145 lines
6.9 KiB
Plaintext
145 lines
6.9 KiB
Plaintext
= Userfaultfd =
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== Objective ==
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Userfaults allow the implementation of on-demand paging from userland
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and more generally they allow userland to take control of various
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memory page faults, something otherwise only the kernel code could do.
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For example userfaults allows a proper and more optimal implementation
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of the PROT_NONE+SIGSEGV trick.
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== Design ==
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Userfaults are delivered and resolved through the userfaultfd syscall.
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The userfaultfd (aside from registering and unregistering virtual
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memory ranges) provides two primary functionalities:
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1) read/POLLIN protocol to notify a userland thread of the faults
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happening
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2) various UFFDIO_* ioctls that can manage the virtual memory regions
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registered in the userfaultfd that allows userland to efficiently
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resolve the userfaults it receives via 1) or to manage the virtual
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memory in the background
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The real advantage of userfaults if compared to regular virtual memory
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management of mremap/mprotect is that the userfaults in all their
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operations never involve heavyweight structures like vmas (in fact the
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userfaultfd runtime load never takes the mmap_sem for writing).
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Vmas are not suitable for page- (or hugepage) granular fault tracking
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when dealing with virtual address spaces that could span
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Terabytes. Too many vmas would be needed for that.
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The userfaultfd once opened by invoking the syscall, can also be
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passed using unix domain sockets to a manager process, so the same
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manager process could handle the userfaults of a multitude of
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different processes without them being aware about what is going on
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(well of course unless they later try to use the userfaultfd
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themselves on the same region the manager is already tracking, which
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is a corner case that would currently return -EBUSY).
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== API ==
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When first opened the userfaultfd must be enabled invoking the
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UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or
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a later API version) which will specify the read/POLLIN protocol
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userland intends to speak on the UFFD and the uffdio_api.features
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userland requires. The UFFDIO_API ioctl if successful (i.e. if the
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requested uffdio_api.api is spoken also by the running kernel and the
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requested features are going to be enabled) will return into
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uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of
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respectively all the available features of the read(2) protocol and
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the generic ioctl available.
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Once the userfaultfd has been enabled the UFFDIO_REGISTER ioctl should
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be invoked (if present in the returned uffdio_api.ioctls bitmask) to
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register a memory range in the userfaultfd by setting the
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uffdio_register structure accordingly. The uffdio_register.mode
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bitmask will specify to the kernel which kind of faults to track for
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the range (UFFDIO_REGISTER_MODE_MISSING would track missing
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pages). The UFFDIO_REGISTER ioctl will return the
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uffdio_register.ioctls bitmask of ioctls that are suitable to resolve
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userfaults on the range registered. Not all ioctls will necessarily be
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supported for all memory types depending on the underlying virtual
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memory backend (anonymous memory vs tmpfs vs real filebacked
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mappings).
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Userland can use the uffdio_register.ioctls to manage the virtual
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address space in the background (to add or potentially also remove
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memory from the userfaultfd registered range). This means a userfault
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could be triggering just before userland maps in the background the
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user-faulted page.
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The primary ioctl to resolve userfaults is UFFDIO_COPY. That
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atomically copies a page into the userfault registered range and wakes
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up the blocked userfaults (unless uffdio_copy.mode &
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UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to
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UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an
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half copied page since it'll keep userfaulting until the copy has
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finished.
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== QEMU/KVM ==
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QEMU/KVM is using the userfaultfd syscall to implement postcopy live
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migration. Postcopy live migration is one form of memory
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externalization consisting of a virtual machine running with part or
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all of its memory residing on a different node in the cloud. The
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userfaultfd abstraction is generic enough that not a single line of
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KVM kernel code had to be modified in order to add postcopy live
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migration to QEMU.
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Guest async page faults, FOLL_NOWAIT and all other GUP features work
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just fine in combination with userfaults. Userfaults trigger async
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page faults in the guest scheduler so those guest processes that
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aren't waiting for userfaults (i.e. network bound) can keep running in
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the guest vcpus.
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It is generally beneficial to run one pass of precopy live migration
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just before starting postcopy live migration, in order to avoid
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generating userfaults for readonly guest regions.
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The implementation of postcopy live migration currently uses one
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single bidirectional socket but in the future two different sockets
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will be used (to reduce the latency of the userfaults to the minimum
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possible without having to decrease /proc/sys/net/ipv4/tcp_wmem).
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The QEMU in the source node writes all pages that it knows are missing
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in the destination node, into the socket, and the migration thread of
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the QEMU running in the destination node runs UFFDIO_COPY|ZEROPAGE
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ioctls on the userfaultfd in order to map the received pages into the
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guest (UFFDIO_ZEROCOPY is used if the source page was a zero page).
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A different postcopy thread in the destination node listens with
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poll() to the userfaultfd in parallel. When a POLLIN event is
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generated after a userfault triggers, the postcopy thread read() from
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the userfaultfd and receives the fault address (or -EAGAIN in case the
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userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run
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by the parallel QEMU migration thread).
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After the QEMU postcopy thread (running in the destination node) gets
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the userfault address it writes the information about the missing page
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into the socket. The QEMU source node receives the information and
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roughly "seeks" to that page address and continues sending all
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remaining missing pages from that new page offset. Soon after that
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(just the time to flush the tcp_wmem queue through the network) the
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migration thread in the QEMU running in the destination node will
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receive the page that triggered the userfault and it'll map it as
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usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it
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was spontaneously sent by the source or if it was an urgent page
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requested through an userfault).
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By the time the userfaults start, the QEMU in the destination node
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doesn't need to keep any per-page state bitmap relative to the live
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migration around and a single per-page bitmap has to be maintained in
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the QEMU running in the source node to know which pages are still
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missing in the destination node. The bitmap in the source node is
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checked to find which missing pages to send in round robin and we seek
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over it when receiving incoming userfaults. After sending each page of
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course the bitmap is updated accordingly. It's also useful to avoid
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sending the same page twice (in case the userfault is read by the
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postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration
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thread).
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