Ubuntu Manpage: mlock, mlock2, munlock, mlockall, munlockall

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 can be swapped out again 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 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() 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
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.

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 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).

VERSIONS

   Linux
       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.

STANDARDS

       mlock()
       munlock()
       mlockall()
       munlockall()
              POSIX.1-2008.

       mlock2()
              Linux.

       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).)

HISTORY

       mlock()
       munlock()
       mlockall()
       munlockall()
              POSIX.1-2001, POSIX.1-2008, SVr4.

       mlock2()
              Linux 4.4, glibc 2.27.

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, 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.

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  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.