mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory
Standard C library (
libc,
-lc)
#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);
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() 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 guaranteed 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 memory range can be moved to external swap space again
by the kernel.
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 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_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.
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.
- 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 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 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).
mlock2() is available since Linux 4.4; glibc support was added in glibc
2.27.
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).)
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, 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 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.
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.
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.
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_MEMLOCK resource limit is encountered.
mincore(2),
mmap(2),
setrlimit(2),
shmctl(2),
sysconf(3),
proc(5),
capabilities(7)