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MADVISE(2)                 Linux Programmer's ManualMADVISE(2)NAME       madvise - give advice about use of memorySYNOPSIS       #include <sys/mman.h>       int madvise(void *addr, size_t length, int advice);   Feature Test Macro Requirements for glibc (seefeature_test_macros(7)):       madvise():           Since glibc 2.19:               _DEFAULT_SOURCE           Up to and including glibc 2.19:               _BSD_SOURCEDESCRIPTION       The  madvise()  system call is used to give advice or directions to the       kernel about the address range beginning at address addr and with  size       length  bytes In most cases, the goal of such advice is to improve sys-       tem or application performance.       Initially, the system call supported a  set  of  "conventional"  advice       values,  which  are  also  available  on several other implementations.       (Note, though, that madvise()  is  not  specified  in  POSIX.)   Subse-       quently, a number of Linux-specific advice values have been added.   Conventional advice values       The  advice values listed below allow 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.  These       advice values do not influence the semantics of the application (except       in  the case of MADV_DONTNEED), but may influence its performance.  All       of the advice values listed here have analogs  in  the  POSIX-specifiedposix_madvise(3)  function, and the values have the same meanings, with       the exception of MADV_DONTNEED.       The advice is indicated in the advice argument, which  is  one  of  the       following:       MADV_NORMAL              No special treatment.  This is the default.       MADV_RANDOM              Expect  page references in random order.  (Hence, read ahead may              be less useful than normally.)       MADV_SEQUENTIAL              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.)       MADV_WILLNEED              Expect access in the near future.  (Hence, it might  be  a  good              idea to read some pages ahead.)       MADV_DONTNEED              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.)              After  a  successful  MADV_DONTNEED  operation, the semantics of              memory access in the specified region  are  changed:  subsequent              accesses  of pages in the range will succeed, but will result in              either repopulating the memory contents from the up-to-date con-              tents  of  the underlying mapped file (for shared file mappings,              shared anonymous mappings, and shmem-based  techniques  such  as              System  V  shared  memory segments) or zero-fill-on-demand pages              for anonymous private mappings.              Note that, when applied to shared mappings, MADV_DONTNEED  might              not  lead  to  immediate freeing of the pages in the range.  The              kernel is free to delay freeing the pages until  an  appropriate              moment.  The resident set size (RSS) of the calling process will              be immediately reduced however.              MADV_DONTNEED cannot be applied to locked pages, Huge TLB pages,              or  VM_PFNMAP  pages.   (Pages  marked  with the kernel-internal              VM_PFNMAP flag are special memory areas that are not managed  by              the  virtual memory subsystem.  Such pages are typically created              by device drivers that map the pages into user space.)   Linux-specific advice values       The following Linux-specific advice values have no counterparts in  the       POSIX-specifiedposix_madvise(3), and may or may not have counterparts       in the madvise() interface available on  other  implementations.   Note       that some of these operations change the semantics of memory accesses.       MADV_REMOVE (since Linux 2.6.16)              Free up a given range of pages and its associated backing store.              This is equivalent to punching a hole in the corresponding  byte              range  of  the backing store (seefallocate(2)).  Subsequent ac-              cesses in the specified address range will see bytes  containing              zero.              The  specified address range must be mapped shared and writable.              This flag cannot be applied to locked pages, Huge TLB pages,  or              VM_PFNMAP pages.              In  the  initial  implementation,  onlytmpfs(5)  was supported              MADV_REMOVE; but since Linux 3.5, any filesystem which  supports              thefallocate(2)   FALLOC_FL_PUNCH_HOLE   mode  also  supports              MADV_REMOVE.  Hugetlbfs fails with the error  EINVAL  and  other              filesystems fail with the error EOPNOTSUPP.       MADV_DONTFORK (since Linux 2.6.16)              Do not make the pages in this range available to the child after              afork(2).  This is useful to  prevent  copy-on-write  semantics              from  changing  the  physical  location  of a page if the parent              writes to it after afork(2).   (Such  page  relocations  cause              problems for hardware that DMAs into the page.)       MADV_DOFORK (since Linux 2.6.16)              Undo  the  effect of MADV_DONTFORK, restoring the default behav-              ior, whereby a mapping is inherited acrossfork(2).       MADV_HWPOISON (since Linux 2.6.32)              Poison the pages in the range specified by addr and  length  and              handle subsequent references to those pages like a hardware mem-              ory 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  available  only 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 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 (seemmap(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   Linux   kernel   source   file  Documentation/admin-              guide/mm/ksm.rst 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_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 configured with              CONFIG_MEMORY_FAILURE.       MADV_HUGEPAGE (since Linux 2.6.38)              Enable Transparent Huge Pages (THP) for pages in the range spec-              ified  by  addr  and  length.  Currently, Transparent Huge Pages              work only with private anonymous pages (seemmap(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 (seeposix_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 2 MB mapping that only ever ac-              cesses 1 byte will result in 2 MB of wired memory instead of one              4 KB  page).  See the Linux kernel source file Documentation/ad-              min-guide/mm/transhuge.rst for more details.              The MADV_HUGEPAGE and MADV_NOHUGEPAGE operations  are  available              only   if   the  kernel  was  configured  with  CONFIG_TRANSPAR-              ENT_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.       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  (seecore(5)).       MADV_DODUMP (since Linux 3.4)              Undo the effect of an earlier MADV_DONTDUMP.       MADV_FREE (since Linux 4.5)              The application no longer requires the pages in the range speci-              fied by addr and len.  The kernel can thus free these pages, but              the freeing could be delayed until memory pressure occurs.   For              each  of  the pages that has been marked to be freed but has not              yet been freed, the free  operation  will  be  canceled  if  the              caller writes into the page.  After a successful MADV_FREE oper-              ation, any stale data (i.e., dirty,  unwritten  pages)  will  be              lost  when  the  kernel  frees  the  pages.  However, subsequent              writes to pages in the range will succeed and then kernel cannot              free those dirtied pages, so that the caller can always see just              written data.  If there is no subsequent write, the  kernel  can              free  the  pages at any time.  Once pages in the range have been              freed, the caller will see zero-fill-on-demand pages upon subse-              quent page references.              The MADV_FREE operation can be applied only to private anonymous              pages (seemmap(2)).  In Linux before version 4.12, when freeing              pages  on  a  swapless  system, the pages in the given range are              freed instantly, regardless of memory pressure.       MADV_WIPEONFORK (since Linux 4.14)              Present the child process with zero-filled memory in this  range              after  afork(2).  This is useful in forking servers in order to              ensure that sensitive per-process data (for example, PRNG seeds,              cryptographic  secrets,  and  so on) is not handed to child pro-              cesses.              The MADV_WIPEONFORK operation can be  applied  only  to  private              anonymous pages (seemmap(2)).              Within the child created byfork(2), the MADV_WIPEONFORK setting              remains in place on the specified address range.   This  setting              is cleared duringexecve(2).       MADV_KEEPONFORK (since Linux 4.14)              Undo the effect of an earlier MADV_WIPEONFORK.RETURN VALUE       On  success, madvise() returns zero.  On error, it returns -1 and errno       is set appropriately.ERRORS       EACCES advice is MADV_REMOVE, but the specified address range is not  a              shared writable mapping.       EAGAIN A kernel resource was temporarily unavailable.       EBADF  The map exists, but the area maps something that isn't a file.       EINVAL addr is not page-aligned or length is negative.       EINVAL advice is not a valid.       EINVAL advice is MADV_DONTNEED or MADV_REMOVE and the specified address              range includes locked, Huge TLB pages, or VM_PFNMAP pages.       EINVAL advice is MADV_MERGEABLE or MADV_UNMERGEABLE, but the kernel was              not configured with CONFIG_KSM.       EINVAL advice is MADV_FREE or MADV_WIPEONFORK but the specified address              range includes file, Huge TLB, MAP_SHARED, or VM_PFNMAP ranges.       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.       EPERM  advice is MADV_HWPOISON,  but  the  caller  does  not  have  the              CAP_SYS_ADMIN capability.VERSIONS       Since  Linux  3.18, support for this system call is optional, depending       on the setting of the CONFIG_ADVISE_SYSCALLS configuration option.CONFORMING TO       madvise() is not specified by any standards.  Versions of  this  system       call, implementing a wide variety of advice values, exist on many other       implementations.  Other implementations typically  implement  at  least       the  flags  listed  above  under Conventional advice flags, albeit with       some variation in semantics.       POSIX.1-2001 describesposix_madvise(3) with constants  POSIX_MADV_NOR-       MAL, POSIX_MADV_RANDOM, POSIX_MADV_SEQUENTIAL, POSIX_MADV_WILLNEED, and       POSIX_MADV_DONTNEED, and so on, with behavior close  to  the  similarly       named flags listed above.NOTES   Linux notes       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).SEE ALSOgetrlimit(2),mincore(2),mmap(2),mprotect(2),msync(2),munmap(2),prctl(2),posix_madvise(3),core(5)COLOPHON       This page is part of release 5.05 of the Linux  man-pages  project.   A       description  of  the project, information about reporting bugs, and the       latest    version    of    this    page,    can     be     found     at       https://www.kernel.org/doc/man-pages/.Linux                             2019-03-06MADVISE(2)