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MADVISE(2)		   Linux Programmer's Manual		    MADVISE(2)

       madvise - give advice about use of memory

       #include <sys/mman.h>

       int madvise(void *addr, size_t length, int advice);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       madvise(): _BSD_SOURCE

       The madvise() system call advises the kernel about how to handle paging
       input/output in the address range beginning at address  addr  and  with
       size  length bytes.  It allows an application to tell the kernel how it
       expects to use some mapped or shared memory areas, so that  the	kernel
       can  choose  appropriate	 read-ahead and caching techniques.  This call
       does not influence the semantics of the application (except in the case
       of  MADV_DONTNEED),  but	 may influence its performance.	 The kernel is
       free to ignore the advice.

       The advice is indicated in the advice argument which can be

	      No special treatment.  This is the default.

	      Expect page references in random order.  (Hence, read ahead  may
	      be less useful than normally.)

	      Expect  page  references	in sequential order.  (Hence, pages in
	      the given range can be aggressively read ahead, and may be freed
	      soon after they are accessed.)

	      Expect  access  in  the near future.  (Hence, it might be a good
	      idea to read some pages ahead.)

	      Do not expect access in the near future.	(For the  time	being,
	      the  application is finished with the given range, so the kernel
	      can free resources associated with it.)  Subsequent accesses  of
	      pages  in	 this  range  will  succeed, but will result either in
	      reloading of the memory contents from the underlying mapped file
	      (see  mmap(2)) or zero-fill-on-demand pages for mappings without
	      an underlying file.

       MADV_REMOVE (since Linux 2.6.16)
	      Free up a given range of pages and its associated backing store.
	      Currently,  only	shmfs/tmpfs  supports  this; other filesystems
	      return with the error ENOSYS.

       MADV_DONTFORK (since Linux 2.6.16)
	      Do not make the pages in this range available to the child after
	      a	 fork(2).   This  is useful to prevent copy-on-write semantics
	      from changing the physical location of a page(s) if  the	parent
	      writes  to  it  after  a	fork(2).  (Such page relocations cause
	      problems for hardware that DMAs into the page(s).)

       MADV_DOFORK (since Linux 2.6.16)
	      Undo the effect of MADV_DONTFORK, restoring the  default	behav‐
	      ior, whereby a mapping is inherited across fork(2).

       MADV_HWPOISON (since Linux 2.6.32)
	      Poison  a	 page and handle it like a hardware memory corruption.
	      This operation is available only for privileged  (CAP_SYS_ADMIN)
	      processes.   This	 operation  may	 result in the calling process
	      receiving a SIGBUS and the page being unmapped.  This feature is
	      intended for testing of memory error-handling code; it is avail‐
	      able only if the kernel was configured with  CONFIG_MEMORY_FAIL‐

       MADV_SOFT_OFFLINE (since Linux 2.6.33)
	      Soft  offline  the  pages	 in  the  range	 specified by addr and
	      length.  The memory of each page in the specified range is  pre‐
	      served (i.e., when next accessed, the same content will be visi‐
	      ble, but in a new physical page frame), and the original page is
	      offlined	(i.e.,	no longer used, and taken out of normal memory
	      management).  The effect of the MADV_SOFT_OFFLINE	 operation  is
	      invisible	 to (i.e., does not change the semantics of) the call‐
	      ing process.  This feature is intended  for  testing  of	memory
	      error-handling code; it is available only if the kernel was con‐
	      figured with CONFIG_MEMORY_FAILURE.

       MADV_MERGEABLE (since Linux 2.6.32)
	      Enable Kernel Samepage Merging (KSM) for the pages in the	 range
	      specified	 by addr and length.  The kernel regularly scans those
	      areas of user memory that have been marked as mergeable, looking
	      for  pages with identical content.  These are replaced by a sin‐
	      gle write-protected page (which is  automatically	 copied	 if  a
	      process  later  wants  to	 update the content of the page).  KSM
	      merges only private anonymous pages (see mmap(2)).  The KSM fea‐
	      ture  is	intended for applications that generate many instances
	      of the same data (e.g., virtualization systems such as KVM).  It
	      can  consume  a lot of processing power; use with care.  See the
	      Linux  kernel  source  file  Documentation/vm/ksm.txt  for  more
	      details.	The MADV_MERGEABLE and MADV_UNMERGEABLE operations are
	      available only if the kernel was configured with CONFIG_KSM.

       MADV_UNMERGEABLE (since Linux 2.6.32)
	      Undo the effect of an earlier MADV_MERGEABLE  operation  on  the
	      specified	 address  range;  KSM  unmerges	 whatever pages it had
	      merged in the address range specified by addr and length.

       MADV_HUGEPAGE (since Linux 2.6.38)
	      Enables Transparent Huge Pages (THP)  for	 pages	in  the	 range
	      specified by addr and length.  Currently, Transparent Huge Pages
	      work only with private anonymous pages (see mmap(2)).  The  ker‐
	      nel will regularly scan the areas marked as huge page candidates
	      to replace them with huge pages.	The kernel will also  allocate
	      huge  pages directly when the region is naturally aligned to the
	      huge page size (see posix_memalign(2)).  This feature is primar‐
	      ily  aimed  at  applications that use large mappings of data and
	      access large regions of that memory at a time (e.g., virtualiza‐
	      tion  systems  such  as  QEMU).  It can very easily waste memory
	      (e.g., a 2MB mapping that only ever accesses 1 byte will	result
	      in  2MB of wired memory instead of one 4KB page).	 See the Linux
	      kernel  source  file  Documentation/vm/transhuge.txt  for	  more
	      details.	 The  MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are
	      available only if the kernel was configured  with	 CONFIG_TRANS‐

       MADV_NOHUGEPAGE (since Linux 2.6.38)
	      Ensures  that  memory in the address range specified by addr and
	      length will not be collapsed into huge pages.

       MADV_DONTDUMP (since Linux 3.4)
	      Exclude from a core dump those pages in the range	 specified  by
	      addr and length.	This is useful in applications that have large
	      areas of memory that are known not to be useful in a core	 dump.
	      The  effect  of MADV_DONTDUMP takes precedence over the bit mask
	      that  is	set  via  the  /proc/PID/coredump_filter   file	  (see

       MADV_DODUMP (since Linux 3.4)
	      Undo the effect of an earlier MADV_DONTDUMP.

       On  success  madvise() returns zero.  On error, it returns -1 and errno
       is set appropriately.

       EAGAIN A kernel resource was temporarily unavailable.

       EBADF  The map exists, but the area maps something that isn't a file.

       EINVAL This error can occur for the following reasons:

	      *	 The value len is negative.

	      *	 addr is not page-aligned.

	      *	 advice is not a valid value

	      *	 The application is attempting to  release  locked  or	shared
		 pages (with MADV_DONTNEED).

	      *	 MADV_MERGEABLE	 or  MADV_UNMERGEABLE was specified in advice,
		 but the kernel was not configured with CONFIG_KSM.

       EIO    (for  MADV_WILLNEED)  Paging  in	this  area  would  exceed  the
	      process's maximum resident set size.

       ENOMEM (for MADV_WILLNEED) Not enough memory: paging in failed.

       ENOMEM Addresses	 in  the  specified range are not currently mapped, or
	      are outside the address space of the process.

       POSIX.1b.   POSIX.1-2001	 describes  posix_madvise(3)  with   constants
       POSIX_MADV_NORMAL,  POSIX_MADV_RANDOM, and so on, with a behavior close
       to that described here.	There is a similar posix_fadvise(2)  for  file

       and MADV_UNMERGEABLE are Linux-specific.

   Linux notes
       The current Linux implementation (2.4.0) views this system call more as
       a  command  than as advice and hence may return an error when it cannot
       do what it usually would do in  response	 to  this  advice.   (See  the
       ERRORS description above.)  This is nonstandard behavior.

       The  Linux  implementation  requires  that  the	address	 addr be page-
       aligned, and allows length to be zero.  If there are some parts of  the
       specified  address range that are not mapped, the Linux version of mad‐
       vise() ignores them and applies the  call  to  the  rest	 (but  returns
       ENOMEM from the system call, as it should).

       getrlimit(2),  mincore(2),  mmap(2),  mprotect(2), msync(2), munmap(2),
       prctl(2), core(5)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-04-20			    MADVISE(2)

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