.\" Copyright (c) 2016, IBM Corporation. .\" Written by Mike Rapoport .\" and Copyright (C) 2016 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" .TH IOCTL_USERFAULTFD 2 2020-06-09 "Linux" "Linux Programmer's Manual" .SH NAME ioctl_userfaultfd \- create a file descriptor for handling page faults in user space .SH SYNOPSIS .nf .B #include .PP .BI "int ioctl(int " fd ", int " cmd ", ...);" .fi .SH DESCRIPTION Various .BR ioctl (2) operations can be performed on a userfaultfd object (created by a call to .BR userfaultfd (2)) using calls of the form: .PP .in +4n .EX ioctl(fd, cmd, argp); .EE .in In the above, .I fd is a file descriptor referring to a userfaultfd object, .I cmd is one of the commands listed below, and .I argp is a pointer to a data structure that is specific to .IR cmd . .PP The various .BR ioctl (2) operations are described below. The .BR UFFDIO_API , .BR UFFDIO_REGISTER , and .BR UFFDIO_UNREGISTER operations are used to .I 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 .IR range operations. These operations enable the calling application to resolve page-fault events. .\" .SS UFFDIO_API (Since Linux 4.3.) Enable operation of the userfaultfd and perform API handshake. .PP The .I argp argument is a pointer to a .IR uffdio_api structure, defined as: .PP .in +4n .EX struct uffdio_api { __u64 api; /* Requested API version (input) */ __u64 features; /* Requested features (input/output) */ __u64 ioctls; /* Available ioctl() operations (output) */ }; .EE .in .PP The .I api field denotes the API version requested by the application. .PP The kernel verifies that it can support the requested API version, and sets the .I features and .I ioctls fields to bit masks representing all the available features and the generic .BR ioctl (2) operations available. .PP For Linux kernel versions before 4.11, the .I features field must be initialized to zero before the call to .BR UFFDIO_API , and zero (i.e., no feature bits) is placed in the .I features field by the kernel upon return from .BR ioctl (2). .PP Starting from Linux 4.11, the .I 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 .I features field. .PP To enable userfaultfd features the application should set a bit corresponding to each feature it wants to enable in the .I features field. If the kernel supports all the requested features it will enable them. Otherwise it will zero out the returned .I uffdio_api structure and return .BR EINVAL . .\" FIXME add more details about feature negotiation and enablement .PP The following feature bits may be set: .TP .BR 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 .BR fork (2) and a .B UFFD_EVENT_FORK event is delivered to the userfaultfd monitor .TP .BR UFFD_FEATURE_EVENT_REMAP " (since Linux 4.11)" If this feature is enabled, when the faulting process invokes .BR mremap (2), the userfaultfd monitor will receive an event of type .BR UFFD_EVENT_REMAP . .TP .BR UFFD_FEATURE_EVENT_REMOVE " (since Linux 4.11)" If this feature is enabled, when the faulting process calls .BR madvise (2) with the .B MADV_DONTNEED or .B MADV_REMOVE advice value to free a virtual memory area the userfaultfd monitor will receive an event of type .BR UFFD_EVENT_REMOVE . .TP .BR UFFD_FEATURE_EVENT_UNMAP " (since Linux 4.11)" If this feature is enabled, when the faulting process unmaps virtual memory either explicitly with .BR munmap (2), or implicitly during either .BR mmap (2) or .BR mremap (2). the userfaultfd monitor will receive an event of type .BR UFFD_EVENT_UNMAP . .TP .BR 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 .TP .BR 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, .BR tmpfs (5), shared mappings of .IR /dev/zero , .BR mmap (2) with the .B MAP_SHARED flag set, .BR memfd_create (2), and so on. .TP .BR UFFD_FEATURE_SIGBUS " (since Linux 4.14)" .\" commit 2d6d6f5a09a96cc1fec7ed992b825e05f64cb50e If this feature bit is set, no page-fault events .RB ( UFFD_EVENT_PAGEFAULT ) will be delivered. Instead, a .B 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. .PP The returned .I ioctls field can contain the following bits: .\" FIXME This user-space API seems not fully polished. Why are there .\" not constants defined for each of the bit-mask values listed below? .TP .B 1 << _UFFDIO_API The .B UFFDIO_API operation is supported. .TP .B 1 << _UFFDIO_REGISTER The .B UFFDIO_REGISTER operation is supported. .TP .B 1 << _UFFDIO_UNREGISTER The .B UFFDIO_UNREGISTER operation is supported. .PP This .BR ioctl (2) operation returns 0 on success. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .TP .B EFAULT .I argp refers to an address that is outside the calling process's accessible address space. .TP .B EINVAL The userfaultfd has already been enabled by a previous .BR UFFDIO_API operation. .TP .B EINVAL The API version requested in the .I api field is not supported by this kernel, or the .I features field passed to the kernel includes feature bits that are not supported by the current kernel version. .\" FIXME In the above error case, the returned 'uffdio_api' structure is .\" zeroed out. Why is this done? This should be explained in the manual page. .\" .\" Mike Rapoport: .\" In my understanding the uffdio_api .\" structure is zeroed to allow the caller .\" to distinguish the reasons for -EINVAL. .\" .SS UFFDIO_REGISTER (Since Linux 4.3.) Register a memory address range with the userfaultfd object. The pages in the range must be "compatible". .PP Up to Linux kernel 4.11, only private anonymous ranges are compatible for registering with .BR UFFDIO_REGISTER . .PP Since Linux 4.11, hugetlbfs and shared memory ranges are also compatible with .BR UFFDIO_REGISTER . .PP The .I argp argument is a pointer to a .I uffdio_register structure, defined as: .PP .in +4n .EX 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) */ }; .EE .in .PP The .I range field defines a memory range starting at .I start and continuing for .I len bytes that should be handled by the userfaultfd. .PP The .I 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: .TP .B UFFDIO_REGISTER_MODE_MISSING Track page faults on missing pages. .TP .B UFFDIO_REGISTER_MODE_WP Track page faults on write-protected pages. .PP Currently, the only supported mode is .BR UFFDIO_REGISTER_MODE_MISSING . .PP If the operation is successful, the kernel modifies the .I ioctls bit-mask field to indicate which .BR ioctl (2) operations are available for the specified range. This returned bit mask is as for .BR UFFDIO_API . .PP This .BR ioctl (2) operation returns 0 on success. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .\" FIXME Is the following error list correct? .\" .TP .B EBUSY A mapping in the specified range is registered with another userfaultfd object. .TP .B EFAULT .I argp refers to an address that is outside the calling process's accessible address space. .TP .B EINVAL An invalid or unsupported bit was specified in the .I mode field; or the .I mode field was zero. .TP .B EINVAL There is no mapping in the specified address range. .TP .B EINVAL .I range.start or .I range.len is not a multiple of the system page size; or, .I range.len is zero; or these fields are otherwise invalid. .TP .B EINVAL There as an incompatible mapping in the specified address range. .\" Mike Rapoport: .\" ENOMEM if the process is exiting and the .\" mm_struct has gone by the time userfault grabs it. .SS 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 .BR UFFDIO_REGISTER .) .PP The address range to unregister is specified in the .IR uffdio_range structure pointed to by .IR argp . .PP This .BR ioctl (2) operation returns 0 on success. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .TP .B EINVAL Either the .I start or the .I len field of the .I ufdio_range structure was not a multiple of the system page size; or the .I len field was zero; or these fields were otherwise invalid. .TP .B EINVAL There as an incompatible mapping in the specified address range. .TP .B EINVAL There was no mapping in the specified address range. .\" .SS 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 .IR src ", " dst ", and " len fields of the .I uffdio_copy structure pointed to by .IR argp : .PP .in +4n .EX 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 */ }; .EE .in .PP The following value may be bitwise ORed in .IR mode to change the behavior of the .B UFFDIO_COPY operation: .TP .B UFFDIO_COPY_MODE_DONTWAKE Do not wake up the thread that waits for page-fault resolution .PP The .I copy field is used by the kernel to return the number of bytes that was actually copied, or an error (a negated .IR errno -style value). .\" FIXME Above: Why is the 'copy' field used to return error values? .\" This should be explained in the manual page. If the value returned in .I copy doesn't match the value that was specified in .IR len , the operation fails with the error .BR EAGAIN . The .I copy field is output-only; it is not read by the .B UFFDIO_COPY operation. .PP This .BR ioctl (2) operation returns 0 on success. In this case, the entire area was copied. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .TP .B EAGAIN The number of bytes copied (i.e., the value returned in the .I copy field) does not equal the value that was specified in the .I len field. .TP .B EINVAL Either .I dst or .I len was not a multiple of the system page size, or the range specified by .IR src and .IR len or .IR dst and .IR len was invalid. .TP .B EINVAL An invalid bit was specified in the .IR mode field. .TP .BR ENOENT " (since Linux 4.11)" The faulting process has changed its virtual memory layout simultaneously with an outstanding .B UFFDIO_COPY operation. .TP .BR ENOSPC " (from Linux 4.11 until Linux 4.13)" The faulting process has exited at the time of a .B UFFDIO_COPY operation. .TP .BR ESRCH " (since Linux 4.13)" The faulting process has exited at the time of a .B UFFDIO_COPY operation. .\" .SS UFFDIO_ZEROPAGE (Since Linux 4.3.) Zero out a memory range registered with userfaultfd. .PP The requested range is specified by the .I range field of the .I uffdio_zeropage structure pointed to by .IR argp : .PP .in +4n .EX struct uffdio_zeropage { struct uffdio_range range; __u64 mode; /* Flags controlling behavior of copy */ __s64 zeropage; /* Number of bytes zeroed, or negated error */ }; .EE .in .PP The following value may be bitwise ORed in .IR mode to change the behavior of the .B UFFDIO_ZEROPAGE operation: .TP .B UFFDIO_ZEROPAGE_MODE_DONTWAKE Do not wake up the thread that waits for page-fault resolution. .PP The .I 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 .BR UFFDIO_COPY . .\" FIXME Why is the 'zeropage' field used to return error values? .\" This should be explained in the manual page. If the value returned in the .I zeropage field doesn't match the value that was specified in .IR range.len , the operation fails with the error .BR EAGAIN . The .I zeropage field is output-only; it is not read by the .B UFFDIO_ZEROPAGE operation. .PP This .BR ioctl (2) operation returns 0 on success. In this case, the entire area was zeroed. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .TP .B EAGAIN The number of bytes zeroed (i.e., the value returned in the .I zeropage field) does not equal the value that was specified in the .I range.len field. .TP .B EINVAL Either .I range.start or .I range.len was not a multiple of the system page size; or .I range.len was zero; or the range specified was invalid. .TP .B EINVAL An invalid bit was specified in the .IR mode field. .TP .BR ESRCH " (since Linux 4.13)" The faulting process has exited at the time of a .B UFFDIO_ZEROPAGE operation. .\" .SS UFFDIO_WAKE (Since Linux 4.3.) Wake up the thread waiting for page-fault resolution on a specified memory address range. .PP The .B UFFDIO_WAKE operation is used in conjunction with .BR UFFDIO_COPY and .BR UFFDIO_ZEROPAGE operations that have the .BR UFFDIO_COPY_MODE_DONTWAKE or .BR UFFDIO_ZEROPAGE_MODE_DONTWAKE bit set in the .I mode field. The userfault monitor can perform several .BR UFFDIO_COPY and .BR UFFDIO_ZEROPAGE operations in a batch and then explicitly wake up the faulting thread using .BR UFFDIO_WAKE . .PP The .I argp argument is a pointer to a .I uffdio_range structure (shown above) that specifies the address range. .PP This .BR ioctl (2) operation returns 0 on success. On error, \-1 is returned and .I errno is set to indicate the cause of the error. Possible errors include: .TP .B EINVAL The .I start or the .I len field of the .I ufdio_range structure was not a multiple of the system page size; or .I len was zero; or the specified range was otherwise invalid. .SH RETURN VALUE See descriptions of the individual operations, above. .SH ERRORS See descriptions of the individual operations, above. In addition, the following general errors can occur for all of the operations described above: .TP .B EFAULT .I argp does not point to a valid memory address. .TP .B EINVAL (For all operations except .BR UFFDIO_API .) The userfaultfd object has not yet been enabled (via the .BR UFFDIO_API operation). .SH CONFORMING TO These .BR ioctl (2) operations are Linux-specific. .SH 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 .BR UFFDIO_API operation that queries features availability and reopened before the second .BR UFFDIO_API operation that actually enables the desired features. .SH EXAMPLES See .BR userfaultfd (2). .SH SEE ALSO .BR ioctl (2), .BR mmap (2), .BR userfaultfd (2) .PP .IR Documentation/admin-guide/mm/userfaultfd.rst in the Linux kernel source tree .SH COLOPHON This page is part of release 5.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.