add boot partition size

hc

2023-12-11 d2ccde1c8e90d38cee87a1b0309ad2827f3fd30d

 kernel/Documentation/admin-guide/mm/userfaultfd.rst
..
..
@@ -12,114 +12,192 @@
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12
 memory page faults, something otherwise only the kernel code could do.
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13
 
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14
 For example userfaults allows a proper and more optimal implementation
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-of the PROT_NONE+SIGSEGV trick.
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+of the ``PROT_NONE+SIGSEGV`` trick.
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16
 
17
17
 Design
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18
 ======
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19
 
20
-Userfaults are delivered and resolved through the userfaultfd syscall.
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+Userfaults are delivered and resolved through the ``userfaultfd`` syscall.
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21
 
22
-The userfaultfd (aside from registering and unregistering virtual
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+The ``userfaultfd`` (aside from registering and unregistering virtual
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23
 memory ranges) provides two primary functionalities:
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24
 
25
-1) read/POLLIN protocol to notify a userland thread of the faults
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+1) ``read/POLLIN`` protocol to notify a userland thread of the faults
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26
    happening
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27
 
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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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+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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31
    memory in the background
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32
 
33
33
 The real advantage of userfaults if compared to regular virtual memory
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34
 management of mremap/mprotect is that the userfaults in all their
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35
 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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+``userfaultfd`` runtime load never takes the mmap_lock for writing).
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37
 
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38
 Vmas are not suitable for page- (or hugepage) granular fault tracking
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39
 when dealing with virtual address spaces that could span
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40
 Terabytes. Too many vmas would be needed for that.
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41
 
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-The userfaultfd once opened by invoking the syscall, can also be
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+The ``userfaultfd`` once opened by invoking the syscall, can also be
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43
 passed using unix domain sockets to a manager process, so the same
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44
 manager process could handle the userfaults of a multitude of
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45
 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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+(well of course unless they later try to use the ``userfaultfd``
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47
 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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+is a corner case that would currently return ``-EBUSY``).
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49
 
50
50
 API
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51
 ===
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52
 
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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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+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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59
 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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+``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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63
 
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-The uffdio_api.features bitmask returned by the UFFDIO_API ioctl
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-defines what memory types are supported by the userfaultfd and what
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-events, except page fault notifications, may be generated.
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+The ``uffdio_api.features`` bitmask returned by the ``UFFDIO_API`` ioctl
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+defines what memory types are supported by the ``userfaultfd`` and what
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+events, except page fault notifications, may be generated:
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67
 
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-If the kernel supports registering userfaultfd ranges on hugetlbfs
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-virtual memory areas, UFFD_FEATURE_MISSING_HUGETLBFS will be set in
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-uffdio_api.features. Similarly, UFFD_FEATURE_MISSING_SHMEM will be
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-set if the kernel supports registering userfaultfd ranges on shared
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-memory (covering all shmem APIs, i.e. tmpfs, IPCSHM, /dev/zero
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-MAP_SHARED, memfd_create, etc).
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+- The ``UFFD_FEATURE_EVENT_*`` flags indicate that various other events
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+  other than page faults are supported. These events are described in more
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+  detail below in the `Non-cooperative userfaultfd`_ section.
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71
 
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-The userland application that wants to use userfaultfd with hugetlbfs
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-or shared memory need to set the corresponding flag in
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-uffdio_api.features to enable those features.
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+- ``UFFD_FEATURE_MISSING_HUGETLBFS`` and ``UFFD_FEATURE_MISSING_SHMEM``
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+  indicate that the kernel supports ``UFFDIO_REGISTER_MODE_MISSING``
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+  registrations for hugetlbfs and shared memory (covering all shmem APIs,
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+  i.e. tmpfs, ``IPCSHM``, ``/dev/zero``, ``MAP_SHARED``, ``memfd_create``,
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+  etc) virtual memory areas, respectively.
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77
 
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-If the userland desires to receive notifications for events other than
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-page faults, it has to verify that uffdio_api.features has appropriate
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-UFFD_FEATURE_EVENT_* bits set. These events are described in more
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-detail below in "Non-cooperative userfaultfd" section.
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+- ``UFFD_FEATURE_MINOR_HUGETLBFS`` indicates that the kernel supports
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+  ``UFFDIO_REGISTER_MODE_MINOR`` registration for hugetlbfs virtual memory
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+  areas. ``UFFD_FEATURE_MINOR_SHMEM`` is the analogous feature indicating
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+  support for shmem virtual memory areas.
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82
 
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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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+The userland application should set the feature flags it intends to use
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+when invoking the ``UFFDIO_API`` ioctl, to request that those features be
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+enabled if supported.
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+
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+Once the ``userfaultfd`` API has been enabled the ``UFFDIO_REGISTER``
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+ioctl should be invoked (if present in the returned ``uffdio_api.ioctls``
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+bitmask) to 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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+the range. 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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+supported for all memory types (e.g. anonymous memory vs. shmem vs.
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+hugetlbfs), or all types of intercepted faults.
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97
 
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-Userland can use the uffdio_register.ioctls to manage the virtual
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+Userland can use the ``uffdio_register.ioctls`` to manage the virtual
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99
 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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+memory from the ``userfaultfd`` registered range). This means a userfault
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101
 could be triggering just before userland maps in the background the
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102
 user-faulted page.
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103
 
103
-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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+Resolving Userfaults
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+--------------------
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+
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+There are three basic ways to resolve userfaults:
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+
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+- ``UFFDIO_COPY`` atomically copies some existing page contents from
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+  userspace.
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+
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+- ``UFFDIO_ZEROPAGE`` atomically zeros the new page.
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+
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+- ``UFFDIO_CONTINUE`` maps an existing, previously-populated page.
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+
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+These operations are atomic in the sense that they guarantee nothing can
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+see a half-populated page, since readers will keep userfaulting until the
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+operation has finished.
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+
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+By default, these wake up userfaults blocked on the range in question.
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+They support a ``UFFDIO_*_MODE_DONTWAKE`` ``mode`` flag, which indicates
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+that waking will be done separately at some later time.
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+
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+Which ioctl to choose depends on the kind of page fault, and what we'd
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+like to do to resolve it:
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+
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+- For ``UFFDIO_REGISTER_MODE_MISSING`` faults, the fault needs to be
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+  resolved by either providing a new page (``UFFDIO_COPY``), or mapping
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+  the zero page (``UFFDIO_ZEROPAGE``). By default, the kernel would map
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+  the zero page for a missing fault. With userfaultfd, userspace can
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+  decide what content to provide before the faulting thread continues.
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+
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+- For ``UFFDIO_REGISTER_MODE_MINOR`` faults, there is an existing page (in
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+  the page cache). Userspace has the option of modifying the page's
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+  contents before resolving the fault. Once the contents are correct
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+  (modified or not), userspace asks the kernel to map the page and let the
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+  faulting thread continue with ``UFFDIO_CONTINUE``.
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+
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+Notes:
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+
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+- You can tell which kind of fault occurred by examining
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+  ``pagefault.flags`` within the ``uffd_msg``, checking for the
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+  ``UFFD_PAGEFAULT_FLAG_*`` flags.
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+
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+- None of the page-delivering ioctls default to the range that you
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+  registered with.  You must fill in all fields for the appropriate
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+  ioctl struct including the range.
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+
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+- You get the address of the access that triggered the missing page
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+  event out of a struct uffd_msg that you read in the thread from the
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+  uffd.  You can supply as many pages as you want with these IOCTLs.
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+  Keep in mind that unless you used DONTWAKE then the first of any of
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+  those IOCTLs wakes up the faulting thread.
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+
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+- Be sure to test for all errors including
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+  (``pollfd[0].revents & POLLERR``).  This can happen, e.g. when ranges
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+  supplied were incorrect.
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+
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+Write Protect Notifications
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+---------------------------
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+
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+This is equivalent to (but faster than) using mprotect and a SIGSEGV
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+signal handler.
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+
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+Firstly you need to register a range with ``UFFDIO_REGISTER_MODE_WP``.
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+Instead of using mprotect(2) you use
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+``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)``
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+while ``mode = UFFDIO_WRITEPROTECT_MODE_WP``
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+in the struct passed in.  The range does not default to and does not
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+have to be identical to the range you registered with.  You can write
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+protect as many ranges as you like (inside the registered range).
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+Then, in the thread reading from uffd the struct will have
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+``msg.arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WP`` set. Now you send
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+``ioctl(uffd, UFFDIO_WRITEPROTECT, struct *uffdio_writeprotect)``
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+again while ``pagefault.mode`` does not have ``UFFDIO_WRITEPROTECT_MODE_WP``
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+set. This wakes up the thread which will continue to run with writes. This
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+allows you to do the bookkeeping about the write in the uffd reading
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+thread before the ioctl.
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+
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+If you registered with both ``UFFDIO_REGISTER_MODE_MISSING`` and
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+``UFFDIO_REGISTER_MODE_WP`` then you need to think about the sequence in
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+which you supply a page and undo write protect.  Note that there is a
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+difference between writes into a WP area and into a !WP area.  The
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+former will have ``UFFD_PAGEFAULT_FLAG_WP`` set, the latter
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+``UFFD_PAGEFAULT_FLAG_WRITE``.  The latter did not fail on protection but
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+you still need to supply a page when ``UFFDIO_REGISTER_MODE_MISSING`` was
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+used.
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188
 
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189
 QEMU/KVM
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190
 ========
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191
 
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-QEMU/KVM is using the userfaultfd syscall to implement postcopy live
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+QEMU/KVM is using the ``userfaultfd`` syscall to implement postcopy live
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193
 migration. Postcopy live migration is one form of memory
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194
 externalization consisting of a virtual machine running with part or
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195
 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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+``userfaultfd`` abstraction is generic enough that not a single line of
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197
 KVM kernel code had to be modified in order to add postcopy live
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198
 migration to QEMU.
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199
 
122
-Guest async page faults, FOLL_NOWAIT and all other GUP features work
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+Guest async page faults, ``FOLL_NOWAIT`` and all other ``GUP*`` features work
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201
 just fine in combination with userfaults. Userfaults trigger async
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202
 page faults in the guest scheduler so those guest processes that
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203
 aren't waiting for userfaults (i.e. network bound) can keep running in
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..
@@ -132,19 +210,19 @@
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210
 The implementation of postcopy live migration currently uses one
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211
 single bidirectional socket but in the future two different sockets
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212
 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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+possible without having to decrease ``/proc/sys/net/ipv4/tcp_wmem``).
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214
 
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215
 The QEMU in the source node writes all pages that it knows are missing
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216
 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
141
-guest (UFFDIO_ZEROCOPY is used if the source page was a zero page).
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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
219
+guest (``UFFDIO_ZEROCOPY`` is used if the source page was a zero page).
142
220
 
143
221
 A different postcopy thread in the destination node listens with
144
-poll() to the userfaultfd in parallel. When a POLLIN event is
222
+poll() to the ``userfaultfd`` in parallel. When a ``POLLIN`` event is
145
223
 generated after a userfault triggers, the postcopy thread read() from
146
-the userfaultfd and receives the fault address (or -EAGAIN in case the
147
-userfault was already resolved and waken by a UFFDIO_COPY|ZEROPAGE run
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+the ``userfaultfd`` and receives the fault address (or ``-EAGAIN`` in case the
225
+userfault was already resolved and waken by a ``UFFDIO_COPY|ZEROPAGE`` run
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226
 by the parallel QEMU migration thread).
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227
 
150
228
 After the QEMU postcopy thread (running in the destination node) gets
..
..
@@ -155,7 +233,7 @@
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233
 (just the time to flush the tcp_wmem queue through the network) the
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234
 migration thread in the QEMU running in the destination node will
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235
 receive the page that triggered the userfault and it'll map it as
158
-usual with the UFFDIO_COPY|ZEROPAGE (without actually knowing if it
236
+usual with the ``UFFDIO_COPY|ZEROPAGE`` (without actually knowing if it
159
237
 was spontaneously sent by the source or if it was an urgent page
160
238
 requested through a userfault).
161
239
 
..
..
@@ -168,74 +246,74 @@
168
246
 over it when receiving incoming userfaults. After sending each page of
169
247
 course the bitmap is updated accordingly. It's also useful to avoid
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248
 sending the same page twice (in case the userfault is read by the
171
-postcopy thread just before UFFDIO_COPY|ZEROPAGE runs in the migration
249
+postcopy thread just before ``UFFDIO_COPY|ZEROPAGE`` runs in the migration
172
250
 thread).
173
251
 
174
252
 Non-cooperative userfaultfd
175
253
 ===========================
176
254
 
177
-When the userfaultfd is monitored by an external manager, the manager
255
+When the ``userfaultfd`` is monitored by an external manager, the manager
178
256
 must be able to track changes in the process virtual memory
179
257
 layout. Userfaultfd can notify the manager about such changes using
180
258
 the same read(2) protocol as for the page fault notifications. The
181
259
 manager has to explicitly enable these events by setting appropriate
182
-bits in uffdio_api.features passed to UFFDIO_API ioctl:
260
+bits in ``uffdio_api.features`` passed to ``UFFDIO_API`` ioctl:
183
261
 
184
-UFFD_FEATURE_EVENT_FORK
185
-	enable userfaultfd hooks for fork(). When this feature is
186
-	enabled, the userfaultfd context of the parent process is
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
187
265
 	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.
266
+	receives ``UFFD_EVENT_FORK`` with file descriptor of the new
267
+	``userfaultfd`` context in the ``uffd_msg.fork``.
190
268
 
191
-UFFD_FEATURE_EVENT_REMAP
269
+``UFFD_FEATURE_EVENT_REMAP``
192
270
 	enable notifications about mremap() calls. When the
193
271
 	non-cooperative process moves a virtual memory area to a
194
272
 	different location, the manager will receive
195
-	UFFD_EVENT_REMAP. The uffd_msg.remap will contain the old and
273
+	``UFFD_EVENT_REMAP``. The ``uffd_msg.remap`` will contain the old and
196
274
 	new addresses of the area and its original length.
197
275
 
198
-UFFD_FEATURE_EVENT_REMOVE
276
+``UFFD_FEATURE_EVENT_REMOVE``
199
277
 	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
278
+	madvise(MADV_DONTNEED) calls. The event ``UFFD_EVENT_REMOVE`` will
279
+	be generated upon these calls to madvise(). The ``uffd_msg.remove``
202
280
 	will contain start and end addresses of the removed area.
203
281
 
204
-UFFD_FEATURE_EVENT_UNMAP
282
+``UFFD_FEATURE_EVENT_UNMAP``
205
283
 	enable notifications about memory unmapping. The manager will
206
-	get UFFD_EVENT_UNMAP with uffd_msg.remove containing start and
284
+	get ``UFFD_EVENT_UNMAP`` with ``uffd_msg.remove`` containing start and
207
285
 	end addresses of the unmapped area.
208
286
 
209
-Although the UFFD_FEATURE_EVENT_REMOVE and UFFD_FEATURE_EVENT_UNMAP
287
+Although the ``UFFD_FEATURE_EVENT_REMOVE`` and ``UFFD_FEATURE_EVENT_UNMAP``
210
288
 are pretty similar, they quite differ in the action expected from the
211
-userfaultfd manager. In the former case, the virtual memory is
289
+``userfaultfd`` manager. In the former case, the virtual memory is
212
290
 removed, but the area is not, the area remains monitored by the
213
-userfaultfd, and if a page fault occurs in that area it will be
291
+``userfaultfd``, and if a page fault occurs in that area it will be
214
292
 delivered to the manager. The proper resolution for such page fault is
215
293
 to zeromap the faulting address. However, in the latter case, when an
216
294
 area is unmapped, either explicitly (with munmap() system call), or
217
295
 implicitly (e.g. during mremap()), the area is removed and in turn the
218
-userfaultfd context for such area disappears too and the manager will
296
+``userfaultfd`` context for such area disappears too and the manager will
219
297
 not get further userland page faults from the removed area. Still, the
220
298
 notification is required in order to prevent manager from using
221
-UFFDIO_COPY on the unmapped area.
299
+``UFFDIO_COPY`` on the unmapped area.
222
300
 
223
301
 Unlike userland page faults which have to be synchronous and require
224
302
 explicit or implicit wakeup, all the events are delivered
225
303
 asynchronously and the non-cooperative process resumes execution as
226
-soon as manager executes read(). The userfaultfd manager should
227
-carefully synchronize calls to UFFDIO_COPY with the events
228
-processing. To aid the synchronization, the UFFDIO_COPY ioctl will
229
-return -ENOSPC when the monitored process exits at the time of
230
-UFFDIO_COPY, and -ENOENT, when the non-cooperative process has changed
231
-its virtual memory layout simultaneously with outstanding UFFDIO_COPY
304
+soon as manager executes read(). The ``userfaultfd`` manager should
305
+carefully synchronize calls to ``UFFDIO_COPY`` with the events
306
+processing. To aid the synchronization, the ``UFFDIO_COPY`` ioctl will
307
+return ``-ENOSPC`` when the monitored process exits at the time of
308
+``UFFDIO_COPY``, and ``-ENOENT``, when the non-cooperative process has changed
309
+its virtual memory layout simultaneously with outstanding ``UFFDIO_COPY``
232
310
 operation.
233
311
 
234
312
 The current asynchronous model of the event delivery is optimal for
235
-single threaded non-cooperative userfaultfd manager implementations. A
313
+single threaded non-cooperative ``userfaultfd`` manager implementations. A
236
314
 synchronous event delivery model can be added later as a new
237
-userfaultfd feature to facilitate multithreading enhancements of the
238
-non cooperative manager, for example to allow UFFDIO_COPY ioctls to
315
+``userfaultfd`` feature to facilitate multithreading enhancements of the
316
+non cooperative manager, for example to allow ``UFFDIO_COPY`` ioctls to
239
317
 run in parallel to the event reception. Single threaded
240
318
 implementations should continue to use the current async event
241
319
 delivery model instead.