= Userfaultfd =

== Objective ==

Userfaults allow the implementation of on-demand paging from userland
and more generally they allow userland to take control of various
memory page faults, something otherwise only the kernel code could do.

For example userfaults allows a proper and more optimal implementation
of the PROT_NONE+SIGSEGV trick.

== Design ==

Userfaults are delivered and resolved through the userfaultfd syscall.

The userfaultfd (aside from registering and unregistering virtual
memory ranges) provides two primary functionalities:

1) read/POLLIN protocol to notify a userland thread of the faults
   happening

2) various UFFDIO_* ioctls that can manage the virtual memory regions
   registered in the userfaultfd that allows userland to efficiently
   resolve the userfaults it receives via 1) or to manage the virtual
   memory in the background

The real advantage of userfaults if compared to regular virtual memory
management of mremap/mprotect is that the userfaults in all their
operations never involve heavyweight structures like vmas (in fact the
userfaultfd runtime load never takes the mmap_sem for writing).

Vmas are not suitable for page- (or hugepage) granular fault tracking
when dealing with virtual address spaces that could span
Terabytes. Too many vmas would be needed for that.

The userfaultfd once opened by invoking the syscall, can also be
passed using unix domain sockets to a manager process, so the same
manager process could handle the userfaults of a multitude of
different processes without them being aware about what is going on
(well of course unless they later try to use the userfaultfd
themselves on the same region the manager is already tracking, which
is a corner case that would currently return -EBUSY).

== API ==

When first opened the userfaultfd must be enabled invoking the
UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or
a later API version) which will specify the read/POLLIN protocol
userland intends to speak on the UFFD and the uffdio_api.features
userland requires. The UFFDIO_API ioctl if successful (i.e. if the
requested uffdio_api.api is spoken also by the running kernel and the
requested features are going to be enabled) will return into
uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
respectively all the available features of the read(2) protocol and
the generic ioctl available.

The uffdio_api.features bitmask returned by the UFFDIO_API ioctl
defines what memory types are supported by the userfaultfd and what
events, except page fault notifications, may be generated:

- The UFFD_FEATURE_EVENT_* flags indicate that various other events
  other than page faults are supported. These events are described in more
  detail below in the Non-cooperative userfaultfd section.

- UFFD_FEATURE_MISSING_HUGETLBFS and UFFD_FEATURE_MISSING_SHMEM
  indicate that the kernel supports UFFDIO_REGISTER_MODE_MISSING
  registrations for hugetlbfs and shared memory (covering all shmem APIs,
  i.e. tmpfs, IPCSHM, /dev/zero, MAP_SHARED, memfd_create,
  etc) virtual memory areas, respectively.

- UFFD_FEATURE_MINOR_HUGETLBFS indicates that the kernel supports
  UFFDIO_REGISTER_MODE_MINOR registration for hugetlbfs virtual memory
  areas.

The userland application should set the feature flags it intends to use
when invoking the UFFDIO_API ioctl, to request that those features be
enabled if supported.

Once the userfaultfd API has been enabled the UFFDIO_REGISTER
ioctl should be invoked (if present in the returned uffdio_api.ioctls
bitmask) to register a memory range in the userfaultfd by setting the
uffdio_register structure accordingly. The uffdio_register.mode
bitmask will specify to the kernel which kind of faults to track for
the range. The UFFDIO_REGISTER ioctl will return the
uffdio_register.ioctls bitmask of ioctls that are suitable to resolve
userfaults on the range registered. Not all ioctls will necessarily be
supported for all memory types (e.g. anonymous memory vs. shmem vs.
hugetlbfs), or all types of intercepted faults.

Userland can use the uffdio_register.ioctls to manage the virtual
address space in the background (to add or potentially also remove
memory from the userfaultfd registered range). This means a userfault
could be triggering just before userland maps in the background the
user-faulted page.

Resolving Userfaults
--------------------

There are three basic ways to resolve userfaults:

- UFFDIO_COPY atomically copies some existing page contents from
  userspace.

- UFFDIO_ZEROPAGE atomically zeros the new page.

- UFFDIO_CONTINUE maps an existing, previously-populated page.

These operations are atomic in the sense that they guarantee nothing can
see a half-populated page, since readers will keep userfaulting until the
operation has finished.

By default, these wake up userfaults blocked on the range in question.
They support a UFFDIO_*_MODE_DONTWAKE mode flag, which indicates
that waking will be done separately at some later time.

Which ioctl to choose depends on the kind of page fault, and what we'd
like to do to resolve it:

- For UFFDIO_REGISTER_MODE_MISSING faults, the fault needs to be
  resolved by either providing a new page (UFFDIO_COPY), or mapping
  the zero page (UFFDIO_ZEROPAGE). By default, the kernel would map
  the zero page for a missing fault. With userfaultfd, userspace can
  decide what content to provide before the faulting thread continues.

- For UFFDIO_REGISTER_MODE_MINOR faults, there is an existing page (in
  the page cache). Userspace has the option of modifying the page's
  contents before resolving the fault. Once the contents are correct
  (modified or not), userspace asks the kernel to map the page and let the
  faulting thread continue with UFFDIO_CONTINUE.

Notes:

- You can tell which kind of fault occurred by examining
  pagefault.flags within the uffd_msg, checking for the
  UFFD_PAGEFAULT_FLAG_* flags.

- None of the page-delivering ioctls default to the range that you
  registered with.  You must fill in all fields for the appropriate
  ioctl struct including the range.

- You get the address of the access that triggered the missing page
  event out of a struct uffd_msg that you read in the thread from the
  uffd.  You can supply as many pages as you want with these IOCTLs.
  Keep in mind that unless you used DONTWAKE then the first of any of
  those IOCTLs wakes up the faulting thread.

- Be sure to test for all errors including
  (pollfd[0].revents & POLLERR).  This can happen, e.g. when ranges
  supplied were incorrect.

== QEMU/KVM ==

QEMU/KVM is using the userfaultfd syscall to implement postcopy live
migration. Postcopy live migration is one form of memory
externalization consisting of a virtual machine running with part or
all of its memory residing on a different node in the cloud. The
userfaultfd abstraction is generic enough that not a single line of
KVM kernel code had to be modified in order to add postcopy live
migration to QEMU.

Guest async page faults, FOLL_NOWAIT and all other GUP features work
just fine in combination with userfaults. Userfaults trigger async
page faults in the guest scheduler so those guest processes that
aren't waiting for userfaults (i.e. network bound) can keep running in
the guest vcpus.

It is generally beneficial to run one pass of precopy live migration
just before starting postcopy live migration, in order to avoid
generating userfaults for readonly guest regions.

The implementation of postcopy live migration currently uses one
single bidirectional socket but in the future two different sockets
will be used (to reduce the latency of the userfaults to the minimum
possible without having to decrease /proc/sys/net/ipv4/tcp_wmem).

The QEMU in the source node writes all pages that it knows are missing
in the destination node, into the socket, and the migration thread of
the QEMU running in the destination node runs UFFDIO_COPY|ZEROPAGE
ioctls on the userfaultfd in order to map the received pages into the
guest (UFFDIO_ZEROCOPY is used if the source page was a zero page).

A different postcopy thread in the destination node listens with
poll() to the userfaultfd in parallel. When a POLLIN event is
generated after a userfault triggers, the postcopy thread read() from
the userfaultfd and receives the fault address (or -EAGAIN in case the
userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run
by the parallel QEMU migration thread).

After the QEMU postcopy thread (running in the destination node) gets
the userfault address it writes the information about the missing page
into the socket. The QEMU source node receives the information and
roughly "seeks" to that page address and continues sending all
remaining missing pages from that new page offset. Soon after that
(just the time to flush the tcp_wmem queue through the network) the
migration thread in the QEMU running in the destination node will
receive the page that triggered the userfault and it'll map it as
usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it
was spontaneously sent by the source or if it was an urgent page
requested through a userfault).

By the time the userfaults start, the QEMU in the destination node
doesn't need to keep any per-page state bitmap relative to the live
migration around and a single per-page bitmap has to be maintained in
the QEMU running in the source node to know which pages are still
missing in the destination node. The bitmap in the source node is
checked to find which missing pages to send in round robin and we seek
over it when receiving incoming userfaults. After sending each page of
course the bitmap is updated accordingly. It's also useful to avoid
sending the same page twice (in case the userfault is read by the
postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration
thread).

== Non-cooperative userfaultfd ==

When the userfaultfd is monitored by an external manager, the manager
must be able to track changes in the process virtual memory
layout. Userfaultfd can notify the manager about such changes using
the same read(2) protocol as for the page fault notifications. The
manager has to explicitly enable these events by setting appropriate
bits in uffdio_api.features passed to UFFDIO_API ioctl:

UFFD_FEATURE_EVENT_FORK - enable userfaultfd hooks for fork(). When
this feature is enabled, the userfaultfd context of the parent process
is duplicated into the newly created process. The manager receives
UFFD_EVENT_FORK with file descriptor of the new userfaultfd context in
the uffd_msg.fork.

UFFD_FEATURE_EVENT_REMAP - enable notifications about mremap()
calls. When the non-cooperative process moves a virtual memory area to
a different location, the manager will receive UFFD_EVENT_REMAP. The
uffd_msg.remap will contain the old and new addresses of the area and
its original length.

UFFD_FEATURE_EVENT_REMOVE - enable notifications about
madvise(MADV_REMOVE) and madvise(MADV_DONTNEED) calls. The event
UFFD_EVENT_REMOVE will be generated upon these calls to madvise. The
uffd_msg.remove will contain start and end addresses of the removed
area.

UFFD_FEATURE_EVENT_UNMAP - enable notifications about memory
unmapping. The manager will get UFFD_EVENT_UNMAP with uffd_msg.remove
containing start and end addresses of the unmapped area.

Although the UFFD_FEATURE_EVENT_REMOVE and UFFD_FEATURE_EVENT_UNMAP
are pretty similar, they quite differ in the action expected from the
userfaultfd manager. In the former case, the virtual memory is
removed, but the area is not, the area remains monitored by the
userfaultfd, and if a page fault occurs in that area it will be
delivered to the manager. The proper resolution for such page fault is
to zeromap the faulting address. However, in the latter case, when an
area is unmapped, either explicitly (with munmap() system call), or
implicitly (e.g. during mremap()), the area is removed and in turn the
userfaultfd context for such area disappears too and the manager will
not get further userland page faults from the removed area. Still, the
notification is required in order to prevent manager from using
UFFDIO_COPY on the unmapped area.

Unlike userland page faults which have to be synchronous and require
explicit or implicit wakeup, all the events are delivered
asynchronously and the non-cooperative process resumes execution as
soon as manager executes read(). The userfaultfd manager should
carefully synchronize calls to UFFDIO_COPY with the events
processing. To aid the synchronization, the UFFDIO_COPY ioctl will
return -ENOSPC when the monitored process exits at the time of
UFFDIO_COPY, and -ENOENT, when the non-cooperative process has changed
its virtual memory layout simultaneously with outstanding UFFDIO_COPY
operation.

The current asynchronous model of the event delivery is optimal for
single threaded non-cooperative userfaultfd manager implementations. A
synchronous event delivery model can be added later as a new
userfaultfd feature to facilitate multithreading enhancements of the
non cooperative manager, for example to allow UFFDIO_COPY ioctls to
run in parallel to the event reception. Single threaded
implementations should continue to use the current async event
delivery model instead.