man2/mlock.2

   1 .\" Copyright (C) Michael Kerrisk, 2004
   2 .\"     using some material drawn from earlier man pages
   3 .\"     written by Thomas Kuhn, Copyright 1996
   4 .\"
   5 .\" %%%LICENSE_START(GPLv2+_DOC_FULL)
   6 .\" This is free documentation; you can redistribute it and/or
   7 .\" modify it under the terms of the GNU General Public License as
   8 .\" published by the Free Software Foundation; either version 2 of
   9 .\" the License, or (at your option) any later version.
  10 .\"
  11 .\" The GNU General Public License's references to "object code"
  12 .\" and "executables" are to be interpreted as the output of any
  13 .\" document formatting or typesetting system, including
  14 .\" intermediate and printed output.
  15 .\"
  16 .\" This manual is distributed in the hope that it will be useful,
  17 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 .\" GNU General Public License for more details.
  20 .\"
  21 .\" You should have received a copy of the GNU General Public
  22 .\" License along with this manual; if not, see
  23 .\" <http://www.gnu.org/licenses/>.
  24 .\" %%%LICENSE_END
  25 .\"
  26 .TH MLOCK 2 2015-08-28 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 mlock, mlock2, munlock, mlockall, munlockall \- lock and unlock memory
  29 .SH SYNOPSIS
  30 .nf
  31 .B #include <sys/mman.h>
  32 .sp
  33 .BI "int mlock(const void *" addr ", size_t " len );
  34 .BI "int mlock2(const void *" addr ", size_t " len ", int " flags );
  35 .BI "int munlock(const void *" addr ", size_t " len );
  36 .sp
  37 .BI "int mlockall(int " flags );
  38 .B int munlockall(void);
  39 .fi
  40 .SH DESCRIPTION
  41 .BR mlock (),
  42 .BR mlock2 (),
  43 and
  44 .BR mlockall ()
  45 respectively lock part or all of the calling process's virtual address
  46 space into RAM, preventing that memory from being paged to the
  47 swap area.
  48 .BR munlock ()
  49 and
  50 .BR munlockall ()
  51 perform the converse operation,
  52 respectively unlocking part or all of the calling process's virtual
  53 address space, so that pages in the specified virtual address range may
  54 once more to be swapped out if required by the kernel memory manager.
  55 Memory locking and unlocking are performed in units of whole pages.
  56 .SS mlock(), mlock2(), and munlock()
  57 .BR mlock ()
  58 locks pages in the address range starting at
  59 .I addr
  60 and continuing for
  61 .I len
  62 bytes.
  63 All pages that contain a part of the specified address range are
  64 guaranteed to be resident in RAM when the call returns successfully;
  65 the pages are guaranteed to stay in RAM until later unlocked.
  66
  67 .BR mlock2 ()
  68 also locks pages in the specified range starting at
  69 .I addr
  70 and continuing for
  71 .I len
  72 bytes.
  73 However, the state of the pages contained in that range after the call
  74 returns successfully will depend on the value in the
  75 .I flags
  76 argument.
  77
  78 The
  79 .I flags
  80 argument can be either 0 or the following constant:
  81 .TP 1.2i
  82 .B MLOCK_ONFAULT
  83 Lock pages that are currently resident and mark the entire range to have
  84 pages locked when they are populated by the page fault.
  85 .PP
  86
  87 If
  88 .I flags
  89 is 0,
  90 .BR mlock2 ()
  91 will function exactly as
  92 .BR mlock ()
  93 would.
  94
  95 Note: Currently, there is not a glibc wrapper for
  96 .BR mlock2 ()
  97 so it will need to be invoked using
  98 .BR syscall (2)
  99
 100 .BR munlock ()
 101 unlocks pages in the address range starting at
 102 .I addr
 103 and continuing for
 104 .I len
 105 bytes.
 106 After this call, all pages that contain a part of the specified
 107 memory range can be moved to external swap space again by the kernel.
 108 .SS mlockall() and munlockall()
 109 .BR mlockall ()
 110 locks all pages mapped into the address space of the
 111 calling process.
 112 This includes the pages of the code, data and stack
 113 segment, as well as shared libraries, user space kernel data, shared
 114 memory, and memory-mapped files.
 115 All mapped pages are guaranteed
 116 to be resident in RAM when the call returns successfully;
 117 the pages are guaranteed to stay in RAM until later unlocked.
 118
 119 The
 120 .I flags
 121 argument is constructed as the bitwise OR of one or more of the
 122 following constants:
 123 .TP 1.2i
 124 .B MCL_CURRENT
 125 Lock all pages which are currently mapped into the address space of
 126 the process.
 127 .TP
 128 .B MCL_FUTURE
 129 Lock all pages which will become mapped into the address space of the
 130 process in the future.
 131 These could be, for instance, new pages required
 132 by a growing heap and stack as well as new memory-mapped files or
 133 shared memory regions.
 134 .TP
 135 .BR MCL_ONFAULT " (since Linux 4.4)"
 136 Used together with
 137 .BR MCL_CURRENT ,
 138 .BR MCL_FUTURE ,
 139 or both.
 140 Mark all current (with
 141 .BR MCL_CURRENT )
 142 or future (with
 143 .BR MCL_FUTURE )
 144 mappings to lock pages when they are faulted in.
 145 When used with
 146 .BR MCL_CURRENT ,
 147 all present pages are locked, but
 148 .BR mlockall ()
 149 will not fault in non-present pages.
 150 When used with
 151 .BR MCL_FUTURE ,
 152 all future mappings will be marked to lock pages when they are faulted
 153 in, but they will not be populated by the lock when the mapping is
 154 created.
 155 .B MCL_ONFAULT
 156 must be used with either
 157 .B MCL_CURRENT
 158 or
 159 .B MCL_FUTURE
 160 or both.
 161 .PP
 162 If
 163 .B MCL_FUTURE
 164 has been specified, then a later system call (e.g.,
 165 .BR mmap (2),
 166 .BR sbrk (2),
 167 .BR malloc (3)),
 168 may fail if it would cause the number of locked bytes to exceed
 169 the permitted maximum (see below).
 170 In the same circumstances, stack growth may likewise fail:
 171 the kernel will deny stack expansion and deliver a
 172 .B SIGSEGV
 173 signal to the process.
 174
 175 .BR munlockall ()
 176 unlocks all pages mapped into the address space of the
 177 calling process.
 178 .SH RETURN VALUE
 179 On success, these system calls return 0.
 180 On error, \-1 is returned,
 181 .I errno
 182 is set appropriately, and no changes are made to any locks in the
 183 address space of the process.
 184 .SH ERRORS
 185 .TP
 186 .B ENOMEM
 187 (Linux 2.6.9 and later) the caller had a nonzero
 188 .B RLIMIT_MEMLOCK
 189 soft resource limit, but tried to lock more memory than the limit
 190 permitted.
 191 This limit is not enforced if the process is privileged
 192 .RB ( CAP_IPC_LOCK ).
 193 .TP
 194 .B ENOMEM
 195 (Linux 2.4 and earlier) the calling process tried to lock more than
 196 half of RAM.
 197 .\" In the case of mlock(), this check is somewhat buggy: it doesn't
 198 .\" take into account whether the to-be-locked range overlaps with
 199 .\" already locked pages.  Thus, suppose we allocate
 200 .\" (num_physpages / 4 + 1) of memory, and lock those pages once using
 201 .\" mlock(), and then lock the *same* page range a second time.
 202 .\" In the case, the second mlock() call will fail, since the check
 203 .\" calculates that the process is trying to lock (num_physpages / 2 + 2)
 204 .\" pages, which of course is not true.  (MTK, Nov 04, kernel 2.4.28)
 205 .TP
 206 .B EPERM
 207 The caller is not privileged, but needs privilege
 208 .RB ( CAP_IPC_LOCK )
 209 to perform the requested operation.
 210 .\"SVr4 documents an additional EAGAIN error code.
 211 .LP
 212 For
 213 .BR mlock (),
 214 .BR mlock2 (),
 215 and
 216 .BR munlock ():
 217 .TP
 218 .B EAGAIN
 219 Some or all of the specified address range could not be locked.
 220 .TP
 221 .B EINVAL
 222 The result of the addition
 223 .IR addr + len
 224 was less than
 225 .IR addr
 226 (e.g., the addition may have resulted in an overflow).
 227 .TP
 228 .B EINVAL
 229 (Not on Linux)
 230 .I addr
 231 was not a multiple of the page size.
 232 .TP
 233 .B ENOMEM
 234 Some of the specified address range does not correspond to mapped
 235 pages in the address space of the process.
 236 .TP
 237 .B ENOMEM
 238 Locking or unlocking a region would result in the total number of
 239 mappings with distinct attributes (e.g., locked versus unlocked)
 240 exceeding the allowed maximum.
 241 .\" I.e., the number of VMAs would exceed the 64kB maximum
 242 (For example, unlocking a range in the middle of a currently locked
 243 mapping would result in three mappings:
 244 two locked mappings at each end and an unlocked mapping in the middle.)
 245 .LP
 246 For
 247 .BR mlock2 ():
 248 .TP
 249 .B EINVAL
 250 Unknown \fIflags\fP were specified.
 251 .LP
 252 For
 253 .BR mlockall ():
 254 .TP
 255 .B EINVAL
 256 Unknown \fIflags\fP were specified or
 257 .B MCL_ONFAULT
 258 was specified without either
 259 .B MCL_FUTURE
 260 or
 261 .BR MCL_CURRENT .
 262 .LP
 263 For
 264 .BR munlockall ():
 265 .TP
 266 .B EPERM
 267 (Linux 2.6.8 and earlier) The caller was not privileged
 268 .RB ( CAP_IPC_LOCK ).
 269 .SH CONFORMING TO
 270 POSIX.1-2001, POSIX.1-2008, SVr4.
 271 .SH AVAILABILITY
 272 On POSIX systems on which
 273 .BR mlock ()
 274 and
 275 .BR munlock ()
 276 are available,
 277 .B _POSIX_MEMLOCK_RANGE
 278 is defined in \fI<unistd.h>\fP and the number of bytes in a page
 279 can be determined from the constant
 280 .B PAGESIZE
 281 (if defined) in \fI<limits.h>\fP or by calling
 282 .IR sysconf(_SC_PAGESIZE) .
 283
 284 On POSIX systems on which
 285 .BR mlockall ()
 286 and
 287 .BR munlockall ()
 288 are available,
 289 .B _POSIX_MEMLOCK
 290 is defined in \fI<unistd.h>\fP to a value greater than 0.
 291 (See also
 292 .BR sysconf (3).)
 293 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
 294 .\" -1: unavailable, 0: ask using sysconf().
 295 .\" glibc defines it to 1.
 296 .SH NOTES
 297 Memory locking has two main applications: real-time algorithms and
 298 high-security data processing.
 299 Real-time applications require
 300 deterministic timing, and, like scheduling, paging is one major cause
 301 of unexpected program execution delays.
 302 Real-time applications will
 303 usually also switch to a real-time scheduler with
 304 .BR sched_setscheduler (2).
 305 Cryptographic security software often handles critical bytes like
 306 passwords or secret keys as data structures.
 307 As a result of paging,
 308 these secrets could be transferred onto a persistent swap store medium,
 309 where they might be accessible to the enemy long after the security
 310 software has erased the secrets in RAM and terminated.
 311 (But be aware that the suspend mode on laptops and some desktop
 312 computers will save a copy of the system's RAM to disk, regardless
 313 of memory locks.)
 314
 315 Real-time processes that are using
 316 .BR mlockall ()
 317 to prevent delays on page faults should reserve enough
 318 locked stack pages before entering the time-critical section,
 319 so that no page fault can be caused by function calls.
 320 This can be achieved by calling a function that allocates a
 321 sufficiently large automatic variable (an array) and writes to the
 322 memory occupied by this array in order to touch these stack pages.
 323 This way, enough pages will be mapped for the stack and can be
 324 locked into RAM.
 325 The dummy writes ensure that not even copy-on-write
 326 page faults can occur in the critical section.
 327
 328 Memory locks are not inherited by a child created via
 329 .BR fork (2)
 330 and are automatically removed (unlocked) during an
 331 .BR execve (2)
 332 or when the process terminates.
 333 The
 334 .BR mlockall ()
 335 .B MCL_FUTURE
 336 and
 337 .B MCL_FUTURE | MCL_ONFAULT
 338 settings are not inherited by a child created via
 339 .BR fork (2)
 340 and are cleared during an
 341 .BR execve (2).
 342
 343 The memory lock on an address range is automatically removed
 344 if the address range is unmapped via
 345 .BR munmap (2).
 346
 347 Memory locks do not stack, that is, pages which have been locked several times
 348 by calls to
 349 .BR mlock (),
 350 .BR mlock2 (),
 351 or
 352 .BR mlockall ()
 353 will be unlocked by a single call to
 354 .BR munlock ()
 355 for the corresponding range or by
 356 .BR munlockall ().
 357 Pages which are mapped to several locations or by several processes stay
 358 locked into RAM as long as they are locked at least at one location or by
 359 at least one process.
 360
 361 If a call to
 362 .BR mlockall ()
 363 which uses the
 364 .B MCL_FUTURE
 365 flag is followed by another call that does not specify this flag, the
 366 changes made by the
 367 .B MCL_FUTURE
 368 call will be lost.
 369 .SS Linux notes
 370 Under Linux,
 371 .BR mlock (),
 372 .BR mlock2 (),
 373 and
 374 .BR munlock ()
 375 automatically round
 376 .I addr
 377 down to the nearest page boundary.
 378 However, POSIX.1 allows an implementation to require that
 379 .I addr
 380 is page aligned, so portable applications should ensure this.
 381
 382 The
 383 .I VmLck
 384 field of the Linux-specific
 385 .I /proc/PID/status
 386 file shows how many kilobytes of memory the process with ID
 387 .I PID
 388 has locked using
 389 .BR mlock (),
 390 .BR mlock2 (),
 391 .BR mlockall (),
 392 and
 393 .BR mmap (2)
 394 .BR MAP_LOCKED .
 395 .SS Limits and permissions
 396 In Linux 2.6.8 and earlier,
 397 a process must be privileged
 398 .RB ( CAP_IPC_LOCK )
 399 in order to lock memory and the
 400 .B RLIMIT_MEMLOCK
 401 soft resource limit defines a limit on how much memory the process may lock.
 402
 403 Since Linux 2.6.9, no limits are placed on the amount of memory
 404 that a privileged process can lock and the
 405 .B RLIMIT_MEMLOCK
 406 soft resource limit instead defines a limit on how much memory an
 407 unprivileged process may lock.
 408 .SH BUGS
 409 In the 2.4 series Linux kernels up to and including 2.4.17,
 410 a bug caused the
 411 .BR mlockall ()
 412 .B MCL_FUTURE
 413 flag to be inherited across a
 414 .BR fork (2).
 415 This was rectified in kernel 2.4.18.
 416
 417 Since kernel 2.6.9, if a privileged process calls
 418 .I mlockall(MCL_FUTURE)
 419 and later drops privileges (loses the
 420 .B CAP_IPC_LOCK
 421 capability by, for example,
 422 setting its effective UID to a nonzero value),
 423 then subsequent memory allocations (e.g.,
 424 .BR mmap (2),
 425 .BR brk (2))
 426 will fail if the
 427 .B RLIMIT_MEMLOCK
 428 resource limit is encountered.
 429 .\" See the following LKML thread:
 430 .\" http://marc.theaimsgroup.com/?l=linux-kernel&m=113801392825023&w=2
 431 .\" "Rationale for RLIMIT_MEMLOCK"
 432 .\" 23 Jan 2006
 433 .SH VERSIONS
 434 .BR mlock2 (2)
 435 is available since Linux 4.4.
 436 .SH SEE ALSO
 437 .BR mmap (2),
 438 .BR setrlimit (2),
 439 .BR shmctl (2),
 440 .BR sysconf (3),
 441 .BR proc (5),
 442 .BR capabilities (7)