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.
Userfaults are delivered and resolved through the userfaultfd syscall.
The userfaultfd (aside from registering and unregistering virtual memory ranges) provides two primary functionalities:
read/POLLIN protocol to notify a userland thread of the faults happening
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).
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.
If the kernel supports registering userfaultfd ranges on hugetlbfs virtual memory areas, UFFD_FEATURE_MISSING_HUGETLBFS will be set in uffdio_api.features. Similarly, UFFD_FEATURE_MISSING_SHMEM will be set if the kernel supports registering userfaultfd ranges on shared memory (covering all shmem APIs, i.e. tmpfs, IPCSHM, /dev/zero MAP_SHARED, memfd_create, etc).
The userland application that wants to use userfaultfd with hugetlbfs or shared memory need to set the corresponding flag in uffdio_api.features to enable those features.
If the userland desires to receive notifications for events other than page faults, it has to verify that uffdio_api.features has appropriate UFFD_FEATURE_EVENT_* bits set. These events are described in more detail below in "Non-cooperative userfaultfd" section.
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 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).
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:
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.
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.
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.
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.