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diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst index c30176e67900..e5cc8848dcb3 100644 --- a/Documentation/admin-guide/mm/userfaultfd.rst +++ b/Documentation/admin-guide/mm/userfaultfd.rst @@ -1,5 +1,3 @@ -.. _userfaultfd: - =========== Userfaultfd =========== @@ -12,109 +10,173 @@ 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. +of the ``PROT_NONE+SIGSEGV`` trick. Design ====== -Userfaults are delivered and resolved through the userfaultfd syscall. +Userspace creates a new userfaultfd, initializes it, and registers one or more +regions of virtual memory with it. Then, any page faults which occur within the +region(s) result in a message being delivered to the userfaultfd, notifying +userspace of the fault. -The userfaultfd (aside from registering and unregistering virtual +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 +1) ``read/POLLIN`` protocol to notify a userland thread of the faults happening -2) various UFFDIO_* ioctls that can manage the virtual memory regions - registered in the userfaultfd that allows userland to efficiently +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). - +``userfaultfd`` runtime load never takes the mmap_lock 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 +The ``userfaultfd``, once created, 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 +(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). +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 +Creating a userfaultfd +---------------------- + +There are two ways to create a new userfaultfd, each of which provide ways to +restrict access to this functionality (since historically userfaultfds which +handle kernel page faults have been a useful tool for exploiting the kernel). + +The first way, supported since userfaultfd was introduced, is the +userfaultfd(2) syscall. Access to this is controlled in several ways: + +- Any user can always create a userfaultfd which traps userspace page faults + only. Such a userfaultfd can be created using the userfaultfd(2) syscall + with the flag UFFD_USER_MODE_ONLY. + +- In order to also trap kernel page faults for the address space, either the + process needs the CAP_SYS_PTRACE capability, or the system must have + vm.unprivileged_userfaultfd set to 1. By default, vm.unprivileged_userfaultfd + is set to 0. + +The second way, added to the kernel more recently, is by opening +/dev/userfaultfd and issuing a USERFAULTFD_IOC_NEW ioctl to it. This method +yields equivalent userfaultfds to the userfaultfd(2) syscall. + +Unlike userfaultfd(2), access to /dev/userfaultfd is controlled via normal +filesystem permissions (user/group/mode), which gives fine grained access to +userfaultfd specifically, without also granting other unrelated privileges at +the same time (as e.g. granting CAP_SYS_PTRACE would do). Users who have access +to /dev/userfaultfd can always create userfaultfds that trap kernel page faults; +vm.unprivileged_userfaultfd is not considered. + +Initializing a userfaultfd +-------------------------- + +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 +``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 +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: + +- The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events + other than page faults are supported. These events are described in more + detail below in the `Non-cooperative userfaultfd`_ section. + +- ``UFFD_FEATURE_MISSING_HUGETLBFS`` and ``UFFD_FEATURE_MISSING_SHMEM`` + indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING`` + registrations for hugetlbfs and shared memory (covering all shmem APIs, + i.e. tmpfs, ``IPCSHM``, ``/dev/zero``, ``MAP_SHARED``, ``memfd_create``, + etc) virtual memory areas, respectively. + +- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports + ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory + areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating + support for shmem virtual memory areas. + +- ``UFFD_FEATURE_MOVE`` indicates that the kernel supports moving an + existing page contents from userspace. + +The userland application should set the feature flags it intends to use +when invoking the ``UFFDIO_API`` ioctl, to request that those features be +enabled if supported. + +Once the ``userfaultfd`` API 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 +the range. 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). +supported for all memory types (e.g. anonymous memory vs. shmem vs. +hugetlbfs), or all types of intercepted faults. -Userland can use the uffdio_register.ioctls to manage the virtual +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 +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. +Resolving Userfaults +-------------------- + +There are three basic ways to resolve userfaults: + +- ``UFFDIO_COPY`` atomically copies some existing page contents from + userspace. + +- ``UFFDIO_ZEROPAGE`` atomically zeros the new page. + +- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page. + +These operations are atomic in the sense that they guarantee nothing can +see a half-populated page, since readers will keep userfaulting until the +operation has finished. + +By default, these wake up userfaults blocked on the range in question. +They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates +that waking will be done separately at some later time. + +Which ioctl to choose depends on the kind of page fault, and what we'd +like to do to resolve it: + +- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be + resolved by either providing a new page (``UFFDIO_COPY``), or mapping + the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map + the zero page for a missing fault. With userfaultfd, userspace can + decide what content to provide before the faulting thread continues. + +- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, there is an existing page (in + the page cache). Userspace has the option of modifying the page's + contents before resolving the fault. Once the contents are correct + (modified or not), userspace asks the kernel to map the page and let the + faulting thread continue with ``UFFDIO_CONTINUE``. Notes: -- If you requested UFFDIO_REGISTER_MODE_MISSING when registering then - you must provide some kind of page in your thread after reading from - the uffd. You must provide either UFFDIO_COPY or UFFDIO_ZEROPAGE. - The normal behavior of the OS automatically providing a zero page on - an annonymous mmaping is not in place. +- You can tell which kind of fault occurred by examining + ``pagefault.flags`` within the ``uffd_msg``, checking for the + ``UFFD_PAGEFAULT_FLAG_*`` flags. - None of the page-delivering ioctls default to the range that you registered with. You must fill in all fields for the appropriate @@ -122,13 +184,13 @@ Notes: - You get the address of the access that triggered the missing page event out of a struct uffd_msg that you read in the thread from the - uffd. You can supply as many pages as you want with UFFDIO_COPY or - UFFDIO_ZEROPAGE. Keep in mind that unless you used DONTWAKE then - the first of any of those IOCTLs wakes up the faulting thread. + uffd. You can supply as many pages as you want with these IOCTLs. + Keep in mind that unless you used DONTWAKE then the first of any of + those IOCTLs wakes up the faulting thread. -- Be sure to test for all errors including (pollfd[0].revents & - POLLERR). This can happen, e.g. when ranges supplied were - incorrect. +- Be sure to test for all errors including + (``pollfd[0].revents & POLLERR``). This can happen, e.g. when ranges + supplied were incorrect. Write Protect Notifications --------------------------- @@ -136,41 +198,117 @@ Write Protect Notifications This is equivalent to (but faster than) using mprotect and a SIGSEGV signal handler. -Firstly you need to register a range with UFFDIO_REGISTER_MODE_WP. -Instead of using mprotect(2) you use ioctl(uffd, UFFDIO_WRITEPROTECT, -struct *uffdio_writeprotect) while mode = UFFDIO_WRITEPROTECT_MODE_WP +Firstly you need to register a range with ``UFFDIO_REGISTER_MODE_WP``. +Instead of using mprotect(2) you use +``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)`` +while ``mode = UFFDIO_WRITEPROTECT_MODE_WP`` in the struct passed in. The range does not default to and does not have to be identical to the range you registered with. You can write protect as many ranges as you like (inside the registered range). Then, in the thread reading from uffd the struct will have -msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP set. Now you send -ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect) again -while pagefault.mode does not have UFFDIO_WRITEPROTECT_MODE_WP set. -This wakes up the thread which will continue to run with writes. This +``msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP`` set. Now you send +``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)`` +again while ``pagefault.mode`` does not have ``UFFDIO_WRITEPROTECT_MODE_WP`` +set. This wakes up the thread which will continue to run with writes. This allows you to do the bookkeeping about the write in the uffd reading thread before the ioctl. -If you registered with both UFFDIO_REGISTER_MODE_MISSING and -UFFDIO_REGISTER_MODE_WP then you need to think about the sequence in +If you registered with both ``UFFDIO_REGISTER_MODE_MISSING`` and +``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in which you supply a page and undo write protect. Note that there is a difference between writes into a WP area and into a !WP area. The -former will have UFFD_PAGEFAULT_FLAG_WP set, the latter -UFFD_PAGEFAULT_FLAG_WRITE. The latter did not fail on protection but -you still need to supply a page when UFFDIO_REGISTER_MODE_MISSING was +former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter +``UFFD_PAGEFAULT_FLAG_WRITE``. The latter did not fail on protection but +you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was used. +Userfaultfd write-protect mode currently behave differently on none ptes +(when e.g. page is missing) over different types of memories. + +For anonymous memory, ``ioctl(UFFDIO_WRITEPROTECT)`` will ignore none ptes +(e.g. when pages are missing and not populated). For file-backed memories +like shmem and hugetlbfs, none ptes will be write protected just like a +present pte. In other words, there will be a userfaultfd write fault +message generated when writing to a missing page on file typed memories, +as long as the page range was write-protected before. Such a message will +not be generated on anonymous memories by default. + +If the application wants to be able to write protect none ptes on anonymous +memory, one can pre-populate the memory with e.g. MADV_POPULATE_READ. On +newer kernels, one can also detect the feature UFFD_FEATURE_WP_UNPOPULATED +and set the feature bit in advance to make sure none ptes will also be +write protected even upon anonymous memory. + +When using ``UFFDIO_REGISTER_MODE_WP`` in combination with either +``UFFDIO_REGISTER_MODE_MISSING`` or ``UFFDIO_REGISTER_MODE_MINOR``, when +resolving missing / minor faults with ``UFFDIO_COPY`` or ``UFFDIO_CONTINUE`` +respectively, it may be desirable for the new page / mapping to be +write-protected (so future writes will also result in a WP fault). These ioctls +support a mode flag (``UFFDIO_COPY_MODE_WP`` or ``UFFDIO_CONTINUE_MODE_WP`` +respectively) to configure the mapping this way. + +If the userfaultfd context has ``UFFD_FEATURE_WP_ASYNC`` feature bit set, +any vma registered with write-protection will work in async mode rather +than the default sync mode. + +In async mode, there will be no message generated when a write operation +happens, meanwhile the write-protection will be resolved automatically by +the kernel. It can be seen as a more accurate version of soft-dirty +tracking and it can be different in a few ways: + + - The dirty result will not be affected by vma changes (e.g. vma + merging) because the dirty is only tracked by the pte. + + - It supports range operations by default, so one can enable tracking on + any range of memory as long as page aligned. + + - Dirty information will not get lost if the pte was zapped due to + various reasons (e.g. during split of a shmem transparent huge page). + + - Due to a reverted meaning of soft-dirty (page clean when uffd-wp bit + set; dirty when uffd-wp bit cleared), it has different semantics on + some of the memory operations. For example: ``MADV_DONTNEED`` on + anonymous (or ``MADV_REMOVE`` on a file mapping) will be treated as + dirtying of memory by dropping uffd-wp bit during the procedure. + +The user app can collect the "written/dirty" status by looking up the +uffd-wp bit for the pages being interested in /proc/pagemap. + +The page will not be under track of uffd-wp async mode until the page is +explicitly write-protected by ``ioctl(UFFDIO_WRITEPROTECT)`` with the mode +flag ``UFFDIO_WRITEPROTECT_MODE_WP`` set. Trying to resolve a page fault +that was tracked by async mode userfaultfd-wp is invalid. + +When userfaultfd-wp async mode is used alone, it can be applied to all +kinds of memory. + +Memory Poisioning Emulation +--------------------------- + +In response to a fault (either missing or minor), an action userspace can +take to "resolve" it is to issue a ``UFFDIO_POISON``. This will cause any +future faulters to either get a SIGBUS, or in KVM's case the guest will +receive an MCE as if there were hardware memory poisoning. + +This is used to emulate hardware memory poisoning. Imagine a VM running on a +machine which experiences a real hardware memory error. Later, we live migrate +the VM to another physical machine. Since we want the migration to be +transparent to the guest, we want that same address range to act as if it was +still poisoned, even though it's on a new physical host which ostensibly +doesn't have a memory error in the exact same spot. + QEMU/KVM ======== -QEMU/KVM is using the userfaultfd syscall to implement postcopy live +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 +``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 +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 @@ -183,19 +321,19 @@ 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). +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). +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 +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 +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 @@ -206,7 +344,7 @@ 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 +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). @@ -219,74 +357,74 @@ 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 +postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration thread). Non-cooperative userfaultfd =========================== -When the userfaultfd is monitored by an external manager, the manager +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: +bits in ``uffdio_api.features`` passed to ``UFFDIO_API`` ioctl: -UFFD_FEATURE_EVENT_FORK - enable userfaultfd hooks for fork(). When this feature is - enabled, the userfaultfd context of the parent process is +``UFFD_FEATURE_EVENT_FORK`` + 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. + receives ``UFFD_EVENT_FORK`` with file descriptor of the new + ``userfaultfd`` context in the ``uffd_msg.fork``. -UFFD_FEATURE_EVENT_REMAP +``UFFD_FEATURE_EVENT_REMAP`` 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 + ``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and new addresses of the area and its original length. -UFFD_FEATURE_EVENT_REMOVE +``UFFD_FEATURE_EVENT_REMOVE`` 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 + 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. -UFFD_FEATURE_EVENT_UNMAP +``UFFD_FEATURE_EVENT_UNMAP`` enable notifications about memory unmapping. The manager will - get UFFD_EVENT_UNMAP with uffd_msg.remove containing start and + 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 +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 +``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 +``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 +``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. +``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 +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 +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 +``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. |