.3.)
Register a memory address range with the userfaultfd object.
The pages in the range must be "compatible".
-.PP
-Up to Linux kernel 4.11,
-only private anonymous ranges are compatible for registering with
-.BR UFFDIO_REGISTER .
-.PP
-Since Linux 4.11,
-hugetlbfs and shared memory ranges are also compatible with
-.BR UFFDIO_REGISTER .
+Please refer to the list of register modes below for the compatible memory
+backends for each mode.

Regarding semantic newlines mentioned above:

Here for example, a more sensible point to break the line would be just after (or maybe before, up to you) the first "for".

.PP
The
.I argp
@@ -324,9 +327,16 @@ the specified range:
.TP
.B UFFDIO_REGISTER_MODE_MISSING
Track page faults on missing pages.
+Since Linux 4.3, only private anonymous ranges are compatible.
+Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible.
.TP
.B UFFDIO_REGISTER_MODE_WP
Track page faults on write-protected pages.
+Since Linux 5.7, only private anonymous ranges are compatible.
+.TP
+.B UFFDIO_REGISTER_MODE_MINOR
+Track minor page faults.
+Since Linux 5.13, only hugetlbfs ranges are compatible.
.PP
If the operation is successful, the kernel modifies the
.I ioctls
@@ -735,6 +745,105 @@ or not registered with userfaultfd write-protect mode.
.TP
.B EFAULT
Encountered a generic fault during processing.
+.\"
+.SS UFFDIO_CONTINUE
+(Since Linux 5.13.)
+Resolve a minor page fault by installing page table entries for existing pages
+in the page cache.
+.PP
+The
+.I argp
+argument is a pointer to a
+.I uffdio_continue
+structure as shown below:
+.PP
+.in +4n
+.EX
+struct uffdio_continue {
+ struct uffdio_range range; /* Range to install PTEs for and continue */
+ __u64 mode; /* Flags controlling the behavior of continue */
+ __s64 mapped; /* Number of bytes mapped, or negated error */
+};
+.EE
+.in
+.PP
+The following value may be bitwise ORed in
+.IR mode
+to change the behavior of the
+.B UFFDIO_CONTINUE
+operation:
+.TP
+.B UFFDIO_CONTINUE_MODE_DONTWAKE
+Do not wake up the thread that waits for page-fault resolution.
+.PP
+The
+.I mapped
+field is used by the kernel to return the number of bytes
+that were actually mapped, or an error in the same manner as
+.BR UFFDIO_COPY .
+If the value returned in the
+.I mapped
+field doesn't match the value that was specified in
+.IR range.len ,
+the operation fails with the error
+.BR EAGAIN .
+The
+.I mapped
+field is output-only;
+it is not read by the
+.B UFFDIO_CONTINUE
+operation.
+.PP
+This
+.BR ioctl (2)
+operation returns 0 on success.
+In this case, the entire area was mapped.
+On error, \-1 is returned and
+.I errno
+is set to indicate the error.
+Possible errors include:
+.TP
+.B EAGAIN
+The number of bytes mapped (i.e., the value returned in the
+.I mapped
+field) does not equal the value that was specified in the
+.I range.len
+field.
+.TP
+.B EINVAL
+Either
+.I range.start
+or
+.I range.len
+was not a multiple of the system page size; or
+.I range.len
+was zero; or the range specified was invalid.
+.TP
+.B EINVAL
+An invalid bit was specified in the
+.IR mode
+field.
+.TP
+.B EEXIST
+One or more pages were already mapped in the given range.
+.TP
+.B ENOENT
+The faulting process has changed its virtual memory layout simultaneously with
+an outstanding
+.B UFFDIO_CONTINUE
+operation.
+.TP
+.B ENOMEM
+Allocating memory needed to setup the page table mappings failed.
+.TP
+.B EFAULT
+No existing page could be found in the page cache for the given range.
+.TP
+.BR ESRCH
+The faulting process has exited at the time of a
+.B UFFDIO_CONTINUE
+operation.
+.\"
.SH RETURN VALUE
See descriptions of the individual operations, above.
.SH ERRORS
diff --git a/man2/userfaultfd.2 b/man2/userfaultfd.2
index 593c189d8..07f53c6ff 100644
--- a/man2/userfaultfd.2
+++ b/man2/userfaultfd.2
@@ -78,7 +78,7 @@ all memory ranges that were registered with the object are unregistered
and unread events are flushed.
.\"
.PP
-Userfaultfd supports two modes of registration:
+Userfaultfd supports three modes of registration:
.TP
.BR UFFDIO_REGISTER_MODE_MISSING " (since 4.10)"
When registered with
@@ -92,6 +92,18 @@ or an
.B UFFDIO_ZEROPAGE
ioctl.
.TP
+.BR UFFDIO_REGISTER_MODE_MINOR " (since 5.13)"
+When registered with
+.B UFFDIO_REGISTER_MODE_MINOR
+mode, user-space will receive a page-fault notification

s/user-space/user space/

See the following extract from man-pages(7):

Preferred terms
The following table lists some preferred terms to use in
man pages, mainly to ensure consistency across pages.

Term Avoid using Notes
─────────────────────────────────────────────────────────────
[...]
user space userspace

However, when user space is used as an adjective, per the usual English rules, we write "user-space". Example: "a user-space program".

+when a minor page fault occurs.
+That is, when a backing page is in the page cache, but
+page table entries don't yet exist.
+The faulted thread will be stopped from execution until the page fault is
+resolved from user-space by an
+.B UFFDIO_CONTINUE
+ioctl.
+.TP
.BR UFFDIO_REGISTER_MODE_WP " (since 5.7)"
When registered with
.B UFFDIO_REGISTER_MODE_WP
@@ -212,9 +224,10 @@ a page fault occurring in the requested memory range, and satisfying
the mode defined at the registration time, will be forwarded by the kernel to
the user-space application.
The application can then use the
-.B UFFDIO_COPY
+.B UFFDIO_COPY ,
+.B UFFDIO_ZEROPAGE ,
or
-.B UFFDIO_ZEROPAGE
+.B UFFDIO_CONTINUE
.BR ioctl (2)
operations to resolve the page fault.
.PP
@@ -318,6 +331,43 @@ should have the flag
cleared upon the faulted page or range.
.PP
Write-protect mode supports only private anonymous memory.
+.\"
+.SS Userfaultfd minor fault mode (since 5.13)
+Since Linux 5.13, userfaultfd supports minor fault mode.
+In this mode, fault messages are produced not for major faults (where the
+page was missing), but rather for minor faults, where a page exists in the page
+cache, but the page table entries are not yet present.
+The user needs to first check availability of this feature using
+.B UFFDIO_API
+ioctl against the feature bit
+.B UFFD_FEATURE_MINOR_HUGETLBFS
+before using this feature.
+.PP
+To register with userfaultfd minor fault mode, the user needs to initiate the
+.B UFFDIO_REGISTER
+ioctl with mode
+.B UFFD_REGISTER_MODE_MINOR
+set.
+.PP
+When a minor fault occurs, user-space will receive a page-fault notification
+whose
+.I uffd_msg.pagefault.flags
+will have the
+.B UFFD_PAGEFAULT_FLAG_MINOR
+flag set.
+.PP
+To resolve a minor page fault, the handler should decide whether or not the
+existing page contents need to be modified first.
+If so, this should be done in-place via a second, non-userfaultfd-registered
+mapping to the same backing page (e.g., by mapping the hugetlbfs file twice).
+Once the page is considered "up to date", the fault can be resolved by
+initiating an
+.B UFFDIO_CONTINUE
+ioctl, which installs the page table entries and (by default) wakes up the
+faulting thread(s).
+.PP
+Minor fault mode supports only hugetlbfs-backed memory.
+.\"
.SS Reading from the userfaultfd structure
Each
.BR read (2)
@@ -456,19 +506,20 @@ For
the following flag may appear:
.RS
.TP
-.B UFFD_PAGEFAULT_FLAG_WRITE
-If the address is in a range that was registered with the
-.B UFFDIO_REGISTER_MODE_MISSING
-flag (see
-.BR ioctl_userfaultfd (2))
-and this flag is set, this a write fault;
-otherwise it is a read fault.
+.B UFFD_PAGEFAULT_FLAG_WP
+If this flag is set, then the fault was a write-protect fault.
.TP
+.B UFFD_PAGEFAULT_FLAG_MINOR
+If this flag is set, then the fault was a minor fault.
+.TP
+.B UFFD_PAGEFAULT_FLAG_WRITE
+If this flag is set, then the fault was a write fault.
+.HP

See the following extract from groff_man(7):

Deprecated features
Use of the following is discouraged.

[...]

.HP [indent]
Set up a paragraph with a hanging left indentation.
The indent argument, if present, is handled as with
.TP.

Use of this presentation‐level macro is deprecated.
While it is universally portable to legacy Unix sys‐
tems, a hanging indentation cannot be expressed nat‐
urally under HTML, and many HTML‐based manual view‐
ers simply interpret it as a starter for a normal
paragraph. Thus, any information or distinction you
tried to express with the indentation may be lost.

I'd just use .PP here, I think.


+If neither
.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 is a write-protect fault.
-Otherwise it is a page-missing fault.
+nor
+.B UFFD_PAGEFAULT_FLAG_MINOR
+are set, then the fault was a missing fault.
.RE
.TP
.I pagefault.feat.pid
--
2.32.0.rc1.229.g3e70b5a671-goog





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