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.\" Copyright (C) Michael Kerrisk, 2004
.\" using some material drawn from earlier man pages
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.\"
-.TH MLOCK 2 2008-09-25 "Linux" "Linux Programmer's Manual"
+.TH MLOCK 2 2020-04-11 "Linux" "Linux Programmer's Manual"
.SH NAME
-mlock, munlock, mlockall, munlockall \- lock and unlock memory
+mlock, mlock2, munlock, mlockall, munlockall \- lock and unlock memory
.SH SYNOPSIS
.nf
.B #include <sys/mman.h>
-.sp
+.PP
.BI "int mlock(const void *" addr ", size_t " len );
+.BI "int mlock2(const void *" addr ", size_t " len ", int " flags );
.BI "int munlock(const void *" addr ", size_t " len );
-.sp
+.PP
.BI "int mlockall(int " flags );
.B int munlockall(void);
.fi
.SH DESCRIPTION
-.BR mlock ()
+.BR mlock (),
+.BR mlock2 (),
and
.BR mlockall ()
-respectively lock part or all of the calling process's virtual address
+lock part or all of the calling process's virtual address
space into RAM, preventing that memory from being paged to the
swap area.
+.PP
.BR munlock ()
and
.BR munlockall ()
perform the converse operation,
-respectively unlocking part or all of the calling process's virtual
+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.
+.PP
Memory locking and unlocking are performed in units of whole pages.
-.SS "mlock() and munlock()"
+.SS mlock(), mlock2(), and munlock()
.BR mlock ()
locks pages in the address range starting at
.I addr
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.
-
+.PP
+.BR mlock2 ()
+.\" commit a8ca5d0ecbdde5cc3d7accacbd69968b0c98764e
+.\" commit de60f5f10c58d4f34b68622442c0e04180367f3f
+.\" commit b0f205c2a3082dd9081f9a94e50658c5fa906ff1
+also locks pages in the specified range starting at
+.I addr
+and continuing for
+.I len
+bytes.
+However, the state of the pages contained in that range after the call
+returns successfully will depend on the value in the
+.I flags
+argument.
+.PP
+The
+.I flags
+argument can be either 0 or the following constant:
+.TP
+.B 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.
+.PP
+.PP
+If
+.I flags
+is 0,
+.BR mlock2 ()
+behaves exactly the same as
+.BR mlock ().
+.PP
.BR munlock ()
unlocks pages in the address range starting at
.I addr
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.
-.SS "mlockall() and munlockall()"
+.SS mlockall() and munlockall()
.BR mlockall ()
locks all pages mapped into the address space of the
calling process.
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.
-
+.PP
The
.I flags
argument is constructed as the bitwise OR of one or more of the
following constants:
-.TP 1.2i
+.TP
.B MCL_CURRENT
Lock all pages which are currently mapped into the address space of
the process.
.B 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
+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.
+.TP
+.BR MCL_ONFAULT " (since Linux 4.4)"
+Used together with
+.BR MCL_CURRENT ,
+.BR MCL_FUTURE ,
+or both.
+Mark all current (with
+.BR MCL_CURRENT )
+or future (with
+.BR MCL_FUTURE )
+mappings to lock pages when they are faulted in.
+When used with
+.BR MCL_CURRENT ,
+all present pages are locked, but
+.BR mlockall ()
+will not fault in non-present pages.
+When used with
+.BR 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.
+.B MCL_ONFAULT
+must be used with either
+.B MCL_CURRENT
+or
+.B MCL_FUTURE
+or both.
.PP
If
.B MCL_FUTURE
the kernel will deny stack expansion and deliver a
.B SIGSEGV
signal to the process.
-
+.PP
.BR munlockall ()
unlocks all pages mapped into the address space of the
calling process.
-.SH "RETURN VALUE"
-On success these system calls return 0.
+.SH RETURN VALUE
+On success, these system calls return 0.
On error, \-1 is returned,
.I errno
is set appropriately, and no changes are made to any locks in the
.SH ERRORS
.TP
.B ENOMEM
-(Linux 2.6.9 and later) the caller had a non-zero
+(Linux 2.6.9 and later) the caller had a nonzero
.B RLIMIT_MEMLOCK
soft resource limit, but tried to lock more memory than the limit
permitted.
.\" pages, which of course is not true. (MTK, Nov 04, kernel 2.4.28)
.TP
.B EPERM
-(Linux 2.6.9 and later) the caller was not privileged
+The caller is not privileged, but needs privilege
.RB ( CAP_IPC_LOCK )
-and its
-.B RLIMIT_MEMLOCK
-soft resource limit was 0.
-.TP
-.B EPERM
-(Linux 2.6.8 and earlier)
-The calling process has insufficient privilege to call
-.BR munlockall ().
-Under Linux the
-.B CAP_IPC_LOCK
-capability is required.
+to perform the requested operation.
.\"SVr4 documents an additional EAGAIN error code.
-.LP
+.PP
For
-.BR mlock ()
+.BR mlock (),
+.BR mlock2 (),
and
.BR munlock ():
.TP
Some or all of the specified address range could not be locked.
.TP
.B EINVAL
-.I len
-was negative.
+The result of the addition
+.IR addr + len
+was less than
+.IR addr
+(e.g., the addition may have resulted in an overflow).
.TP
.B EINVAL
(Not on Linux)
.B ENOMEM
Some of the specified address range does not correspond to mapped
pages in the address space of the process.
-.LP
+.TP
+.B 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.
+.\" I.e., the number of VMAs would exceed the 64kB 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.)
+.PP
For
-.BR mlockall ():
+.BR mlock2 ():
.TP
.B EINVAL
Unknown \fIflags\fP were specified.
-.LP
+.PP
+For
+.BR mlockall ():
+.TP
+.B EINVAL
+Unknown \fIflags\fP were specified or
+.B MCL_ONFAULT
+was specified without either
+.B MCL_FUTURE
+or
+.BR MCL_CURRENT .
+.PP
For
.BR munlockall ():
.TP
.B EPERM
(Linux 2.6.8 and earlier) The caller was not privileged
.RB ( CAP_IPC_LOCK ).
-.SH "CONFORMING TO"
-POSIX.1-2001, SVr4.
+.SH VERSIONS
+.BR mlock2 ()
+is available since Linux 4.4;
+glibc support was added in version 2.27.
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, SVr4.
+.PP
+.BR mlock2 ()
+is Linux specific.
.SH AVAILABILITY
On POSIX systems on which
.BR mlock ()
.B PAGESIZE
(if defined) in \fI<limits.h>\fP or by calling
.IR sysconf(_SC_PAGESIZE) .
-
+.PP
On POSIX systems on which
.BR mlockall ()
and
.\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
.\" -1: unavailable, 0: ask using sysconf().
.\" glibc defines it to 1.
-.SH "NOTES"
+.SH NOTES
Memory locking has two main applications: real-time algorithms and
high-security data processing.
Real-time applications require
(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.)
-
+.PP
Real-time processes that are using
.BR mlockall ()
to prevent delays on page faults should reserve enough
locked into RAM.
The dummy writes ensure that not even copy-on-write
page faults can occur in the critical section.
-
+.PP
Memory locks are not inherited by a child created via
.BR fork (2)
and are automatically removed (unlocked) during an
.BR execve (2)
or when the process terminates.
-
+The
+.BR mlockall ()
+.B MCL_FUTURE
+and
+.B MCL_FUTURE | MCL_ONFAULT
+settings are not inherited by a child created via
+.BR fork (2)
+and are cleared during an
+.BR execve (2).
+.PP
+Note that
+.BR 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
+.BR fork (2)
+after an
+.BR mlockall ()
+or
+.BR mlock ()
+operation\(emnot even from a thread which runs at a low priority within
+a process which also has a thread running at elevated priority.
+.PP
The memory lock on an address range is automatically removed
if the address range is unmapped via
.BR munmap (2).
-
+.PP
Memory locks do not stack, that is, pages which have been locked several times
by calls to
-.BR mlock ()
+.BR mlock (),
+.BR mlock2 (),
or
.BR mlockall ()
will be unlocked by a single call to
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.
-.SS "Linux Notes"
+.PP
+If a call to
+.BR mlockall ()
+which uses the
+.B MCL_FUTURE
+flag is followed by another call that does not specify this flag, the
+changes made by the
+.B MCL_FUTURE
+call will be lost.
+.PP
+The
+.BR mlock2 ()
+.B MLOCK_ONFAULT
+flag and the
+.BR mlockall ()
+.B 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.
+.SS Linux notes
Under Linux,
-.BR mlock ()
+.BR mlock (),
+.BR mlock2 (),
and
.BR munlock ()
automatically round
.I addr
down to the nearest page boundary.
-However, POSIX.1-2001 allows an implementation to require that
+However, the POSIX.1 specification of
+.BR mlock ()
+and
+.BR munlock ()
+allows an implementation to require that
.I addr
is page aligned, so portable applications should ensure this.
-.SS "Limits and permissions"
+.PP
+The
+.I VmLck
+field of the Linux-specific
+.I /proc/[pid]/status
+file shows how many kilobytes of memory the process with ID
+.I PID
+has locked using
+.BR mlock (),
+.BR mlock2 (),
+.BR mlockall (),
+and
+.BR mmap (2)
+.BR MAP_LOCKED .
+.SS Limits and permissions
In Linux 2.6.8 and earlier,
a process must be privileged
.RB ( CAP_IPC_LOCK )
in order to lock memory and the
.B RLIMIT_MEMLOCK
soft resource limit defines a limit on how much memory the process may lock.
-
+.PP
Since Linux 2.6.9, no limits are placed on the amount of memory
that a privileged process can lock and the
.B RLIMIT_MEMLOCK
soft resource limit instead defines a limit on how much memory an
unprivileged process may lock.
-.SH "BUGS"
+.SH BUGS
+In Linux 4.8 and earlier,
+a bug in the kernel's accounting of locked memory for unprivileged processes
+(i.e., without
+.BR CAP_IPC_LOCK )
+meant that if the region specified by
+.I addr
+and
+.I 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
+.BR RLIMIT_MEMLOCK
+limit, with the result that
+.BR mlock ()
+and
+.BR mlock2 ()
+would fail on requests that should have succeeded.
+This bug was fixed
+.\" commit 0cf2f6f6dc605e587d2c1120f295934c77e810e8
+in Linux 4.9
+.PP
In the 2.4 series Linux kernels up to and including 2.4.17,
a bug caused the
.BR mlockall ()
flag to be inherited across a
.BR fork (2).
This was rectified in kernel 2.4.18.
-
+.PP
Since kernel 2.6.9, if a privileged process calls
.I mlockall(MCL_FUTURE)
and later drops privileges (loses the
.B CAP_IPC_LOCK
capability by, for example,
-setting its effective UID to a non-zero value),
+setting its effective UID to a nonzero value),
then subsequent memory allocations (e.g.,
.BR mmap (2),
.BR brk (2))
.\" http://marc.theaimsgroup.com/?l=linux-kernel&m=113801392825023&w=2
.\" "Rationale for RLIMIT_MEMLOCK"
.\" 23 Jan 2006
-.SH "SEE ALSO"
+.SH SEE ALSO
+.BR mincore (2),
.BR mmap (2),
.BR setrlimit (2),
.BR shmctl (2),
.BR sysconf (3),
+.BR proc (5),
.BR capabilities (7)
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