xref: /dflybsd-src/lib/libc/sys/mlockall.2 (revision c936cb6fb98d3c40c4f9174ee19179e918a2b544)

.\"     $OpenBSD: mlockall.2,v 1.5 2008/06/26 05:42:05 ray Exp $
.\"     $NetBSD: mlockall.2,v 1.6 2000/06/26 17:00:02 kleink Exp $
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.\" by Jason R. Thorpe of the Numerical Aerospace Simulation Facility,
.\" NASA Ames Research Center.
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.Dd May 16, 2022
.Dt MLOCKALL 2
.Os
.Sh NAME
.Nm mlockall ,
.Nm munlockall
.Nd lock (unlock) the address space of a process
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/mman.h
.Ft int
.Fn mlockall "int flags"
.Ft int
.Fn munlockall "void"
.Sh DESCRIPTION
The
.Fn mlockall
system call locks into memory the physical pages associated with the
address space of a process until the address space is unlocked, the
process exits, fork()s, or execs another program image.  Any pages which
are copy-on-write at the time of the function call will be force faulted.
Locked pages will not be paged to swap backing store.
.Pp
The following flags affect the behavior of
.Fn mlockall :
.Bl -tag -width ".Dv MCL_CURRENT"
.It Dv MCL_CURRENT
Lock all pages currently mapped into the process's address space.
.It Dv MCL_FUTURE
Lock all pages mapped into the process's address space in the future,
at the time the mapping is established.
Note that this may cause future mappings to fail if those mappings
cause resource limits to be exceeded.
.El
.Pp
Since physical memory is a potentially scarce resource, processes are
limited in how much they can lock down.
A single process can lock the minimum of a system-wide
.Dq wired pages
limit and the per-process
.Dv RLIMIT_MEMLOCK
resource limit.
.Pp
.Fn mlockall
has two limitations, necessary to keep it functional on modern systems.
The first is that writable file-backed MAP_PRIVATE pages (such as shared
library mappings) which have not yet been write-faulted will retain their
read-only mapping to the file backing store and not be force-copied.
If we were to force copy these pages, it would cause immense unnecessary
overheads for the program.
So any unmodified but writable pages which are currently in
the pmap read-only will still take a COW fault if written to.
.Pp
The second limitation is that when a fork() is issued, all writable pages
will be made copy-on-write (COW) in both the parent and the child.  The child
of course does not inherit the locked memory state, but this action will
cause any locked pages in the parent to become copy-on-write and they will
be faulted if written to.  So they will not be quite as locked as might have
been intended in this situation.
.Pp
The
.Fn munlockall
call unlocks any locked memory regions in the process address space.
Any regions mapped after an
.Fn munlockall
call will not be locked.
.Sh RETURN VALUES
A return value of 0 indicates that the call
succeeded and all pages in the range have either been locked or unlocked.
A return value of \-1 indicates an error occurred and the locked
status of all pages in the range remains unchanged.
In this case, the global location
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn mlockall
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
The
.Fa flags
argument is zero, or includes unimplemented flags.
.It Bq Er ENOMEM
Locking the indicated range would exceed either the system or per-process
limit for locked memory.
.It Bq Er EPERM
The calling process does not have the appropriate privilege to perform
the requested operation.
.El
.Sh SEE ALSO
.Xr mincore 2 ,
.Xr mlock 2 ,
.Xr setrlimit 2
.Sh STANDARDS
The
.Fn mlockall
and
.Fn munlockall
functions are believed to conform to
.St -p1003.1-2001 .
.Sh HISTORY
The
.Fn mlockall
and
.Fn munlockall
functions first appeared in
.Dx 2.9 .
.Sh BUGS
How could there be any bugs?  This is soooo simple...
.Pp
These system calls are not recommended for general use.  They are obviously
not thread-safe, and the larger application context from which they are
called might be hostile to such actions due to non-deterministic resource
limits in the system.  In a modern system, even semi-realtime and interactive
processes are already detected and handled by the system schedule.
Please use
.Fn mlock
and
.Fn munlock
to lock specific address ranges instead of locking the entire address space.