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
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  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 .\" commit a8ca5d0ecbdde5cc3d7accacbd69968b0c98764e
  69 .\" commit de60f5f10c58d4f34b68622442c0e04180367f3f
  70 .\" commit b0f205c2a3082dd9081f9a94e50658c5fa906ff1
  71 also locks pages in the specified range starting at
  72 .I addr
  73 and continuing for
  74 .I len
  75 bytes.
  76 However, the state of the pages contained in that range after the call
  77 returns successfully will depend on the value in the
  78 .I flags
  79 argument.
  80
  81 The
  82 .I flags
  83 argument can be either 0 or the following constant:
  84 .TP
  85 .B MLOCK_ONFAULT
  86 Lock pages that are currently resident and mark the entire range to have
  87 pages locked when they are populated by the page fault.
  88 .PP
  89
  90 If
  91 .I flags
  92 is 0,
  93 .BR mlock2 ()
  94 behaves exactly the same as
  95 .BR mlock ().
  96
  97 Note: currently, there is not a glibc wrapper for
  98 .BR mlock2 (),
  99 so it will need to be invoked using
 100 .BR syscall (2).
 101
 102 .BR munlock ()
 103 unlocks pages in the address range starting at
 104 .I addr
 105 and continuing for
 106 .I len
 107 bytes.
 108 After this call, all pages that contain a part of the specified
 109 memory range can be moved to external swap space again by the kernel.
 110 .SS mlockall() and munlockall()
 111 .BR mlockall ()
 112 locks all pages mapped into the address space of the
 113 calling process.
 114 This includes the pages of the code, data and stack
 115 segment, as well as shared libraries, user space kernel data, shared
 116 memory, and memory-mapped files.
 117 All mapped pages are guaranteed
 118 to be resident in RAM when the call returns successfully;
 119 the pages are guaranteed to stay in RAM until later unlocked.
 120
 121 The
 122 .I flags
 123 argument is constructed as the bitwise OR of one or more of the
 124 following constants:
 125 .TP 1.2i
 126 .B MCL_CURRENT
 127 Lock all pages which are currently mapped into the address space of
 128 the process.
 129 .TP
 130 .B MCL_FUTURE
 131 Lock all pages which will become mapped into the address space of the
 132 process in the future.
 133 These could be, for instance, new pages required
 134 by a growing heap and stack as well as new memory-mapped files or
 135 shared memory regions.
 136 .TP
 137 .BR MCL_ONFAULT " (since Linux 4.4)"
 138 Used together with
 139 .BR MCL_CURRENT ,
 140 .BR MCL_FUTURE ,
 141 or both.
 142 Mark all current (with
 143 .BR MCL_CURRENT )
 144 or future (with
 145 .BR MCL_FUTURE )
 146 mappings to lock pages when they are faulted in.
 147 When used with
 148 .BR MCL_CURRENT ,
 149 all present pages are locked, but
 150 .BR mlockall ()
 151 will not fault in non-present pages.
 152 When used with
 153 .BR MCL_FUTURE ,
 154 all future mappings will be marked to lock pages when they are faulted
 155 in, but they will not be populated by the lock when the mapping is
 156 created.
 157 .B MCL_ONFAULT
 158 must be used with either
 159 .B MCL_CURRENT
 160 or
 161 .B MCL_FUTURE
 162 or both.
 163 .PP
 164 If
 165 .B MCL_FUTURE
 166 has been specified, then a later system call (e.g.,
 167 .BR mmap (2),
 168 .BR sbrk (2),
 169 .BR malloc (3)),
 170 may fail if it would cause the number of locked bytes to exceed
 171 the permitted maximum (see below).
 172 In the same circumstances, stack growth may likewise fail:
 173 the kernel will deny stack expansion and deliver a
 174 .B SIGSEGV
 175 signal to the process.
 176
 177 .BR munlockall ()
 178 unlocks all pages mapped into the address space of the
 179 calling process.
 180 .SH RETURN VALUE
 181 On success, these system calls return 0.
 182 On error, \-1 is returned,
 183 .I errno
 184 is set appropriately, and no changes are made to any locks in the
 185 address space of the process.
 186 .SH ERRORS
 187 .TP
 188 .B ENOMEM
 189 (Linux 2.6.9 and later) the caller had a nonzero
 190 .B RLIMIT_MEMLOCK
 191 soft resource limit, but tried to lock more memory than the limit
 192 permitted.
 193 This limit is not enforced if the process is privileged
 194 .RB ( CAP_IPC_LOCK ).
 195 .TP
 196 .B ENOMEM
 197 (Linux 2.4 and earlier) the calling process tried to lock more than
 198 half of RAM.
 199 .\" In the case of mlock(), this check is somewhat buggy: it doesn't
 200 .\" take into account whether the to-be-locked range overlaps with
 201 .\" already locked pages.  Thus, suppose we allocate
 202 .\" (num_physpages / 4 + 1) of memory, and lock those pages once using
 203 .\" mlock(), and then lock the *same* page range a second time.
 204 .\" In the case, the second mlock() call will fail, since the check
 205 .\" calculates that the process is trying to lock (num_physpages / 2 + 2)
 206 .\" pages, which of course is not true.  (MTK, Nov 04, kernel 2.4.28)
 207 .TP
 208 .B EPERM
 209 The caller is not privileged, but needs privilege
 210 .RB ( CAP_IPC_LOCK )
 211 to perform the requested operation.
 212 .\"SVr4 documents an additional EAGAIN error code.
 213 .LP
 214 For
 215 .BR mlock (),
 216 .BR mlock2 (),
 217 and
 218 .BR munlock ():
 219 .TP
 220 .B EAGAIN
 221 Some or all of the specified address range could not be locked.
 222 .TP
 223 .B EINVAL
 224 The result of the addition
 225 .IR addr + len
 226 was less than
 227 .IR addr
 228 (e.g., the addition may have resulted in an overflow).
 229 .TP
 230 .B EINVAL
 231 (Not on Linux)
 232 .I addr
 233 was not a multiple of the page size.
 234 .TP
 235 .B ENOMEM
 236 Some of the specified address range does not correspond to mapped
 237 pages in the address space of the process.
 238 .TP
 239 .B ENOMEM
 240 Locking or unlocking a region would result in the total number of
 241 mappings with distinct attributes (e.g., locked versus unlocked)
 242 exceeding the allowed maximum.
 243 .\" I.e., the number of VMAs would exceed the 64kB maximum
 244 (For example, unlocking a range in the middle of a currently locked
 245 mapping would result in three mappings:
 246 two locked mappings at each end and an unlocked mapping in the middle.)
 247 .LP
 248 For
 249 .BR mlock2 ():
 250 .TP
 251 .B EINVAL
 252 Unknown \fIflags\fP were specified.
 253 .LP
 254 For
 255 .BR mlockall ():
 256 .TP
 257 .B EINVAL
 258 Unknown \fIflags\fP were specified or
 259 .B MCL_ONFAULT
 260 was specified without either
 261 .B MCL_FUTURE
 262 or
 263 .BR MCL_CURRENT .
 264 .LP
 265 For
 266 .BR munlockall ():
 267 .TP
 268 .B EPERM
 269 (Linux 2.6.8 and earlier) The caller was not privileged
 270 .RB ( CAP_IPC_LOCK ).
 271 .SH VERSIONS
 272 .BR mlock2 (2)
 273 is available since Linux 4.4.
 274 .SH CONFORMING TO
 275 POSIX.1-2001, POSIX.1-2008, SVr4.
 276 .SH AVAILABILITY
 277 On POSIX systems on which
 278 .BR mlock ()
 279 and
 280 .BR munlock ()
 281 are available,
 282 .B _POSIX_MEMLOCK_RANGE
 283 is defined in \fI<unistd.h>\fP and the number of bytes in a page
 284 can be determined from the constant
 285 .B PAGESIZE
 286 (if defined) in \fI<limits.h>\fP or by calling
 287 .IR sysconf(_SC_PAGESIZE) .
 288
 289 On POSIX systems on which
 290 .BR mlockall ()
 291 and
 292 .BR munlockall ()
 293 are available,
 294 .B _POSIX_MEMLOCK
 295 is defined in \fI<unistd.h>\fP to a value greater than 0.
 296 (See also
 297 .BR sysconf (3).)
 298 .\" POSIX.1-2001: It shall be defined to -1 or 0 or 200112L.
 299 .\" -1: unavailable, 0: ask using sysconf().
 300 .\" glibc defines it to 1.
 301 .SH NOTES
 302 Memory locking has two main applications: real-time algorithms and
 303 high-security data processing.
 304 Real-time applications require
 305 deterministic timing, and, like scheduling, paging is one major cause
 306 of unexpected program execution delays.
 307 Real-time applications will
 308 usually also switch to a real-time scheduler with
 309 .BR sched_setscheduler (2).
 310 Cryptographic security software often handles critical bytes like
 311 passwords or secret keys as data structures.
 312 As a result of paging,
 313 these secrets could be transferred onto a persistent swap store medium,
 314 where they might be accessible to the enemy long after the security
 315 software has erased the secrets in RAM and terminated.
 316 (But be aware that the suspend mode on laptops and some desktop
 317 computers will save a copy of the system's RAM to disk, regardless
 318 of memory locks.)
 319
 320 Real-time processes that are using
 321 .BR mlockall ()
 322 to prevent delays on page faults should reserve enough
 323 locked stack pages before entering the time-critical section,
 324 so that no page fault can be caused by function calls.
 325 This can be achieved by calling a function that allocates a
 326 sufficiently large automatic variable (an array) and writes to the
 327 memory occupied by this array in order to touch these stack pages.
 328 This way, enough pages will be mapped for the stack and can be
 329 locked into RAM.
 330 The dummy writes ensure that not even copy-on-write
 331 page faults can occur in the critical section.
 332
 333 Memory locks are not inherited by a child created via
 334 .BR fork (2)
 335 and are automatically removed (unlocked) during an
 336 .BR execve (2)
 337 or when the process terminates.
 338 The
 339 .BR mlockall ()
 340 .B MCL_FUTURE
 341 and
 342 .B MCL_FUTURE | MCL_ONFAULT
 343 settings are not inherited by a child created via
 344 .BR fork (2)
 345 and are cleared during an
 346 .BR execve (2).
 347
 348 The memory lock on an address range is automatically removed
 349 if the address range is unmapped via
 350 .BR munmap (2).
 351
 352 Memory locks do not stack, that is, pages which have been locked several times
 353 by calls to
 354 .BR mlock (),
 355 .BR mlock2 (),
 356 or
 357 .BR mlockall ()
 358 will be unlocked by a single call to
 359 .BR munlock ()
 360 for the corresponding range or by
 361 .BR munlockall ().
 362 Pages which are mapped to several locations or by several processes stay
 363 locked into RAM as long as they are locked at least at one location or by
 364 at least one process.
 365
 366 If a call to
 367 .BR mlockall ()
 368 which uses the
 369 .B MCL_FUTURE
 370 flag is followed by another call that does not specify this flag, the
 371 changes made by the
 372 .B MCL_FUTURE
 373 call will be lost.
 374 .SS Linux notes
 375 Under Linux,
 376 .BR mlock (),
 377 .BR mlock2 (),
 378 and
 379 .BR munlock ()
 380 automatically round
 381 .I addr
 382 down to the nearest page boundary.
 383 However, the POSIX.1 specification of
 384 .BR mlock ()
 385 and
 386 .BR munlock ()
 387 allows an implementation to require that
 388 .I addr
 389 is page aligned, so portable applications should ensure this.
 390
 391 The
 392 .I VmLck
 393 field of the Linux-specific
 394 .I /proc/PID/status
 395 file shows how many kilobytes of memory the process with ID
 396 .I PID
 397 has locked using
 398 .BR mlock (),
 399 .BR mlock2 (),
 400 .BR mlockall (),
 401 and
 402 .BR mmap (2)
 403 .BR MAP_LOCKED .
 404 .SS Limits and permissions
 405 In Linux 2.6.8 and earlier,
 406 a process must be privileged
 407 .RB ( CAP_IPC_LOCK )
 408 in order to lock memory and the
 409 .B RLIMIT_MEMLOCK
 410 soft resource limit defines a limit on how much memory the process may lock.
 411
 412 Since Linux 2.6.9, no limits are placed on the amount of memory
 413 that a privileged process can lock and the
 414 .B RLIMIT_MEMLOCK
 415 soft resource limit instead defines a limit on how much memory an
 416 unprivileged process may lock.
 417 .SH BUGS
 418 In the 2.4 series Linux kernels up to and including 2.4.17,
 419 a bug caused the
 420 .BR mlockall ()
 421 .B MCL_FUTURE
 422 flag to be inherited across a
 423 .BR fork (2).
 424 This was rectified in kernel 2.4.18.
 425
 426 Since kernel 2.6.9, if a privileged process calls
 427 .I mlockall(MCL_FUTURE)
 428 and later drops privileges (loses the
 429 .B CAP_IPC_LOCK
 430 capability by, for example,
 431 setting its effective UID to a nonzero value),
 432 then subsequent memory allocations (e.g.,
 433 .BR mmap (2),
 434 .BR brk (2))
 435 will fail if the
 436 .B RLIMIT_MEMLOCK
 437 resource limit is encountered.
 438 .\" See the following LKML thread:
 439 .\" http://marc.theaimsgroup.com/?l=linux-kernel&m=113801392825023&w=2
 440 .\" "Rationale for RLIMIT_MEMLOCK"
 441 .\" 23 Jan 2006
 442 .SH SEE ALSO
 443 .BR mmap (2),
 444 .BR setrlimit (2),
 445 .BR shmctl (2),
 446 .BR sysconf (3),
 447 .BR proc (5),
 448 .BR capabilities (7)