MLOCK(2) Linux Programmer's Manual MLOCK(2)
NAME
mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory
SYNOPSIS
#include <sys/mman.h>
int mlock(const void *addr, size_t len);
int mlock2(const void *addr, size_t len, unsigned int flags);
int munlock(const void *addr, size_t len);
int mlockall(int flags);
int munlockall(void);
DESCRIPTION
mlock(), mlock2(), and mlockall() lock part or all of the calling process's virtual address space into RAM, preventing that memory from being paged to the swap
area.
munlock() and munlockall() perform the converse operation, unlocking part or all of the calling process's virtual address space, so that pages in the specified
virtual address range may once more to be swapped out if required by the kernel memory manager.
Memory locking and unlocking are performed in units of whole pages.
mlock(), mlock2(), and munlock()
mlock() locks pages in the address range starting at addr and continuing for len bytes. All pages that contain a part of the specified address range are guaran‐
teed to be resident in RAM when the call returns successfully; the pages are guaranteed to stay in RAM until later unlocked.
mlock2() also locks pages in the specified range starting at addr and continuing for len bytes. However, the state of the pages contained in that range after
the call returns successfully will depend on the value in the flags argument.
The flags argument can be either 0 or the following constant:
MLOCK_ONFAULT
Lock pages that are currently resident and mark the entire range so that the remaining nonresident pages are locked when they are populated by a page
fault.
If flags is 0, mlock2() behaves exactly the same as mlock().
munlock() unlocks pages in the address range starting at addr and continuing for len bytes. After this call, all pages that contain a part of the specified mem‐
ory range can be moved to external swap space again by the kernel.
mlockall() and munlockall()
mlockall() locks all pages mapped into the address space of the calling process. This includes the pages of the code, data, and stack segment, as well as shared
libraries, user space kernel data, shared memory, and memory-mapped files. All mapped pages are guaranteed to be resident in RAM when the call returns success‐
fully; the pages are guaranteed to stay in RAM until later unlocked.
The flags argument is constructed as the bitwise OR of one or more of the following constants:
MCL_CURRENT
Lock all pages which are currently mapped into the address space of the process.
MCL_FUTURE
Lock all pages which will become mapped into the address space of the process in the future. These could be, for instance, new pages required by a grow‐
ing heap and stack as well as new memory-mapped files or shared memory regions.
MCL_ONFAULT (since Linux 4.4)
Used together with MCL_CURRENT, MCL_FUTURE, or both. Mark all current (with MCL_CURRENT) or future (with MCL_FUTURE) mappings to lock pages when they are
faulted in. When used with MCL_CURRENT, all present pages are locked, but mlockall() will not fault in non-present pages. When used with MCL_FUTURE, all
future mappings will be marked to lock pages when they are faulted in, but they will not be populated by the lock when the mapping is created. MCL_ON‐
FAULT must be used with either MCL_CURRENT or MCL_FUTURE or both.
If MCL_FUTURE has been specified, then a later system call (e.g., mmap(2), sbrk(2), malloc(3)), may fail if it would cause the number of locked bytes to exceed
the permitted maximum (see below). In the same circumstances, stack growth may likewise fail: the kernel will deny stack expansion and deliver a SIGSEGV signal
to the process.
munlockall() unlocks all pages mapped into the address space of the calling process.
RETURN VALUE
On success, these system calls return 0. On error, -1 is returned, errno is set to indicate the error, and no changes are made to any locks in the address space
of the process.
ERRORS
EAGAIN (mlock(), mlock2(), and munlock()) Some or all of the specified address range could not be locked.
EINVAL (mlock(), mlock2(), and munlock()) The result of the addition addr+len was less than addr (e.g., the addition may have resulted in an overflow).
EINVAL (mlock2()) Unknown flags were specified.
EINVAL (mlockall()) Unknown flags were specified or MCL_ONFAULT was specified without either MCL_FUTURE or MCL_CURRENT.
EINVAL (Not on Linux) addr was not a multiple of the page size.
ENOMEM (mlock(), mlock2(), and munlock()) Some of the specified address range does not correspond to mapped pages in the address space of the process.
ENOMEM (mlock(), mlock2(), and munlock()) Locking or unlocking a region would result in the total number of mappings with distinct attributes (e.g., locked ver‐
sus unlocked) exceeding the allowed maximum. (For example, unlocking a range in the middle of a currently locked mapping would result in three mappings:
two locked mappings at each end and an unlocked mapping in the middle.)
ENOMEM (Linux 2.6.9 and later) the caller had a nonzero RLIMIT_MEMLOCK soft resource limit, but tried to lock more memory than the limit permitted. This limit
is not enforced if the process is privileged (CAP_IPC_LOCK).
ENOMEM (Linux 2.4 and earlier) the calling process tried to lock more than half of RAM.
EPERM The caller is not privileged, but needs privilege (CAP_IPC_LOCK) to perform the requested operation.
EPERM (munlockall()) (Linux 2.6.8 and earlier) The caller was not privileged (CAP_IPC_LOCK).
VERSIONS
mlock2() is available since Linux 4.4; glibc support was added in version 2.27.
CONFORMING TO
mlock(), munlock(), mlockall(), and munlockall(): POSIX.1-2001, POSIX.1-2008, SVr4.
mlock2() is Linux specific.
On POSIX systems on which mlock() and munlock() are available, _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number of bytes in a page can be determined
from the constant PAGESIZE (if defined) in <limits.h> or by calling sysconf(_SC_PAGESIZE).
On POSIX systems on which mlockall() and munlockall() are available, _POSIX_MEMLOCK is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)
NOTES
Memory locking has two main applications: real-time algorithms and high-security data processing. Real-time applications require deterministic timing, and, like
scheduling, paging is one major cause of unexpected program execution delays. Real-time applications will usually also switch to a real-time scheduler with
sched_setscheduler(2). Cryptographic security software often handles critical bytes like passwords or secret keys as data structures. As a result of paging,
these secrets could be transferred onto a persistent swap store medium, where they might be accessible to the enemy long after the security software has erased
the secrets in RAM and terminated. (But be aware that the suspend mode on laptops and some desktop computers will save a copy of the system's RAM to disk, re‐
gardless of memory locks.)
Real-time processes that are using mlockall() to prevent delays on page faults should reserve enough locked stack pages before entering the time-critical sec‐
tion, so that no page fault can be caused by function calls. This can be achieved by calling a function that allocates a sufficiently large automatic variable
(an array) and writes to the memory occupied by this array in order to touch these stack pages. This way, enough pages will be mapped for the stack and can be
locked into RAM. The dummy writes ensure that not even copy-on-write page faults can occur in the critical section.
Memory locks are not inherited by a child created via fork(2) and are automatically removed (unlocked) during an execve(2) or when the process terminates. The
mlockall() MCL_FUTURE and MCL_FUTURE | MCL_ONFAULT settings are not inherited by a child created via fork(2) and are cleared during an execve(2).
Note that fork(2) will prepare the address space for a copy-on-write operation. The consequence is that any write access that follows will cause a page fault
that in turn may cause high latencies for a real-time process. Therefore, it is crucial not to invoke fork(2) after an mlockall() or mlock() operation—not even
from a thread which runs at a low priority within a process which also has a thread running at elevated priority.
The memory lock on an address range is automatically removed if the address range is unmapped via munmap(2).
Memory locks do not stack, that is, pages which have been locked several times by calls to mlock(), mlock2(), or mlockall() will be unlocked by a single call to
munlock() for the corresponding range or by munlockall(). Pages which are mapped to several locations or by several processes stay locked into RAM as long as
they are locked at least at one location or by at least one process.
If a call to mlockall() which uses the MCL_FUTURE flag is followed by another call that does not specify this flag, the changes made by the MCL_FUTURE call will
be lost.
The mlock2() MLOCK_ONFAULT flag and the mlockall() MCL_ONFAULT flag allow efficient memory locking for applications that deal with large mappings where only a
(small) portion of pages in the mapping are touched. In such cases, locking all of the pages in a mapping would incur a significant penalty for memory locking.
Linux notes
Under Linux, mlock(), mlock2(), and munlock() automatically round addr down to the nearest page boundary. However, the POSIX.1 specification of mlock() and
munlock() allows an implementation to require that addr is page aligned, so portable applications should ensure this.
The VmLck field of the Linux-specific /proc/[pid]/status file shows how many kilobytes of memory the process with ID PID has locked using mlock(), mlock2(),
mlockall(), and mmap(2) MAP_LOCKED.
Limits and permissions
In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK) in order to lock memory and the RLIMIT_MEMLOCK soft resource limit defines a limit on how
much memory the process may lock.
Since Linux 2.6.9, no limits are placed on the amount of memory that a privileged process can lock and the RLIMIT_MEMLOCK soft resource limit instead defines a
limit on how much memory an unprivileged process may lock.
BUGS
In Linux 4.8 and earlier, a bug in the kernel's accounting of locked memory for unprivileged processes (i.e., without CAP_IPC_LOCK) meant that if the region
specified by addr and len overlapped an existing lock, then the already locked bytes in the overlapping region were counted twice when checking against the
limit. Such double accounting could incorrectly calculate a "total locked memory" value for the process that exceeded the RLIMIT_MEMLOCK limit, with the result
that mlock() and mlock2() would fail on requests that should have succeeded. This bug was fixed in Linux 4.9.
In the 2.4 series Linux kernels up to and including 2.4.17, a bug caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2). This was rectified in
kernel 2.4.18.
Since kernel 2.6.9, if a privileged process calls mlockall(MCL_FUTURE) and later drops privileges (loses the CAP_IPC_LOCK capability by, for example, setting its
effective UID to a nonzero value), then subsequent memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEMLOCK resource limit is encountered.
SEE ALSO
mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), capabilities(7)
Linux 2021-08-27 MLOCK(2)