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= 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
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
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
== 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