mlock(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

MLOCK(2)                Linux Programmer's Manual               MLOCK(2)

NAME         top

       mlock, mlock2, munlock, mlockall, munlockall - lock and unlock
       memory

SYNOPSIS         top

       #include <sys/mman.h>

       int mlock(const void *addr, size_t len);
       int mlock2(const void *addr, size_t len, int flags);
       int munlock(const void *addr, size_t len);

       int mlockall(int flags);
       int munlockall(void);

DESCRIPTION         top

       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 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         top

       On success, these system calls return 0.  On error, -1 is
       returned, errno is set appropriately, and no changes are made to
       any locks in the address space of the process.

ERRORS         top

       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.

       For mlock(), mlock2(), and munlock():

       EAGAIN Some or all of the specified address range could not be
              locked.

       EINVAL The result of the addition addr+len was less than addr
              (e.g., the addition may have resulted in an overflow).

       EINVAL (Not on Linux) addr was not a multiple of the page size.

       ENOMEM Some of the specified address range does not correspond to
              mapped pages in the address space of the process.

       ENOMEM 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.)

       For mlock2():

       EINVAL Unknown flags were specified.

       For mlockall():

       EINVAL Unknown flags were specified or MCL_ONFAULT was specified
              without either MCL_FUTURE or MCL_CURRENT.

       For munlockall():

       EPERM  (Linux 2.6.8 and earlier) The caller was not privileged
              (CAP_IPC_LOCK).

VERSIONS         top

       mlock2() is available since Linux 4.4; glibc support was added in
       version 2.27.

CONFORMING TO         top

       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         top

       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.

   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         top

       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         top

       mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3),
       proc(5), capabilities(7)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-04-11                       MLOCK(2)

Pages that refer to this page: execve(2)fork(2)getrlimit(2)mincore(2)mmap(2)mremap(2)perf_event_open(2)shmctl(2)syscalls(2)proc(5)systemd.exec(5)capabilities(7)sched(7)