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Diffstat (limited to 'man2/mlock.2')
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diff --git a/man2/mlock.2 b/man2/mlock.2 deleted file mode 100644 index 965e018..0000000 --- a/man2/mlock.2 +++ /dev/null @@ -1,507 +0,0 @@ -.\" Copyright (C) Michael Kerrisk, 2004 -.\" using some material drawn from earlier man pages -.\" written by Thomas Kuhn, Copyright 1996 -.\" -.\" SPDX-License-Identifier: GPL-2.0-or-later -.\" -.TH mlock 2 2023-10-31 "Linux man-pages 6.7" -.SH NAME -mlock, mlock2, munlock, mlockall, munlockall \- lock and unlock memory -.SH LIBRARY -Standard C library -.RI ( libc ", " \-lc ) -.SH SYNOPSIS -.nf -.B #include <sys/mman.h> -.P -.BI "int mlock(const void " addr [. len "], size_t " len ); -.BI "int mlock2(const void " addr [. len "], size_t " len ", \ -unsigned int " flags ); -.BI "int munlock(const void " addr [. len "], size_t " len ); -.P -.BI "int mlockall(int " flags ); -.B int munlockall(void); -.fi -.SH DESCRIPTION -.BR mlock (), -.BR mlock2 (), -and -.BR mlockall () -lock part or all of the calling process's virtual address -space into RAM, preventing that memory from being paged to the -swap area. -.P -.BR munlock () -and -.BR munlockall () -perform the converse operation, -unlocking part or all of the calling process's virtual address space, -so that pages in the specified virtual address range -can be swapped out again if required by the kernel memory manager. -.P -Memory locking and unlocking are performed in units of whole pages. -.SS mlock(), mlock2(), and munlock() -.BR mlock () -locks pages in the address range starting at -.I addr -and continuing for -.I len -bytes. -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. -.P -.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. -.P -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. -.P -If -.I flags -is 0, -.BR mlock2 () -behaves exactly the same as -.BR mlock (). -.P -.BR munlock () -unlocks pages in the address range starting at -.I addr -and continuing for -.I len -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() -.BR mlockall () -locks all pages mapped into the address space of the -calling process. -This includes the pages of the code, data, and stack -segment, as well as shared libraries, user space kernel data, shared -memory, and memory-mapped files. -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. -.P -The -.I flags -argument is constructed as the bitwise OR of one or more of the -following constants: -.TP -.B MCL_CURRENT -Lock all pages which are currently mapped into the address space of -the process. -.TP -.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 -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. -.P -If -.B MCL_FUTURE -has been specified, then a later system call (e.g., -.BR mmap (2), -.BR sbrk (2), -.BR malloc (3)), -may fail if it would cause the number of locked bytes to exceed -the permitted maximum (see below). -In the same circumstances, stack growth may likewise fail: -the kernel will deny stack expansion and deliver a -.B SIGSEGV -signal to the process. -.P -.BR munlockall () -unlocks all pages mapped into the address space of the -calling process. -.SH RETURN VALUE -On success, these system calls return 0. -On error, \-1 is returned, -.I errno -is set to indicate the error, -and no changes are made to any locks in the -address space of the process. -.SH ERRORS -.\"SVr4 documents an additional EAGAIN error code. -.TP -.B EAGAIN -.RB ( mlock (), -.BR mlock2 (), -and -.BR munlock ()) -Some or all of the specified address range could not be locked. -.TP -.B EINVAL -.RB ( mlock (), -.BR mlock2 (), -and -.BR munlock ()) -The result of the addition -.IR addr + len -was less than -.I addr -(e.g., the addition may have resulted in an overflow). -.TP -.B EINVAL -.RB ( mlock2 ()) -Unknown \fIflags\fP were specified. -.TP -.B EINVAL -.RB ( mlockall ()) -Unknown \fIflags\fP were specified or -.B MCL_ONFAULT -was specified without either -.B MCL_FUTURE -or -.BR MCL_CURRENT . -.TP -.B EINVAL -(Not on Linux) -.I addr -was not a multiple of the page size. -.TP -.B ENOMEM -.RB ( mlock (), -.BR mlock2 (), -and -.BR munlock ()) -Some of the specified address range does not correspond to mapped -pages in the address space of the process. -.TP -.B ENOMEM -.RB ( mlock (), -.BR mlock2 (), -and -.BR munlock ()) -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.) -.TP -.B ENOMEM -(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. -This limit is not enforced if the process is privileged -.RB ( CAP_IPC_LOCK ). -.TP -.B ENOMEM -(Linux 2.4 and earlier) the calling process tried to lock more than -half of RAM. -.\" In the case of mlock(), this check is somewhat buggy: it doesn't -.\" take into account whether the to-be-locked range overlaps with -.\" already locked pages. Thus, suppose we allocate -.\" (num_physpages / 4 + 1) of memory, and lock those pages once using -.\" mlock(), and then lock the *same* page range a second time. -.\" In the case, the second mlock() call will fail, since the check -.\" calculates that the process is trying to lock (num_physpages / 2 + 2) -.\" pages, which of course is not true. (MTK, Nov 04, kernel 2.4.28) -.TP -.B EPERM -The caller is not privileged, but needs privilege -.RB ( CAP_IPC_LOCK ) -to perform the requested operation. -.TP -.B EPERM -.RB ( munlockall ()) -(Linux 2.6.8 and earlier) The caller was not privileged -.RB ( CAP_IPC_LOCK ). -.SH VERSIONS -.SS Linux -Under Linux, -.BR mlock (), -.BR mlock2 (), -and -.BR munlock () -automatically round -.I addr -down to the nearest page boundary. -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. -.P -The -.I VmLck -field of the Linux-specific -.IR /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 . -.SH STANDARDS -.TP -.BR mlock () -.TQ -.BR munlock () -.TQ -.BR mlockall () -.TQ -.BR munlockall () -POSIX.1-2008. -.TP -.BR mlock2 () -Linux. -.P -On POSIX systems on which -.BR mlock () -and -.BR munlock () -are available, -.B _POSIX_MEMLOCK_RANGE -is defined in \fI<unistd.h>\fP and the number of bytes in a page -can be determined from the constant -.B PAGESIZE -(if defined) in \fI<limits.h>\fP or by calling -.IR sysconf(_SC_PAGESIZE) . -.P -On POSIX systems on which -.BR mlockall () -and -.BR munlockall () -are available, -.B _POSIX_MEMLOCK -is defined in \fI<unistd.h>\fP to a value greater than 0. -(See also -.BR sysconf (3).) -.\" 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 HISTORY -.TP -.BR mlock () -.TQ -.BR munlock () -.TQ -.BR mlockall () -.TQ -.BR munlockall () -POSIX.1-2001, POSIX.1-2008, SVr4. -.TP -.BR mlock2 () -Linux 4.4, -glibc 2.27. -.SH NOTES -Memory locking has two main applications: real-time algorithms and -high-security data processing. -Real-time applications require -deterministic timing, and, like scheduling, paging is one major cause -of unexpected program execution delays. -Real-time applications will -usually also switch to a real-time scheduler with -.BR sched_setscheduler (2). -Cryptographic security software often handles critical bytes like -passwords or secret keys as data structures. -As a result of paging, -these secrets could be transferred onto a persistent swap store medium, -where they might be accessible to the enemy long after the security -software has erased the secrets in RAM and terminated. -(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.) -.P -Real-time processes that are using -.BR mlockall () -to prevent delays on page faults should reserve enough -locked stack pages before entering the time-critical section, -so that no page fault can be caused by function calls. -This can be achieved by calling a function that allocates a -sufficiently large automatic variable (an array) and writes to the -memory occupied by this array in order to touch these stack pages. -This way, enough pages will be mapped for the stack and can be -locked into RAM. -The dummy writes ensure that not even copy-on-write -page faults can occur in the critical section. -.P -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). -.P -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\[em]not even from a thread which runs at a low priority within -a process which also has a thread running at elevated priority. -.P -The memory lock on an address range is automatically removed -if the address range is unmapped via -.BR munmap (2). -.P -Memory locks do not stack, that is, pages which have been locked several times -by calls to -.BR mlock (), -.BR mlock2 (), -or -.BR mlockall () -will be unlocked by a single call to -.BR munlock () -for the corresponding range or by -.BR munlockall (). -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. -.P -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. -.P -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 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. -.P -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 -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 -.B 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. -.P -In Linux 2.4 series of kernels up to and including Linux 2.4.17, -a bug caused the -.BR mlockall () -.B MCL_FUTURE -flag to be inherited across a -.BR fork (2). -This was rectified in Linux 2.4.18. -.P -Since Linux 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 nonzero value), -then subsequent memory allocations (e.g., -.BR mmap (2), -.BR brk (2)) -will fail if the -.B RLIMIT_MEMLOCK -resource limit is encountered. -.\" See the following LKML thread: -.\" http://marc.theaimsgroup.com/?l=linux-kernel&m=113801392825023&w=2 -.\" "Rationale for RLIMIT_MEMLOCK" -.\" 23 Jan 2006 -.SH SEE ALSO -.BR mincore (2), -.BR mmap (2), -.BR setrlimit (2), -.BR shmctl (2), -.BR sysconf (3), -.BR proc (5), -.BR capabilities (7) |