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 36``userfaultfd`` runtime load never takes the mmap_lock 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 54``UFFDIO_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 60``uffdio_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 68- The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events 69 other than page faults are supported. These events are described in more 70 detail below in the `Non-cooperative userfaultfd`_ section. 71 72- ``UFFD_FEATURE_MISSING_HUGETLBFS`` and ``UFFD_FEATURE_MISSING_SHMEM`` 73 indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING`` 74 registrations for hugetlbfs and shared memory (covering all shmem APIs, 75 i.e. tmpfs, ``IPCSHM``, ``/dev/zero``, ``MAP_SHARED``, ``memfd_create``, 76 etc) virtual memory areas, respectively. 77 78- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports 79 ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory 80 areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating 81 support for shmem virtual memory areas. 82 83The userland application should set the feature flags it intends to use 84when invoking the ``UFFDIO_API`` ioctl, to request that those features be 85enabled if supported. 86 87Once the ``userfaultfd`` API has been enabled the ``UFFDIO_REGISTER`` 88ioctl should be invoked (if present in the returned ``uffdio_api.ioctls`` 89bitmask) to register a memory range in the ``userfaultfd`` by setting the 90uffdio_register structure accordingly. The ``uffdio_register.mode`` 91bitmask will specify to the kernel which kind of faults to track for 92the range. The ``UFFDIO_REGISTER`` ioctl will return the 93``uffdio_register.ioctls`` bitmask of ioctls that are suitable to resolve 94userfaults on the range registered. Not all ioctls will necessarily be 95supported for all memory types (e.g. anonymous memory vs. shmem vs. 96hugetlbfs), or all types of intercepted faults. 97 98Userland can use the ``uffdio_register.ioctls`` to manage the virtual 99address space in the background (to add or potentially also remove 100memory from the ``userfaultfd`` registered range). This means a userfault 101could be triggering just before userland maps in the background the 102user-faulted page. 103 104Resolving Userfaults 105-------------------- 106 107There are three basic ways to resolve userfaults: 108 109- ``UFFDIO_COPY`` atomically copies some existing page contents from 110 userspace. 111 112- ``UFFDIO_ZEROPAGE`` atomically zeros the new page. 113 114- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page. 115 116These operations are atomic in the sense that they guarantee nothing can 117see a half-populated page, since readers will keep userfaulting until the 118operation has finished. 119 120By default, these wake up userfaults blocked on the range in question. 121They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates 122that waking will be done separately at some later time. 123 124Which ioctl to choose depends on the kind of page fault, and what we'd 125like to do to resolve it: 126 127- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be 128 resolved by either providing a new page (``UFFDIO_COPY``), or mapping 129 the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map 130 the zero page for a missing fault. With userfaultfd, userspace can 131 decide what content to provide before the faulting thread continues. 132 133- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, there is an existing page (in 134 the page cache). Userspace has the option of modifying the page's 135 contents before resolving the fault. Once the contents are correct 136 (modified or not), userspace asks the kernel to map the page and let the 137 faulting thread continue with ``UFFDIO_CONTINUE``. 138 139Notes: 140 141- You can tell which kind of fault occurred by examining 142 ``pagefault.flags`` within the ``uffd_msg``, checking for the 143 ``UFFD_PAGEFAULT_FLAG_*`` flags. 144 145- None of the page-delivering ioctls default to the range that you 146 registered with. You must fill in all fields for the appropriate 147 ioctl struct including the range. 148 149- You get the address of the access that triggered the missing page 150 event out of a struct uffd_msg that you read in the thread from the 151 uffd. You can supply as many pages as you want with these IOCTLs. 152 Keep in mind that unless you used DONTWAKE then the first of any of 153 those IOCTLs wakes up the faulting thread. 154 155- Be sure to test for all errors including 156 (``pollfd[0].revents & POLLERR``). This can happen, e.g. when ranges 157 supplied were incorrect. 158 159Write Protect Notifications 160--------------------------- 161 162This is equivalent to (but faster than) using mprotect and a SIGSEGV 163signal handler. 164 165Firstly you need to register a range with ``UFFDIO_REGISTER_MODE_WP``. 166Instead of using mprotect(2) you use 167``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)`` 168while ``mode = UFFDIO_WRITEPROTECT_MODE_WP`` 169in the struct passed in. The range does not default to and does not 170have to be identical to the range you registered with. You can write 171protect as many ranges as you like (inside the registered range). 172Then, in the thread reading from uffd the struct will have 173``msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP`` set. Now you send 174``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)`` 175again while ``pagefault.mode`` does not have ``UFFDIO_WRITEPROTECT_MODE_WP`` 176set. This wakes up the thread which will continue to run with writes. This 177allows you to do the bookkeeping about the write in the uffd reading 178thread before the ioctl. 179 180If you registered with both ``UFFDIO_REGISTER_MODE_MISSING`` and 181``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in 182which you supply a page and undo write protect. Note that there is a 183difference between writes into a WP area and into a !WP area. The 184former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter 185``UFFD_PAGEFAULT_FLAG_WRITE``. The latter did not fail on protection but 186you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was 187used. 188 189QEMU/KVM 190======== 191 192QEMU/KVM is using the ``userfaultfd`` syscall to implement postcopy live 193migration. Postcopy live migration is one form of memory 194externalization consisting of a virtual machine running with part or 195all of its memory residing on a different node in the cloud. The 196``userfaultfd`` abstraction is generic enough that not a single line of 197KVM kernel code had to be modified in order to add postcopy live 198migration to QEMU. 199 200Guest async page faults, ``FOLL_NOWAIT`` and all other ``GUP*`` features work 201just fine in combination with userfaults. Userfaults trigger async 202page faults in the guest scheduler so those guest processes that 203aren't waiting for userfaults (i.e. network bound) can keep running in 204the guest vcpus. 205 206It is generally beneficial to run one pass of precopy live migration 207just before starting postcopy live migration, in order to avoid 208generating userfaults for readonly guest regions. 209 210The implementation of postcopy live migration currently uses one 211single bidirectional socket but in the future two different sockets 212will be used (to reduce the latency of the userfaults to the minimum 213possible without having to decrease ``/proc/sys/net/ipv4/tcp_wmem``). 214 215The QEMU in the source node writes all pages that it knows are missing 216in the destination node, into the socket, and the migration thread of 217the QEMU running in the destination node runs ``UFFDIO_COPY|ZEROPAGE`` 218ioctls on the ``userfaultfd`` in order to map the received pages into the 219guest (``UFFDIO_ZEROCOPY`` is used if the source page was a zero page). 220 221A different postcopy thread in the destination node listens with 222poll() to the ``userfaultfd`` in parallel. When a ``POLLIN`` event is 223generated after a userfault triggers, the postcopy thread read() from 224the ``userfaultfd`` and receives the fault address (or ``-EAGAIN`` in case the 225userfault was already resolved and waken by a ``UFFDIO_COPY|ZEROPAGE`` run 226by the parallel QEMU migration thread). 227 228After the QEMU postcopy thread (running in the destination node) gets 229the userfault address it writes the information about the missing page 230into the socket. The QEMU source node receives the information and 231roughly "seeks" to that page address and continues sending all 232remaining missing pages from that new page offset. Soon after that 233(just the time to flush the tcp_wmem queue through the network) the 234migration thread in the QEMU running in the destination node will 235receive the page that triggered the userfault and it'll map it as 236usual with the ``UFFDIO_COPY|ZEROPAGE`` (without actually knowing if it 237was spontaneously sent by the source or if it was an urgent page 238requested through a userfault). 239 240By the time the userfaults start, the QEMU in the destination node 241doesn't need to keep any per-page state bitmap relative to the live 242migration around and a single per-page bitmap has to be maintained in 243the QEMU running in the source node to know which pages are still 244missing in the destination node. The bitmap in the source node is 245checked to find which missing pages to send in round robin and we seek 246over it when receiving incoming userfaults. After sending each page of 247course the bitmap is updated accordingly. It's also useful to avoid 248sending the same page twice (in case the userfault is read by the 249postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration 250thread). 251 252Non-cooperative userfaultfd 253=========================== 254 255When the ``userfaultfd`` is monitored by an external manager, the manager 256must be able to track changes in the process virtual memory 257layout. Userfaultfd can notify the manager about such changes using 258the same read(2) protocol as for the page fault notifications. The 259manager has to explicitly enable these events by setting appropriate 260bits in ``uffdio_api.features`` passed to ``UFFDIO_API`` ioctl: 261 262``UFFD_FEATURE_EVENT_FORK`` 263 enable ``userfaultfd`` hooks for fork(). When this feature is 264 enabled, the ``userfaultfd`` context of the parent process is 265 duplicated into the newly created process. The manager 266 receives ``UFFD_EVENT_FORK`` with file descriptor of the new 267 ``userfaultfd`` context in the ``uffd_msg.fork``. 268 269``UFFD_FEATURE_EVENT_REMAP`` 270 enable notifications about mremap() calls. When the 271 non-cooperative process moves a virtual memory area to a 272 different location, the manager will receive 273 ``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and 274 new addresses of the area and its original length. 275 276``UFFD_FEATURE_EVENT_REMOVE`` 277 enable notifications about madvise(MADV_REMOVE) and 278 madvise(MADV_DONTNEED) calls. The event ``UFFD_EVENT_REMOVE`` will 279 be generated upon these calls to madvise(). The ``uffd_msg.remove`` 280 will contain start and end addresses of the removed area. 281 282``UFFD_FEATURE_EVENT_UNMAP`` 283 enable notifications about memory unmapping. The manager will 284 get ``UFFD_EVENT_UNMAP`` with ``uffd_msg.remove`` containing start and 285 end addresses of the unmapped area. 286 287Although the ``UFFD_FEATURE_EVENT_REMOVE`` and ``UFFD_FEATURE_EVENT_UNMAP`` 288are pretty similar, they quite differ in the action expected from the 289``userfaultfd`` manager. In the former case, the virtual memory is 290removed, but the area is not, the area remains monitored by the 291``userfaultfd``, and if a page fault occurs in that area it will be 292delivered to the manager. The proper resolution for such page fault is 293to zeromap the faulting address. However, in the latter case, when an 294area is unmapped, either explicitly (with munmap() system call), or 295implicitly (e.g. during mremap()), the area is removed and in turn the 296``userfaultfd`` context for such area disappears too and the manager will 297not get further userland page faults from the removed area. Still, the 298notification is required in order to prevent manager from using 299``UFFDIO_COPY`` on the unmapped area. 300 301Unlike userland page faults which have to be synchronous and require 302explicit or implicit wakeup, all the events are delivered 303asynchronously and the non-cooperative process resumes execution as 304soon as manager executes read(). The ``userfaultfd`` manager should 305carefully synchronize calls to ``UFFDIO_COPY`` with the events 306processing. To aid the synchronization, the ``UFFDIO_COPY`` ioctl will 307return ``-ENOSPC`` when the monitored process exits at the time of 308``UFFDIO_COPY``, and ``-ENOENT``, when the non-cooperative process has changed 309its virtual memory layout simultaneously with outstanding ``UFFDIO_COPY`` 310operation. 311 312The current asynchronous model of the event delivery is optimal for 313single threaded non-cooperative ``userfaultfd`` manager implementations. A 314synchronous event delivery model can be added later as a new 315``userfaultfd`` feature to facilitate multithreading enhancements of the 316non cooperative manager, for example to allow ``UFFDIO_COPY`` ioctls to 317run in parallel to the event reception. Single threaded 318implementations should continue to use the current async event 319delivery model instead. 320