munlock

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



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

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

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

COLOPHON
       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)