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       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 file systems
              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
              behavior, 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 only available 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 only
              available    if    the    kernel     was     configured     with

       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
              preserved  (i.e.,  when  next accessed, the same content will be
              visible, 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 calling process.  This feature is intended  for  testing
              of  memory  error-handling  code;  it  is  only available if the
              kernel was configured 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
              single write-protected page (which is automatically copied if  a
              process  later  wants  to  update the content of the page).  KSM
              only merges private anonymous  pages  (see  mmap(2)).   The  KSM
              feature   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 kernel source  file  Documentation/vm/ksm.txt  for  more
              details.  The MADV_MERGEABLE and MADV_UNMERGEABLE operations are
              only available 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
              only work with  private  anonymous  pages  (see  mmap(2)).   The
              kernel  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  primarily  aimed  at  applications  that  use large
              mappings of data and access large regions of that  memory  at  a
              time  (e.g.  virtualization  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       kernel       source       file
              Documentation/vm/transhuge.txt    for    more    details.    The
              MADV_HUGEPAGE and MADV_NOHUGEPAGE operations are only  available
              if the kernel was configured with CONFIG_TRANSPARENT_HUGEPAGE.

       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.


       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, etc., with a behavior close to that described  here.
       There is a similar posix_fadvise(2) for file access.

       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
       madvise() 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)


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