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.