mlock(2) System Calls Manual mlock(2)
NAME
mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/mman.h>
int mlock(const void addr[.len], size_t len);
int mlock2(const void addr[.len], size_t len, unsigned int flags);
int munlock(const void addr[.len], 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 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 continu-
ing for len bytes. All pages that contain a part of the specified ad-
dress range are guaranteed to be resident in RAM when the call returns
successfully; the pages are guaranteed to stay in RAM until later un-
locked.
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 con-
tinuing for len bytes. After this call, all pages that contain a part
of the specified memory 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 successfully; 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 growing heap and stack as well as new mem-
ory-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_CUR-
RENT, all present pages are locked, but mlockall() will not
fault in non-present pages. When used with MCL_FUTURE, all fu-
ture 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_ONFAULT 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, er-
rno 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 re-
sulted 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 at-
tributes (e.g., locked versus 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 mid-
dle.)
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 glibc
2.27.
STANDARDS
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 deter-
ministic timing, and, like scheduling, paging is one major cause of un-
expected program execution delays. Real-time applications will usually
also switch to a real-time scheduler with sched_setscheduler(2). Cryp-
tographic security software often handles critical bytes like passwords
or secret keys as data structures. As a result of paging, these se-
crets 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, regardless 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 section, 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 mem-
ory 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_ON-
FAULT 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 ele-
vated priority.
The memory lock on an address range is automatically removed if the ad-
dress range is unmapped via munmap(2).
Memory locks do not stack, that is, pages which have been locked sev-
eral times by calls to mlock(), mlock2(), or mlockall() will be un-
locked by a single call to munlock() for the corresponding range or by
munlockall(). Pages which are mapped to several locations or by sev-
eral 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 al-
low efficient memory locking for applications that deal with large map-
pings 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 sig-
nificant 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 de-
fines 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 Linux 2.4 series of kernels up to and including Linux 2.4.17, a bug
caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2).
This was rectified in Linux 2.4.18.
Since Linux 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_MEM-
LOCK resource limit is encountered.
SEE ALSO
mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), ca-
pabilities(7)
Linux man-pages 6.03 2023-02-05 mlock(2)
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