Manpages

NAME

ioctl_userfaultfd - create a file descriptor for handling page faults in user space

SYNOPSIS

#include <sys/ioctl.h>

int ioctl(int fd, int cmd, ...);

DESCRIPTION

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, cmd, argp);

In the above, fd is a file descriptor referring to a userfaultfd object, cmd is one of the commands listed below, and argp is a pointer to a data structure that is specific to cmd.

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.

UFFDIO_API
(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.

For Linux kernel versions before 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).

Starting from Linux 4.11, the features field can be used to ask whether particular features are supported and explicitly enable userfaultfd features that are disabled by default. The kernel always reports all the available features in the features field.

To enable userfaultfd features the application should set a bit corresponding to each feature it wants to enable in the features field. If the kernel supports all the requested features it will enable them. Otherwise it 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.

The returned 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.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the cause of the error. Possible errors include:

UFFDIO_REGISTER
(Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range must be "compatible".

Up to Linux kernel 4.11, only private anonymous ranges are compatible for registering with UFFDIO_REGISTER.

Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible with UFFDIO_REGISTER.

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:
UFFDIO_REGISTER_MODE_MISSING

Track page faults on missing pages.

UFFDIO_REGISTER_MODE_WP

Track page faults on write-protected pages.

Currently, the only supported mode is UFFDIO_REGISTER_MODE_MISSING.

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 is as for UFFDIO_API.

This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the cause of the error. Possible errors include:

UFFDIO_UNREGISTER
(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 cause of the error. Possible errors include:

UFFDIO_COPY
(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:
UFFDIO_COPY_MODE_DONTWAKE

Do not wake up the thread that waits for page-fault resolution

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 cause of the error. Possible errors include:

ENOENT (since Linux 4.11)

The faulting process has changed its virtual memory layout simultaneously with an outstanding UFFDIO_COPY operation.

ENOSPC (from Linux 4.11 until Linux 4.13)

The faulting process has exited at the time of a UFFDIO_COPY operation.

ESRCH (since Linux 4.13)

The faulting process has exited at the time of a UFFDIO_COPY operation.

UFFDIO_ZEROPAGE
(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:
UFFDIO_ZEROPAGE_MODE_DONTWAKE

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 cause of the error. Possible errors include:

ESRCH (since Linux 4.13)

The faulting process has exited at the time of a UFFDIO_ZEROPAGE operation.

UFFDIO_WAKE
(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 cause of the error. Possible errors include:

RETURN VALUE

See descriptions of the individual operations, above.

ERRORS

See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above:

CONFORMING TO

These ioctl(2) operations are Linux-specific.

BUGS

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.

EXAMPLES

See userfaultfd(2).

SEE ALSO

ioctl(2), mmap(2), userfaultfd(2)

Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree

COLOPHON

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