| ioctl_userfaultfd(2) | System Calls Manual | ioctl_userfaultfd(2) |
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:
The returned ioctls field can contain the following bits:
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:
(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:
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:
This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:
(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:
(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:
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:
(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:
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:
(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:
(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:
This ioctl(2) operation returns 0 on success. On error, -1 is returned and errno is set to indicate the error. Possible errors include:
(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:
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:
(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:
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:
See descriptions of the individual operations, above.
See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above:
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.
See userfaultfd(2).
ioctl(2), mmap(2), userfaultfd(2)
Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree
| 2024-03-03 | Linux man-pages 6.7 |