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Subject[PATCH v4 09/10] userfaultfd: update documentation to describe minor fault handling
Reword / reorganize things a little bit into "lists", so new features /
modes / ioctls can sort of just be appended.

to intercept and resolve minor faults. Make it clear that COPY and
ZEROPAGE are used for MISSING faults, whereas CONTINUE is used for MINOR

Signed-off-by: Axel Rasmussen <>
Documentation/admin-guide/mm/userfaultfd.rst | 107 ++++++++++++-------
1 file changed, 66 insertions(+), 41 deletions(-)

diff --git a/Documentation/admin-guide/mm/userfaultfd.rst b/Documentation/admin-guide/mm/userfaultfd.rst
index 65eefa66c0ba..cfd3daf59d0e 100644
--- a/Documentation/admin-guide/mm/userfaultfd.rst
+++ b/Documentation/admin-guide/mm/userfaultfd.rst
@@ -63,36 +63,36 @@ 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
+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.
+ 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.
+The userland application should set the feature flags it intends to use
+when envoking 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
+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
+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
address space in the background (to add or potentially also remove
@@ -100,21 +100,46 @@ 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``.


-- 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 anonymous 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
@@ -122,9 +147,9 @@ 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
 \ /
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