1MLOCK(2) Linux Programmer's Manual MLOCK(2)
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6 mlock, mlock2, munlock, mlockall, munlockall - lock and unlock memory
7
9 #include <sys/mman.h>
10 int mlock(const void *addr, size_t len);
12 int mlock2(const void *addr, size_t len, unsigned int flags);
13 int munlock(const void *addr, size_t len);
14 int mlockall(int flags);
16 int munlockall(void);
17
19 mlock(), mlock2(), and mlockall() lock part or all of the calling
20 process's virtual address space into RAM, preventing that memory from
21 being paged to the swap area.
22 munlock() and munlockall() perform the converse operation, unlocking
24 part or all of the calling process's virtual address space, so that
25 pages in the specified virtual address range may once more to be
26 swapped out if required by the kernel memory manager.
27 Memory locking and unlocking are performed in units of whole pages.
29 mlock(), mlock2(), and munlock()
31 mlock() locks pages in the address range starting at addr and continu‐
32 ing for len bytes. All pages that contain a part of the specified ad‐
33 dress range are guaranteed to be resident in RAM when the call returns
34 successfully; the pages are guaranteed to stay in RAM until later un‐
35 locked.
36 mlock2() also locks pages in the specified range starting at addr and
38 continuing for len bytes. However, the state of the pages contained in
39 that range after the call returns successfully will depend on the value
40 in the flags argument.
41 The flags argument can be either 0 or the following constant:
43 MLOCK_ONFAULT
45 Lock pages that are currently resident and mark the entire range
46 so that the remaining nonresident pages are locked when they are
47 populated by a page fault.
48 If flags is 0, mlock2() behaves exactly the same as mlock().
50 munlock() unlocks pages in the address range starting at addr and con‐
52 tinuing for len bytes. After this call, all pages that contain a part
53 of the specified memory range can be moved to external swap space again
54 by the kernel.
55 mlockall() and munlockall()
57 mlockall() locks all pages mapped into the address space of the calling
58 process. This includes the pages of the code, data, and stack segment,
59 as well as shared libraries, user space kernel data, shared memory, and
60 memory-mapped files. All mapped pages are guaranteed to be resident in
61 RAM when the call returns successfully; the pages are guaranteed to
62 stay in RAM until later unlocked.
63 The flags argument is constructed as the bitwise OR of one or more of
65 the following constants:
66 MCL_CURRENT
68 Lock all pages which are currently mapped into the address space
69 of the process.
70 MCL_FUTURE
72 Lock all pages which will become mapped into the address space
73 of the process in the future. These could be, for instance, new
74 pages required by a growing heap and stack as well as new mem‐
75 ory-mapped files or shared memory regions.
76 MCL_ONFAULT (since Linux 4.4)
78 Used together with MCL_CURRENT, MCL_FUTURE, or both. Mark all
79 current (with MCL_CURRENT) or future (with MCL_FUTURE) mappings
80 to lock pages when they are faulted in. When used with MCL_CUR‐
81 RENT, all present pages are locked, but mlockall() will not
82 fault in non-present pages. When used with MCL_FUTURE, all fu‐
83 ture mappings will be marked to lock pages when they are faulted
84 in, but they will not be populated by the lock when the mapping
85 is created. MCL_ONFAULT must be used with either MCL_CURRENT or
86 MCL_FUTURE or both.
87 If MCL_FUTURE has been specified, then a later system call (e.g.,
89 mmap(2), sbrk(2), malloc(3)), may fail if it would cause the number of
90 locked bytes to exceed the permitted maximum (see below). In the same
91 circumstances, stack growth may likewise fail: the kernel will deny
92 stack expansion and deliver a SIGSEGV signal to the process.
93 munlockall() unlocks all pages mapped into the address space of the
95 calling process.
96
98 On success, these system calls return 0. On error, -1 is returned, er‐
99 rno is set to indicate the error, and no changes are made to any locks
100 in the address space of the process.
101
103 EAGAIN (mlock(), mlock2(), and munlock()) Some or all of the specified
104 address range could not be locked.
105 EINVAL (mlock(), mlock2(), and munlock()) The result of the addition
107 addr+len was less than addr (e.g., the addition may have re‐
108 sulted in an overflow).
109 EINVAL (mlock2()) Unknown flags were specified.
111 EINVAL (mlockall()) Unknown flags were specified or MCL_ONFAULT was
113 specified without either MCL_FUTURE or MCL_CURRENT.
114 EINVAL (Not on Linux) addr was not a multiple of the page size.
116 ENOMEM (mlock(), mlock2(), and munlock()) Some of the specified address
118 range does not correspond to mapped pages in the address space
119 of the process.
120 ENOMEM (mlock(), mlock2(), and munlock()) Locking or unlocking a region
122 would result in the total number of mappings with distinct at‐
123 tributes (e.g., locked versus unlocked) exceeding the allowed
124 maximum. (For example, unlocking a range in the middle of a
125 currently locked mapping would result in three mappings: two
126 locked mappings at each end and an unlocked mapping in the mid‐
127 dle.)
128 ENOMEM (Linux 2.6.9 and later) the caller had a nonzero RLIMIT_MEMLOCK
130 soft resource limit, but tried to lock more memory than the
131 limit permitted. This limit is not enforced if the process is
132 privileged (CAP_IPC_LOCK).
133 ENOMEM (Linux 2.4 and earlier) the calling process tried to lock more
135 than half of RAM.
136 EPERM The caller is not privileged, but needs privilege (CAP_IPC_LOCK)
138 to perform the requested operation.
139 EPERM (munlockall()) (Linux 2.6.8 and earlier) The caller was not
141 privileged (CAP_IPC_LOCK).
142
144 mlock2() is available since Linux 4.4; glibc support was added in ver‐
145 sion 2.27.
146
148 mlock(), munlock(), mlockall(), and munlockall(): POSIX.1-2001,
149 POSIX.1-2008, SVr4.
150 mlock2() is Linux specific.
152 On POSIX systems on which mlock() and munlock() are available,
154 _POSIX_MEMLOCK_RANGE is defined in <unistd.h> and the number of bytes
155 in a page can be determined from the constant PAGESIZE (if defined) in
156 <limits.h> or by calling sysconf(_SC_PAGESIZE).
157 On POSIX systems on which mlockall() and munlockall() are available,
159 _POSIX_MEMLOCK is defined in <unistd.h> to a value greater than 0.
160 (See also sysconf(3).)
161
163 Memory locking has two main applications: real-time algorithms and
164 high-security data processing. Real-time applications require deter‐
165 ministic timing, and, like scheduling, paging is one major cause of un‐
166 expected program execution delays. Real-time applications will usually
167 also switch to a real-time scheduler with sched_setscheduler(2). Cryp‐
168 tographic security software often handles critical bytes like passwords
169 or secret keys as data structures. As a result of paging, these se‐
170 crets could be transferred onto a persistent swap store medium, where
171 they might be accessible to the enemy long after the security software
172 has erased the secrets in RAM and terminated. (But be aware that the
173 suspend mode on laptops and some desktop computers will save a copy of
174 the system's RAM to disk, regardless of memory locks.)
175 Real-time processes that are using mlockall() to prevent delays on page
177 faults should reserve enough locked stack pages before entering the
178 time-critical section, so that no page fault can be caused by function
179 calls. This can be achieved by calling a function that allocates a
180 sufficiently large automatic variable (an array) and writes to the mem‐
181 ory occupied by this array in order to touch these stack pages. This
182 way, enough pages will be mapped for the stack and can be locked into
183 RAM. The dummy writes ensure that not even copy-on-write page faults
184 can occur in the critical section.
185 Memory locks are not inherited by a child created via fork(2) and are
187 automatically removed (unlocked) during an execve(2) or when the
188 process terminates. The mlockall() MCL_FUTURE and MCL_FUTURE | MCL_ON‐
189 FAULT settings are not inherited by a child created via fork(2) and are
190 cleared during an execve(2).
191 Note that fork(2) will prepare the address space for a copy-on-write
193 operation. The consequence is that any write access that follows will
194 cause a page fault that in turn may cause high latencies for a real-
195 time process. Therefore, it is crucial not to invoke fork(2) after an
196 mlockall() or mlock() operation—not even from a thread which runs at a
197 low priority within a process which also has a thread running at ele‐
198 vated priority.
199 The memory lock on an address range is automatically removed if the ad‐
201 dress range is unmapped via munmap(2).
202 Memory locks do not stack, that is, pages which have been locked sev‐
204 eral times by calls to mlock(), mlock2(), or mlockall() will be un‐
205 locked by a single call to munlock() for the corresponding range or by
206 munlockall(). Pages which are mapped to several locations or by sev‐
207 eral processes stay locked into RAM as long as they are locked at least
208 at one location or by at least one process.
209 If a call to mlockall() which uses the MCL_FUTURE flag is followed by
211 another call that does not specify this flag, the changes made by the
212 MCL_FUTURE call will be lost.
213 The mlock2() MLOCK_ONFAULT flag and the mlockall() MCL_ONFAULT flag al‐
215 low efficient memory locking for applications that deal with large map‐
216 pings where only a (small) portion of pages in the mapping are touched.
217 In such cases, locking all of the pages in a mapping would incur a sig‐
218 nificant penalty for memory locking.
219 Linux notes
221 Under Linux, mlock(), mlock2(), and munlock() automatically round addr
222 down to the nearest page boundary. However, the POSIX.1 specification
223 of mlock() and munlock() allows an implementation to require that addr
224 is page aligned, so portable applications should ensure this.
225 The VmLck field of the Linux-specific /proc/[pid]/status file shows how
227 many kilobytes of memory the process with ID PID has locked using
228 mlock(), mlock2(), mlockall(), and mmap(2) MAP_LOCKED.
229 Limits and permissions
231 In Linux 2.6.8 and earlier, a process must be privileged (CAP_IPC_LOCK)
232 in order to lock memory and the RLIMIT_MEMLOCK soft resource limit de‐
233 fines a limit on how much memory the process may lock.
234 Since Linux 2.6.9, no limits are placed on the amount of memory that a
236 privileged process can lock and the RLIMIT_MEMLOCK soft resource limit
237 instead defines a limit on how much memory an unprivileged process may
238 lock.
239
241 In Linux 4.8 and earlier, a bug in the kernel's accounting of locked
242 memory for unprivileged processes (i.e., without CAP_IPC_LOCK) meant
243 that if the region specified by addr and len overlapped an existing
244 lock, then the already locked bytes in the overlapping region were
245 counted twice when checking against the limit. Such double accounting
246 could incorrectly calculate a "total locked memory" value for the
247 process that exceeded the RLIMIT_MEMLOCK limit, with the result that
248 mlock() and mlock2() would fail on requests that should have succeeded.
249 This bug was fixed in Linux 4.9.
250 In the 2.4 series Linux kernels up to and including 2.4.17, a bug
252 caused the mlockall() MCL_FUTURE flag to be inherited across a fork(2).
253 This was rectified in kernel 2.4.18.
254 Since kernel 2.6.9, if a privileged process calls mlockall(MCL_FUTURE)
256 and later drops privileges (loses the CAP_IPC_LOCK capability by, for
257 example, setting its effective UID to a nonzero value), then subsequent
258 memory allocations (e.g., mmap(2), brk(2)) will fail if the RLIMIT_MEM‐
259 LOCK resource limit is encountered.
260
262 mincore(2), mmap(2), setrlimit(2), shmctl(2), sysconf(3), proc(5), ca‐
263 pabilities(7)
264
266 This page is part of release 5.13 of the Linux man-pages project. A
267 description of the project, information about reporting bugs, and the
268 latest version of this page, can be found at
269 https://www.kernel.org/doc/man-pages/.
270Linux 2021-08-27 MLOCK(2)