1.. _userfaultfd: 2 3=========== 4Userfaultfd 5=========== 6 7Objective 8========= 9 10Userfaults allow the implementation of on-demand paging from userland 11and more generally they allow userland to take control of various 12memory page faults, something otherwise only the kernel code could do. 13 14For example userfaults allows a proper and more optimal implementation 15of the PROT_NONE+SIGSEGV trick. 16 17Design 18====== 19 20Userfaults are delivered and resolved through the userfaultfd syscall. 21 22The userfaultfd (aside from registering and unregistering virtual 23memory ranges) provides two primary functionalities: 24 251) read/POLLIN protocol to notify a userland thread of the faults 26 happening 27 282) various UFFDIO_* ioctls that can manage the virtual memory regions 29 registered in the userfaultfd that allows userland to efficiently 30 resolve the userfaults it receives via 1) or to manage the virtual 31 memory in the background 32 33The real advantage of userfaults if compared to regular virtual memory 34management of mremap/mprotect is that the userfaults in all their 35operations never involve heavyweight structures like vmas (in fact the 36userfaultfd runtime load never takes the mmap_sem for writing). 37 38Vmas are not suitable for page- (or hugepage) granular fault tracking 39when dealing with virtual address spaces that could span 40Terabytes. Too many vmas would be needed for that. 41 42The userfaultfd once opened by invoking the syscall, can also be 43passed using unix domain sockets to a manager process, so the same 44manager process could handle the userfaults of a multitude of 45different processes without them being aware about what is going on 46(well of course unless they later try to use the userfaultfd 47themselves on the same region the manager is already tracking, which 48is a corner case that would currently return -EBUSY). 49 50API 51=== 52 53When first opened the userfaultfd must be enabled invoking the 54UFFDIO_API ioctl specifying a uffdio_api.api value set to UFFD_API (or 55a later API version) which will specify the read/POLLIN protocol 56userland intends to speak on the UFFD and the uffdio_api.features 57userland requires. The UFFDIO_API ioctl if successful (i.e. if the 58requested uffdio_api.api is spoken also by the running kernel and the 59requested features are going to be enabled) will return into 60uffdio_api.features and uffdio_api.ioctls two 64bit bitmasks of 61respectively all the available features of the read(2) protocol and 62the generic ioctl available. 63 64The uffdio_api.features bitmask returned by the UFFDIO_API ioctl 65defines what memory types are supported by the userfaultfd and what 66events, except page fault notifications, may be generated. 67 68If the kernel supports registering userfaultfd ranges on hugetlbfs 69virtual memory areas, UFFD_FEATURE_MISSING_HUGETLBFS will be set in 70uffdio_api.features. Similarly, UFFD_FEATURE_MISSING_SHMEM will be 71set if the kernel supports registering userfaultfd ranges on shared 72memory (covering all shmem APIs, i.e. tmpfs, IPCSHM, /dev/zero 73MAP_SHARED, memfd_create, etc). 74 75The userland application that wants to use userfaultfd with hugetlbfs 76or shared memory need to set the corresponding flag in 77uffdio_api.features to enable those features. 78 79If the userland desires to receive notifications for events other than 80page faults, it has to verify that uffdio_api.features has appropriate 81UFFD_FEATURE_EVENT_* bits set. These events are described in more 82detail below in "Non-cooperative userfaultfd" section. 83 84Once the userfaultfd has been enabled the UFFDIO_REGISTER ioctl should 85be invoked (if present in the returned uffdio_api.ioctls bitmask) to 86register a memory range in the userfaultfd by setting the 87uffdio_register structure accordingly. The uffdio_register.mode 88bitmask will specify to the kernel which kind of faults to track for 89the range (UFFDIO_REGISTER_MODE_MISSING would track missing 90pages). The UFFDIO_REGISTER ioctl will return the 91uffdio_register.ioctls bitmask of ioctls that are suitable to resolve 92userfaults on the range registered. Not all ioctls will necessarily be 93supported for all memory types depending on the underlying virtual 94memory backend (anonymous memory vs tmpfs vs real filebacked 95mappings). 96 97Userland can use the uffdio_register.ioctls to manage the virtual 98address space in the background (to add or potentially also remove 99memory from the userfaultfd registered range). This means a userfault 100could be triggering just before userland maps in the background the 101user-faulted page. 102 103The primary ioctl to resolve userfaults is UFFDIO_COPY. That 104atomically copies a page into the userfault registered range and wakes 105up the blocked userfaults (unless uffdio_copy.mode & 106UFFDIO_COPY_MODE_DONTWAKE is set). Other ioctl works similarly to 107UFFDIO_COPY. They're atomic as in guaranteeing that nothing can see an 108half copied page since it'll keep userfaulting until the copy has 109finished. 110 111QEMU/KVM 112======== 113 114QEMU/KVM is using the userfaultfd syscall to implement postcopy live 115migration. Postcopy live migration is one form of memory 116externalization consisting of a virtual machine running with part or 117all of its memory residing on a different node in the cloud. The 118userfaultfd abstraction is generic enough that not a single line of 119KVM kernel code had to be modified in order to add postcopy live 120migration to QEMU. 121 122Guest async page faults, FOLL_NOWAIT and all other GUP features work 123just fine in combination with userfaults. Userfaults trigger async 124page faults in the guest scheduler so those guest processes that 125aren't waiting for userfaults (i.e. network bound) can keep running in 126the guest vcpus. 127 128It is generally beneficial to run one pass of precopy live migration 129just before starting postcopy live migration, in order to avoid 130generating userfaults for readonly guest regions. 131 132The implementation of postcopy live migration currently uses one 133single bidirectional socket but in the future two different sockets 134will be used (to reduce the latency of the userfaults to the minimum 135possible without having to decrease /proc/sys/net/ipv4/tcp_wmem). 136 137The QEMU in the source node writes all pages that it knows are missing 138in the destination node, into the socket, and the migration thread of 139the QEMU running in the destination node runs UFFDIO_COPY|ZEROPAGE 140ioctls on the userfaultfd in order to map the received pages into the 141guest (UFFDIO_ZEROCOPY is used if the source page was a zero page). 142 143A different postcopy thread in the destination node listens with 144poll() to the userfaultfd in parallel. When a POLLIN event is 145generated after a userfault triggers, the postcopy thread read() from 146the userfaultfd and receives the fault address (or -EAGAIN in case the 147userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run 148by the parallel QEMU migration thread). 149 150After the QEMU postcopy thread (running in the destination node) gets 151the userfault address it writes the information about the missing page 152into the socket. The QEMU source node receives the information and 153roughly "seeks" to that page address and continues sending all 154remaining missing pages from that new page offset. Soon after that 155(just the time to flush the tcp_wmem queue through the network) the 156migration thread in the QEMU running in the destination node will 157receive the page that triggered the userfault and it'll map it as 158usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it 159was spontaneously sent by the source or if it was an urgent page 160requested through a userfault). 161 162By the time the userfaults start, the QEMU in the destination node 163doesn't need to keep any per-page state bitmap relative to the live 164migration around and a single per-page bitmap has to be maintained in 165the QEMU running in the source node to know which pages are still 166missing in the destination node. The bitmap in the source node is 167checked to find which missing pages to send in round robin and we seek 168over it when receiving incoming userfaults. After sending each page of 169course the bitmap is updated accordingly. It's also useful to avoid 170sending the same page twice (in case the userfault is read by the 171postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration 172thread). 173 174Non-cooperative userfaultfd 175=========================== 176 177When the userfaultfd is monitored by an external manager, the manager 178must be able to track changes in the process virtual memory 179layout. Userfaultfd can notify the manager about such changes using 180the same read(2) protocol as for the page fault notifications. The 181manager has to explicitly enable these events by setting appropriate 182bits in uffdio_api.features passed to UFFDIO_API ioctl: 183 184UFFD_FEATURE_EVENT_FORK 185 enable userfaultfd hooks for fork(). When this feature is 186 enabled, the userfaultfd context of the parent process is 187 duplicated into the newly created process. The manager 188 receives UFFD_EVENT_FORK with file descriptor of the new 189 userfaultfd context in the uffd_msg.fork. 190 191UFFD_FEATURE_EVENT_REMAP 192 enable notifications about mremap() calls. When the 193 non-cooperative process moves a virtual memory area to a 194 different location, the manager will receive 195 UFFD_EVENT_REMAP. The uffd_msg.remap will contain the old and 196 new addresses of the area and its original length. 197 198UFFD_FEATURE_EVENT_REMOVE 199 enable notifications about madvise(MADV_REMOVE) and 200 madvise(MADV_DONTNEED) calls. The event UFFD_EVENT_REMOVE will 201 be generated upon these calls to madvise. The uffd_msg.remove 202 will contain start and end addresses of the removed area. 203 204UFFD_FEATURE_EVENT_UNMAP 205 enable notifications about memory unmapping. The manager will 206 get UFFD_EVENT_UNMAP with uffd_msg.remove containing start and 207 end addresses of the unmapped area. 208 209Although the UFFD_FEATURE_EVENT_REMOVE and UFFD_FEATURE_EVENT_UNMAP 210are pretty similar, they quite differ in the action expected from the 211userfaultfd manager. In the former case, the virtual memory is 212removed, but the area is not, the area remains monitored by the 213userfaultfd, and if a page fault occurs in that area it will be 214delivered to the manager. The proper resolution for such page fault is 215to zeromap the faulting address. However, in the latter case, when an 216area is unmapped, either explicitly (with munmap() system call), or 217implicitly (e.g. during mremap()), the area is removed and in turn the 218userfaultfd context for such area disappears too and the manager will 219not get further userland page faults from the removed area. Still, the 220notification is required in order to prevent manager from using 221UFFDIO_COPY on the unmapped area. 222 223Unlike userland page faults which have to be synchronous and require 224explicit or implicit wakeup, all the events are delivered 225asynchronously and the non-cooperative process resumes execution as 226soon as manager executes read(). The userfaultfd manager should 227carefully synchronize calls to UFFDIO_COPY with the events 228processing. To aid the synchronization, the UFFDIO_COPY ioctl will 229return -ENOSPC when the monitored process exits at the time of 230UFFDIO_COPY, and -ENOENT, when the non-cooperative process has changed 231its virtual memory layout simultaneously with outstanding UFFDIO_COPY 232operation. 233 234The current asynchronous model of the event delivery is optimal for 235single threaded non-cooperative userfaultfd manager implementations. A 236synchronous event delivery model can be added later as a new 237userfaultfd feature to facilitate multithreading enhancements of the 238non cooperative manager, for example to allow UFFDIO_COPY ioctls to 239run in parallel to the event reception. Single threaded 240implementations should continue to use the current async event 241delivery model instead. 242