= 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.
  
  Once the userfaultfd 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 (UFFDIO_REGISTER_MODE_MISSING would track missing
  pages). 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 depending on the underlying virtual
  memory backend (anonymous memory vs tmpfs vs real filebacked
  mappings).
  
  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.
  
  The primary ioctl to resolve userfaults is UFFDIO_COPY. That
  atomically copies a page into the userfault registered range and wakes
  up the blocked userfaults (unless uffdio_copy.mode &
  UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to
  UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an
  half copied page since it'll keep userfaulting until the copy has
  finished.
  
  == 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 an 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).