ioctl_userfaultfd - create a file descriptor for handling page faults in user space
Standard C library (libc, -lc)
#include <linux/userfaultfd.h> /* Definition of UFFD* constants */
#include <sys/ioctl.h>
int ioctl(int fd, int op, ...);Various ioctl(2) operations can be performed on a userfaultfd object (created by a call to userfaultfd(2)) using calls of the form:
ioctl(fd, op, argp);In the above, fd is a file descriptor referring to a
userfaultfd object, op is one of the operations listed below,
and argp is a pointer to a data structure that is specific to
op.
The various ioctl(2) operations are described below.
The UFFDIO_API, UFFDIO_REGISTER, and
UFFDIO_UNREGISTER operations are used to
configure userfaultfd behavior. These operations allow the
caller to choose what features will be enabled and what kinds of events
will be delivered to the application. The remaining operations are
range operations. These operations enable the calling
application to resolve page-fault events.
(Since Linux 4.3.) Enable operation of the userfaultfd and perform API handshake.
The argp argument is a pointer to a uffdio_api
structure, defined as:
struct uffdio_api {
__u64 api; /* Requested API version (input) */
__u64 features; /* Requested features (input/output) */
__u64 ioctls; /* Available ioctl() operations (output) */
};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:
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
If this feature is enabled, when the faulting process invokes mremap(2), the userfaultfd monitor will receive an event of type UFFD_EVENT_REMAP.
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.
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.
If this feature bit is set, the kernel supports registering userfaultfd ranges on hugetlbfs virtual memory areas
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.
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.
If this feature bit is set, uffd_msg.pagefault.feat.ptid
will be set to the faulted thread ID for each page-fault message.
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.)
If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on hugetlbfs-backed memory areas.
If this feature bit is set, the kernel supports registering userfaultfd ranges in minor mode on shmem-backed memory areas.
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.
If this feature bit is set, userfaultfd supports write-protect faults for hugetlbfs and shmem / tmpfs memory.
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.
If this feature bit is set, the kernel supports resolving faults with the UFFDIO_POISON ioctl.
If this feature bit is set, the write protection faults would be asynchronously resolved by the kernel.
The returned ioctls field can contain the following
bits:
The UFFDIO_API operation is supported.
The UFFDIO_REGISTER operation is supported.
The UFFDIO_UNREGISTER operation is supported.
This ioctl(2) operation returns 0 on success. On
error, -1 is returned and errno is set to indicate the error.
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. Possible errors include:
argp refers to an address that is outside the calling
process's accessible address space.
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.
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.
The UFFD_FEATURE_EVENT_FORK feature was enabled, but the calling process doesn't have the CAP_SYS_PTRACE capability.
(Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range must be “compatible”. Please refer to the list of register modes below for the compatible memory backends for each mode.
The argp argument is a pointer to a uffdio_register
structure, defined as:
struct uffdio_range {
__u64 start; /* Start of range */
__u64 len; /* Length of range (bytes) */
};
struct uffdio_register {
struct uffdio_range range;
__u64 mode; /* Desired mode of operation (input) */
__u64 ioctls; /* Available ioctl() operations (output) */
};The range field defines a memory range starting at
start and continuing for len bytes that should be
handled by the userfaultfd.
The mode field defines the mode of operation desired for
this memory region. The following values may be bitwise ORed to set the
userfaultfd mode for the specified range:
Track page faults on missing pages. Since Linux 4.3, only private anonymous ranges are compatible. Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible.
Track page faults on write-protected pages. Since Linux 5.7, only private anonymous ranges are compatible.
Track minor page faults. Since Linux 5.13, only hugetlbfs ranges are compatible. Since Linux 5.14, compatibility with shmem ranges was added.
If the operation is successful, the kernel modifies the
ioctls bit-mask field to indicate which
ioctl(2) operations are available for the specified
range. This returned bit mask can contain the following bits:
The UFFDIO_COPY operation is supported.
The UFFDIO_WAKE operation is supported.
The UFFDIO_WRITEPROTECT operation is supported.
The UFFDIO_ZEROPAGE operation is supported.
The UFFDIO_CONTINUE operation is supported.
The UFFDIO_POISON operation is supported.
This ioctl(2) operation returns 0 on success. On
error, -1 is returned and errno is set to indicate the error.
Possible errors include:
A mapping in the specified range is registered with another userfaultfd object.
argp refers to an address that is outside the calling
process's accessible address space.
An invalid or unsupported bit was specified in the mode
field; or the mode field was zero.
There is no mapping in the specified address range.
range.start or range.len is not a multiple of the
system page size; or, range.len is zero; or these fields are
otherwise invalid.
There as an incompatible mapping in the specified address range.
(Since Linux 4.3.) Unregister a memory address range from userfaultfd. The pages in the range must be “compatible” (see the description of UFFDIO_REGISTER.)
The address range to unregister is specified in the
uffdio_range structure pointed to by argp.
This ioctl(2) operation returns 0 on success. On
error, -1 is returned and errno is set to indicate the error.
Possible errors include:
Either the start or the len field of the
ufdio_range structure was not a multiple of the system page
size; or the len field was zero; or these fields were otherwise
invalid.
There as an incompatible mapping in the specified address range.
There was no mapping in the specified address range.
(Since Linux 4.3.) Atomically copy a continuous memory chunk into the
userfault registered range and optionally wake up the blocked thread.
The source and destination addresses and the number of bytes to copy are
specified by the src, dst, and len fields of
the uffdio_copy structure pointed to by argp:
struct uffdio_copy {
__u64 dst; /* Destination of copy */
__u64 src; /* Source of copy */
__u64 len; /* Number of bytes to copy */
__u64 mode; /* Flags controlling behavior of copy */
__s64 copy; /* Number of bytes copied, or negated error */
};The following value may be bitwise ORed in mode to change
the behavior of the UFFDIO_COPY operation:
Do not wake up the thread that waits for page-fault resolution
Copy the page with read-only permission. This allows the user to trap the next write to the page, which will block and generate another write-protect userfault message. This is used only when both UFFDIO_REGISTER_MODE_MISSING and UFFDIO_REGISTER_MODE_WP modes are enabled for the registered range.
The copy field is used by the kernel to return the number of
bytes that was actually copied, or an error (a negated
errno-style value). If the value returned in copy
doesn't match the value that was specified in len, the
operation fails with the error EAGAIN. The
copy field is output-only; it is not read by the
UFFDIO_COPY operation.
This ioctl(2) operation returns 0 on success. In
this case, the entire area was copied. On error, -1 is returned and
errno is set to indicate the error. Possible errors
include:
The number of bytes copied (i.e., the value returned in the
copy field) does not equal the value that was specified in the
len field.
Either dst or len was not a multiple of the system
page size, or the range specified by src and len or
dst and len was invalid.
An invalid bit was specified in the mode field.
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_COPY operation.
The faulting process has exited at the time of a UFFDIO_COPY operation.
The faulting process has exited at the time of a UFFDIO_COPY operation.
(Since Linux 4.3.) Zero out a memory range registered with userfaultfd.
The requested range is specified by the range field of the
uffdio_zeropage structure pointed to by argp:
struct uffdio_zeropage {
struct uffdio_range range;
__u64 mode; /* Flags controlling behavior of copy */
__s64 zeropage; /* Number of bytes zeroed, or negated error */
};The following value may be bitwise ORed in mode to change
the behavior of the UFFDIO_ZEROPAGE operation:
Do not wake up the thread that waits for page-fault resolution.
The zeropage field is used by the kernel to return the
number of bytes that was actually zeroed, or an error in the same manner
as UFFDIO_COPY. If the value returned in the
zeropage field doesn't match the value that was specified in
range.len, the operation fails with the error
EAGAIN. The zeropage field is output-only; it
is not read by the UFFDIO_ZEROPAGE operation.
This ioctl(2) operation returns 0 on success. In
this case, the entire area was zeroed. On error, -1 is returned and
errno is set to indicate the error. Possible errors
include:
The number of bytes zeroed (i.e., the value returned in the
zeropage field) does not equal the value that was specified in
the range.len field.
Either range.start or range.len was not a multiple
of the system page size; or range.len was zero; or the range
specified was invalid.
An invalid bit was specified in the mode field.
The faulting process has exited at the time of a UFFDIO_ZEROPAGE operation.
(Since Linux 4.3.) Wake up the thread waiting for page-fault resolution on a specified memory address range.
The UFFDIO_WAKE operation is used in conjunction
with UFFDIO_COPY and UFFDIO_ZEROPAGE
operations that have the UFFDIO_COPY_MODE_DONTWAKE or
UFFDIO_ZEROPAGE_MODE_DONTWAKE bit set in the
mode field. The userfault monitor can perform several
UFFDIO_COPY and UFFDIO_ZEROPAGE
operations in a batch and then explicitly wake up the faulting thread
using UFFDIO_WAKE.
The argp argument is a pointer to a uffdio_range
structure (shown above) that specifies the address range.
This ioctl(2) operation returns 0 on success. On
error, -1 is returned and errno is set to indicate the error.
Possible errors include:
The start or the len field of the
ufdio_range structure was not a multiple of the system page
size; or len was zero; or the specified range was otherwise
invalid.
(Since Linux 5.7.) Write-protect or write-unprotect a userfaultfd-registered memory range registered with mode UFFDIO_REGISTER_MODE_WP.
The argp argument is a pointer to a uffdio_range
structure as shown below:
struct uffdio_writeprotect {
struct uffdio_range range; /* Range to change write permission*/
__u64 mode; /* Mode to change write permission */
};There are two mode bits that are supported in this structure:
When this mode bit is set, the ioctl will be a write-protect
operation upon the memory range specified by range. Otherwise
it will be a write-unprotect operation upon the specified range, which
can be used to resolve a userfaultfd write-protect page fault.
When this mode bit is set, do not wake up any thread that waits for page-fault resolution after the operation. This can be specified only if UFFDIO_WRITEPROTECT_MODE_WP is not specified.
This ioctl(2) operation returns 0 on success. On
error, -1 is returned and errno is set to indicate the error.
Possible errors include:
The start or the len field of the
ufdio_range structure was not a multiple of the system page
size; or len was zero; or the specified range was otherwise
invalid.
The process was interrupted; retry this call.
The range specified in range is not valid. For example, the
virtual address does not exist, or not registered with userfaultfd
write-protect mode.
Encountered a generic fault during processing.
(Since Linux 5.13.) Resolve a minor page fault by installing page table entries for existing pages in the page cache.
The argp argument is a pointer to a uffdio_continue
structure as shown below:
struct uffdio_continue {
struct uffdio_range range;
/* Range to install PTEs for and continue */
__u64 mode; /* Flags controlling the behavior of continue */
__s64 mapped; /* Number of bytes mapped, or negated error */
};The following value may be bitwise ORed in mode to change
the behavior of the UFFDIO_CONTINUE operation:
Do not wake up the thread that waits for page-fault resolution.
The mapped field is used by the kernel to return the number
of bytes that were actually mapped, or an error in the same manner as
UFFDIO_COPY. If the value returned in the
mapped field doesn't match the value that was specified in
range.len, the operation fails with the error
EAGAIN. The mapped field is output-only; it is
not read by the UFFDIO_CONTINUE operation.
This ioctl(2) operation returns 0 on success. In
this case, the entire area was mapped. On error, -1 is returned and
errno is set to indicate the error. Possible errors
include:
The number of bytes mapped (i.e., the value returned in the
mapped field) does not equal the value that was specified in
the range.len field.
One or more pages were already mapped in the given range.
No existing page could be found in the page cache for the given range.
Either range.start or range.len was not a multiple
of the system page size; or range.len was zero; or the range
specified was invalid.
An invalid bit was specified in the mode field.
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_CONTINUE operation.
Allocating memory needed to setup the page table mappings failed.
The faulting process has exited at the time of a UFFDIO_CONTINUE operation.
(Since Linux 6.6.) Mark an address range as "poisoned". Future accesses to these addresses will raise a SIGBUS signal. Unlike MADV_HWPOISON this works by installing page table entries, rather than "really" poisoning the underlying physical pages. This means it only affects this particular address space.
The argp argument is a pointer to a uffdio_poison
structure as shown below:
struct uffdio_poison {
struct uffdio_range range;
/* Range to install poison PTE markers in */
__u64 mode; /* Flags controlling the behavior of poison */
__s64 updated; /* Number of bytes poisoned, or negated error */
};The following value may be bitwise ORed in mode to change
the behavior of the UFFDIO_POISON operation:
Do not wake up the thread that waits for page-fault resolution.
The updated field is used by the kernel to return the number
of bytes that were actually poisoned, or an error in the same manner as
UFFDIO_COPY. If the value returned in the
updated field doesn't match the value that was specified in
range.len, the operation fails with the error
EAGAIN. The updated field is output-only; it
is not read by the UFFDIO_POISON operation.
This ioctl(2) operation returns 0 on success. In
this case, the entire area was poisoned. On error, -1 is returned and
errno is set to indicate the error. Possible errors
include:
The number of bytes mapped (i.e., the value returned in the
updated field) does not equal the value that was specified in
the range.len field.
Either range.start or range.len was not a multiple
of the system page size; or range.len was zero; or the range
specified was invalid.
An invalid bit was specified in the mode field.
One or more pages were already mapped in the given range.
The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_POISON operation.
Allocating memory for page table entries failed.
The faulting process has exited at the time of a UFFDIO_POISON operation.
See descriptions of the individual operations, above.
See userfaultfd(2).
See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above:
argp does not point to a valid memory address.
(For all operations except UFFDIO_API.) The userfaultfd object has not yet been enabled (via the UFFDIO_API operation).
Linux.
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.
ioctl(2), mmap(2), userfaultfd(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux
kernel source tree