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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | CAVEATS | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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UFFDIO_API(2const) UFFDIO_API(2const)
UFFDIO_API - enable operation of the userfaultfd and perform API
handshake
Standard C library (libc, -lc)
#include <linux/userfaultfd.h> /* Definition of UFFD* constants */
#include <sys/ioctl.h>
int ioctl(int fd, UFFDIO_API, struct uffdio_api *argp);
#include <linux/userfaultfd.h>
struct uffdio_api {
__u64 api; /* Requested API version (input) */
__u64 features; /* Requested features (input/output) */
__u64 ioctls; /* Available ioctl() operations (output) */
};
Enable operation of the userfaultfd and perform API handshake.
The api field denotes the API version requested by the
application. The kernel verifies that it can support the
requested API version, and sets the features and ioctls fields to
bit masks representing all the available features and the generic
ioctl(2) operations available.
Since Linux 4.11, applications should use the features field to
perform a two-step handshake. First, UFFDIO_API is called with
the features field set to zero. The kernel responds by setting
all supported feature bits.
Applications which do not require any specific features can begin
using the userfaultfd immediately. Applications which do need
specific features should call UFFDIO_API again with a subset of
the reported feature bits set to enable those features.
Before Linux 4.11, the features field must be initialized to zero
before the call to UFFDIO_API, and zero (i.e., no feature bits)
is placed in the features field by the kernel upon return from
ioctl(2).
If the application sets unsupported feature bits, the kernel will
zero out the returned uffdio_api structure and return EINVAL.
The following feature bits may be set:
UFFD_FEATURE_EVENT_FORK (since Linux 4.11)
When this feature is enabled, the userfaultfd objects
associated with a parent process are duplicated into the
child process during fork(2) and a UFFD_EVENT_FORK event
is delivered to the userfaultfd monitor
UFFD_FEATURE_EVENT_REMAP (since Linux 4.11)
If this feature is enabled, when the faulting process
invokes mremap(2), the userfaultfd monitor will receive an
event of type UFFD_EVENT_REMAP.
UFFD_FEATURE_EVENT_REMOVE (since Linux 4.11)
If this feature is enabled, when the faulting process
calls madvise(2) with the MADV_DONTNEED or MADV_REMOVE
advice value to free a virtual memory area the userfaultfd
monitor will receive an event of type UFFD_EVENT_REMOVE.
UFFD_FEATURE_EVENT_UNMAP (since Linux 4.11)
If this feature is enabled, when the faulting process
unmaps virtual memory either explicitly with munmap(2), or
implicitly during either mmap(2) or mremap(2), the
userfaultfd monitor will receive an event of type
UFFD_EVENT_UNMAP.
UFFD_FEATURE_MISSING_HUGETLBFS (since Linux 4.11)
If this feature bit is set, the kernel supports
registering userfaultfd ranges on hugetlbfs virtual memory
areas
UFFD_FEATURE_MISSING_SHMEM (since Linux 4.11)
If this feature bit is set, the kernel supports
registering userfaultfd ranges on shared memory areas.
This includes all kernel shared memory APIs: System V
shared memory, tmpfs(5), shared mappings of /dev/zero,
mmap(2) with the MAP_SHARED flag set, memfd_create(2), and
so on.
UFFD_FEATURE_SIGBUS (since Linux 4.14)
If this feature bit is set, no page-fault events
(UFFD_EVENT_PAGEFAULT) will be delivered. Instead, a
SIGBUS signal will be sent to the faulting process.
Applications using this feature will not require the use
of a userfaultfd monitor for processing memory accesses to
the regions registered with userfaultfd.
UFFD_FEATURE_THREAD_ID (since Linux 4.14)
If this feature bit is set, uffd_msg.pagefault.feat.ptid
will be set to the faulted thread ID for each page-fault
message.
UFFD_FEATURE_PAGEFAULT_FLAG_WP (since Linux 5.10)
If this feature bit is set, userfaultfd supports write-
protect faults for anonymous memory. (Note that shmem /
hugetlbfs support is indicated by a separate feature.)
UFFD_FEATURE_MINOR_HUGETLBFS (since Linux 5.13)
If this feature bit is set, the kernel supports
registering userfaultfd ranges in minor mode on hugetlbfs-
backed memory areas.
UFFD_FEATURE_MINOR_SHMEM (since Linux 5.14)
If this feature bit is set, the kernel supports
registering userfaultfd ranges in minor mode on shmem-
backed memory areas.
UFFD_FEATURE_EXACT_ADDRESS (since Linux 5.18)
If this feature bit is set, uffd_msg.pagefault.address
will be set to the exact page-fault address that was
reported by the hardware, and will not mask the offset
within the page. Note that old Linux versions might
indicate the exact address as well, even though the
feature bit is not set.
UFFD_FEATURE_WP_HUGETLBFS_SHMEM (since Linux 5.19)
If this feature bit is set, userfaultfd supports write-
protect faults for hugetlbfs and shmem / tmpfs memory.
UFFD_FEATURE_WP_UNPOPULATED (since Linux 6.4)
If this feature bit is set, the kernel will handle
anonymous memory the same way as file memory, by allowing
the user to write-protect unpopulated page table entries.
UFFD_FEATURE_POISON (since Linux 6.6)
If this feature bit is set, the kernel supports resolving
faults with the UFFDIO_POISON ioctl.
UFFD_FEATURE_WP_ASYNC (since Linux 6.7)
If this feature bit is set, the write protection faults
would be asynchronously resolved by the kernel.
The returned argp->ioctls field can contain the following bits:
1 << _UFFDIO_API
The UFFDIO_API operation is supported.
1 << _UFFDIO_REGISTER
The UFFDIO_REGISTER operation is supported.
1 << _UFFDIO_UNREGISTER
The UFFDIO_UNREGISTER operation is supported.
On success, 0 is returned.
On error, -1 is returned and errno is set to indicate the error.
EFAULT argp refers to an address that is outside the calling
process's accessible address space.
EINVAL The API version requested in the api field is not
supported by this kernel, or the features field passed to
the kernel includes feature bits that are not supported by
the current kernel version.
EINVAL A previous UFFDIO_API call already enabled one or more
features for this userfaultfd. Calling UFFDIO_API twice,
the first time with no features set, is explicitly allowed
as per the two-step feature detection handshake.
EPERM The UFFD_FEATURE_EVENT_FORK feature was enabled, but the
calling process doesn't have the CAP_SYS_PTRACE
capability.
Linux.
Linux 4.3.
If an error occurs, the kernel may zero the provided uffdio_api
structure. The caller should treat its contents as unspecified,
and reinitialize it before re-attempting another UFFDIO_API call.
In order to detect available userfault features and enable some
subset of those features the userfaultfd file descriptor must be
closed after the first UFFDIO_API operation that queries features
availability and reopened before the second UFFDIO_API operation
that actually enables the desired features.
See userfaultfd(2).
ioctl(2), ioctl_userfaultfd(2), mmap(2), userfaultfd(2)
linux.git/Documentation/admin-guide/mm/userfaultfd.rst
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
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Linux man-pages 6.9.1 2024-06-17 UFFDIO_API(2const)
Pages that refer to this page: ioctl_userfaultfd(2)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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