# madvise

## NAME



           madvise - give advice about use of memory
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/mman.h>**
    
           **int madvise(** size_t size;
                       **void** _addr_**[**_size_**], size_t** _size_**, int** _advice_**);**
    
       Feature Test Macro Requirements for glibc (see
       feature_test_macros(7)):
    
           **madvise**():
               Since glibc 2.19:
                   _DEFAULT_SOURCE
               Up to and including glibc 2.19:
                   _BSD_SOURCE
    


## DESCRIPTION



           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 _size_.  **madvise**() only operates on whole pages, therefore
           _addr_ must be page-aligned.  The value of _size_ is rounded up to a
           multiple of page size.  In most cases, the goal of such advice is
           to improve system 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.)  Subsequently, 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-specified posix_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 contents 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, 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.)
    
                  Support for Huge TLB pages was added in Linux v5.18.
                  Addresses within a mapping backed by Huge TLB pages must be
                  aligned to the underlying Huge TLB page size, and the range
                  size is rounded up to a multiple of the underlying Huge TLB
                  page size.
    
       **Linux-specific advice values**
           The following Linux-specific _advice_ values have no counterparts in
           the POSIX-specified posix_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 range of the backing store (see
                  fallocate(2)).  Subsequent accesses in the specified
                  address range will see data with a value of zero.
    
                  The specified address range must be mapped shared and
                  writable.  This flag cannot be applied to locked pages, or
                  **VM_PFNMAP** pages.
    
                  In the initial implementation, only tmpfs(5) supported
                  **MADV_REMOVE** ; but since Linux 3.5, any filesystem which
                  supports the fallocate(2) **FALLOC_FL_PUNCH_HOLE** mode also
                  supports **MADV_REMOVE**.  Filesystems which do not support
                  **MADV_REMOVE** fail with the error **EOPNOTSUPP**.
    
                  Support for the Huge TLB filesystem was added in Linux
                  v4.3.
    
           **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 if
                  the parent writes to it after a fork(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
                  behavior, whereby a mapping is inherited across fork(2).
    
           **MADV_HWPOISON**(since Linux 2.6.32)
                  Poison the pages in the range specified by _addr_ and _size_
                  and handle subsequent references to those pages 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 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 _size_.  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 merges only 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 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 _size_.
    
           **MADV_SOFT_OFFLINE**(since Linux 2.6.33)
                  Soft offline the pages in the range specified by _addr_ and
                  _size_.  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 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
                  specified by _addr_ and _size_.  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 2 MB
                  mapping that only ever accesses 1 byte will result in 2 MB
                  of wired memory instead of one 4 KB page).  See the Linux
                  kernel source file
                  _Documentation/admin-guide/mm/transhuge.rst_ for more
                  details.
    
                  Most common kernels configurations provide **MADV_HUGEPAGE** -
                  style behavior by default, and thus **MADV_HUGEPAGE** is
                  normally not necessary.  It is mostly intended for embedded
                  systems, where **MADV_HUGEPAGE** -style behavior may not be
                  enabled by default in the kernel.  On such systems, this
                  flag can be used in order to selectively enable THP.
                  Whenever **MADV_HUGEPAGE** is used, it should always be in
                  regions of memory with an access pattern that the developer
                  knows in advance won't risk to increase the memory
                  footprint of the application when transparent hugepages are
                  enabled.
    
                  Since Linux 5.4, automatic scan of eligible areas and
                  replacement by huge pages works with private anonymous
                  pages (see mmap(2)), shmem pages, and file-backed pages.
                  For all memory types, memory may only be replaced by huge
                  pages on hugepage-aligned boundaries.  For file-mapped
                  memory —including tmpfs (see **tmpfs**(2))— the mapping must
                  also be naturally hugepage-aligned within the file.
                  Additionally, for file-backed, non-tmpfs memory, the file
                  must not be open for write and the mapping must be
                  executable.
    
                  The VMA must not be marked **VM_NOHUGEPAGE** , **VM_HUGETLB** ,
                  **VM_IO** , **VM_DONTEXPAND** , **VM_MIXEDMAP** , or **VM_PFNMAP** , nor can it
                  be stack memory or backed by a DAX-enabled device (unless
                  the DAX device is hot-plugged as System RAM).  The process
                  must also not have **PR_SET_THP_DISABLE** set (see prctl(2)).
    
                  The **MADV_HUGEPAGE** , **MADV_NOHUGEPAGE** , and **MADV_COLLAPSE**
                  operations are available only if the kernel was configured
                  with **CONFIG_TRANSPARENT_HUGEPAGE** and file/shmem memory is
                  only supported if the kernel was configured with
                  **CONFIG_READ_ONLY_THP_FOR_FS**.
    
           **MADV_NOHUGEPAGE**(since Linux 2.6.38)
                  Ensures that memory in the address range specified by _addr_
                  and _size_ will not be backed by transparent hugepages.
    
           **MADV_COLLAPSE**(since Linux 6.1)
                  Perform a best-effort synchronous collapse of the native
                  pages mapped by the memory range into Transparent Huge
                  Pages (THPs).  **MADV_COLLAPSE** operates on the current state
                  of memory of the calling process and makes no persistent
                  changes or guarantees on how pages will be mapped,
                  constructed, or faulted in the future.
    
                  **MADV_COLLAPSE** supports private anonymous pages (see
                  mmap(2)), shmem pages, and file-backed pages.  See
                  **MADV_HUGEPAGE** for general information on memory
                  requirements for THP.  If the range provided spans multiple
                  VMAs, the semantics of the collapse over each VMA is
                  independent from the others.  If collapse of a given huge
                  page-aligned/sized region fails, the operation may continue
                  to attempt collapsing the remainder of the specified
                  memory.  **MADV_COLLAPSE** will automatically clamp the
                  provided range to be hugepage-aligned.
    
                  All non-resident pages covered by the range will first be
                  swapped/faulted-in, before being copied onto a freshly
                  allocated hugepage.  If the native pages compose the same
                  PTE-mapped hugepage, and are suitably aligned, allocation
                  of a new hugepage may be elided and collapse may happen in-
                  place.  Unmapped pages will have their data directly
                  initialized to 0 in the new hugepage.  However, for every
                  eligible hugepage-aligned/sized region to be collapsed, at
                  least one page must currently be backed by physical memory.
    
                  **MADV_COLLAPSE** is independent of any sysfs (see sysfs(5))
                  setting under _/sys/kernel/mm/transparent_hugepage_ , both in
                  terms of determining THP eligibility, and allocation
                  semantics.  See Linux kernel source file
                  _Documentation/admin-guide/mm/transhuge.rst_ for more
                  information.  **MADV_COLLAPSE** also ignores **huge=** tmpfs mount
                  when operating on tmpfs files.  Allocation for the new
                  hugepage may enter direct reclaim and/or compaction,
                  regardless of VMA flags (though **VM_NOHUGEPAGE** is still
                  respected).
    
                  When the system has multiple NUMA nodes, the hugepage will
                  be allocated from the node providing the most native pages.
    
                  If all hugepage-sized/aligned regions covered by the
                  provided range were either successfully collapsed, or were
                  already PMD-mapped THPs, this operation will be deemed
                  successful.  Note that this doesn't guarantee anything
                  about other possible mappings of the memory.  In the event
                  multiple hugepage-aligned/sized areas fail to collapse,
                  only the most-recently–failed code will be set in _errno_.
    
           **MADV_DONTDUMP**(since Linux 3.4)
                  Exclude from a core dump those pages in the range specified
                  by _addr_ and _size_.  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 core(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
                  specified by _addr_ and _size_.  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** operation, 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 subsequent page references.
    
                  The **MADV_FREE** operation can be applied only to private
                  anonymous pages (see mmap(2)).  Before Linux 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 a fork(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 processes.
    
                  The **MADV_WIPEONFORK** operation can be applied only to
                  private anonymous pages (see mmap(2)).
    
                  Within the child created by fork(2), the **MADV_WIPEONFORK**
                  setting remains in place on the specified address range.
                  This setting is cleared during execve(2).
    
           **MADV_KEEPONFORK**(since Linux 4.14)
                  Undo the effect of an earlier **MADV_WIPEONFORK**.
    
           **MADV_COLD**(since Linux 5.4)
                  Deactivate a given range of pages.  This will make the
                  pages a more probable reclaim target should there be a
                  memory pressure.  This is a nondestructive operation.  The
                  advice might be ignored for some pages in the range when it
                  is not applicable.
    
           **MADV_PAGEOUT**(since Linux 5.4)
                  Reclaim a given range of pages.  This is done to free up
                  memory occupied by these pages.  If a page is anonymous, it
                  will be swapped out.  If a page is file-backed and dirty,
                  it will be written back to the backing storage.  The advice
                  might be ignored for some pages in the range when it is not
                  applicable.
    
           **MADV_POPULATE_READ**(since Linux 5.14)
                  "Populate (prefault) page tables readable, faulting in all
                  pages in the range just as if manually reading from each
                  page; however, avoid the actual memory access that would
                  have been performed after handling the fault.
    
                  In contrast to **MAP_POPULATE** , **MADV_POPULATE_READ** does not
                  hide errors, can be applied to (parts of) existing mappings
                  and will always populate (prefault) page tables readable.
                  One example use case is prefaulting a file mapping, reading
                  all file content from disk; however, pages won't be dirtied
                  and consequently won't have to be written back to disk when
                  evicting the pages from memory.
    
                  Depending on the underlying mapping, map the shared
                  zeropage, preallocate memory or read the underlying file;
                  files with holes might or might not preallocate blocks.  If
                  populating fails, a **SIGBUS** signal is not generated;
                  instead, an error is returned.
    
                  If **MADV_POPULATE_READ** succeeds, all page tables have been
                  populated (prefaulted) readable once.  If
                  **MADV_POPULATE_READ** fails, some page tables might have been
                  populated.
    
                  **MADV_POPULATE_READ** cannot be applied to mappings without
                  read permissions and special mappings, for example,
                  mappings marked with kernel-internal flags such as
                  **VM_PFNMAP** or **VM_IO** , or secret memory regions created using
                  memfd_secret(2).
    
                  Note that with **MADV_POPULATE_READ** , the process can be
                  killed at any moment when the system runs out of memory.
    
           **MADV_POPULATE_WRITE**(since Linux 5.14)
                  Populate (prefault) page tables writable, faulting in all
                  pages in the range just as if manually writing to each each
                  page; however, avoid the actual memory access that would
                  have been performed after handling the fault.
    
                  In contrast to **MAP_POPULATE** , MADV_POPULATE_WRITE does not
                  hide errors, can be applied to (parts of) existing mappings
                  and will always populate (prefault) page tables writable.
                  One example use case is preallocating memory, breaking any
                  CoW (Copy on Write).
    
                  Depending on the underlying mapping, preallocate memory or
                  read the underlying file; files with holes will preallocate
                  blocks.  If populating fails, a **SIGBUS** signal is not
                  generated; instead, an error is returned.
    
                  If **MADV_POPULATE_WRITE** succeeds, all page tables have been
                  populated (prefaulted) writable once.  If
                  **MADV_POPULATE_WRITE** fails, some page tables might have been
                  populated.
    
                  **MADV_POPULATE_WRITE** cannot be applied to mappings without
                  write permissions and special mappings, for example,
                  mappings marked with kernel-internal flags such as
                  **VM_PFNMAP** or **VM_IO** , or secret memory regions created using
                  memfd_secret(2).
    
                  Note that with **MADV_POPULATE_WRITE** , the process can be
                  killed at any moment when the system runs out of memory.
    
           **MADV_GUARD_INSTALL**(since Linux 6.13)
                  Install a lightweight guard region into the range specified
                  by _addr_ and _size_ , causing any read or write in the range to
                  result in a **SIGSEGV** signal being raised.
    
                  If the region maps memory pages those mappings will be
                  replaced as part of the operation, though if
                  **MADV_GUARD_INSTALL** is applied to regions containing pre-
                  existing lightweight guard regions, they are left in place.
    
                  Prior to Linux 6.15, this operation was supported only for
                  writable anonymous private mappings.  Since Linux 6.15,
                  both anonymous and file-backed mappings are supported,
                  including read-only mappings.
    
                  The mapping must not be mlock'd, map hugetlb ranges, nor
                  contain special mappings.  For example, mappings marked
                  with kernel-internal flags such as **VM_PFNMAP** or **VM_IO** , or
                  secret memory regions created using memfd_secret(2).
    
                  An **EINVAL** error is returned if it is attempted on any other
                  kind of mapping.
    
                  This operation is more efficient than mapping a new region
                  of memory **PROT_NONE** , as it does not require the
                  establishment of new mappings.  Instead, regions of an
                  existing mapping simply have their page tables manipulated
                  to establish the desired behavior.  No additional memory is
                  used.
    
                  Lightweight guard regions remain on fork (except for any
                  parts which have had **MADV_WIPEONFORK** applied to them), and
                  are not removed by **MADV_DONTNEED** , **MADV_FREE** , **MADV_PAGEOUT** ,
                  or **MADV_COLD**.
    
                  Attempting to mlock(2) lightweight guard regions will fail,
                  as will **MADV_POPULATE_READ** or **MADV_POPULATE_WRITE**.
    
                  If the mapping has its attributes changed, or is split or
                  partially unmapped, any existing guard regions remain in
                  place (except if they are unmapped).
    
                  If a mapping is moved using mremap(2), lightweight guard
                  regions are moved with it.
    
                  Lightweight guard regions are removed when unmapped, on
                  process teardown, or when the **MADV_GUARD_REMOVE** operation
                  is applied to them.
    
           **MADV_GUARD_REMOVE**(since Linux 6.13)
                  Remove any lightweight guard regions which exist in the
                  range specified by _addr_ and _size_.
    
                  All mappings in the range other than lightweight guard
                  regions are left in place.  The operation is supported on
                  those mappings permitted by **MADV_GUARD_INSTALL** in addition
                  to mlock()'d mappings, returning an **EINVAL** error otherwise.
    
                  When lightweight guard regions are removed, they act as
                  empty regions of the containing mapping.  Therefore,
                  anonymous private mappings become zero-fill-on-demand
                  pages, and file-backed mappings are repopulated with the
                  memory contents from the up-to-date contents of the
                  underlying mapped file.
    
                  If any transparent huge pages are encountered in the
                  operation, they are left in place.
    


## RETURN VALUE



           On success, **madvise**() returns zero.  On error, it returns -1 and
           _errno_ is set to indicate the error.
    


## 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.
    
           **EBUSY**(for **MADV_COLLAPSE**) Could not charge hugepage to cgroup:
                  cgroup limit exceeded.
    
           **EBUSY**(for **MADV_SOFT_OFFLINE**) Any pages within the specified
                  address range could not be offlined.  This might occur if
                  the page is currently in use or locked.
    
           **EFAULT** _advice_ is **MADV_POPULATE_READ** or **MADV_POPULATE_WRITE** , and
                  populating (prefaulting) page tables failed because a
                  **SIGBUS** would have been generated on actual memory access
                  and the reason is not a HW poisoned page (HW poisoned pages
                  can, for example, be created using the **MADV_HWPOISON** flag
                  described elsewhere in this page).
    
           **EINVAL** _addr_ is not page-aligned or _size_ is negative.
    
           **EINVAL** _advice_ is not a valid.
    
           **EINVAL** _advice_ is **MADV_COLD** or **MADV_PAGEOUT** and the specified
                  address range includes locked, Huge TLB pages, or **VM_PFNMAP**
                  pages.
    
           **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.
    
           **EINVAL** _advice_ is **MADV_POPULATE_READ** or **MADV_POPULATE_WRITE** , but
                  the specified address range includes ranges with
                  insufficient permissions or special mappings, for example,
                  mappings marked with kernel-internal flags such a **VM_IO** or
                  **VM_PFNMAP** , or secret memory regions created using
                  memfd_secret(2).
    
           **EINVAL** _advice_ is **MADV_GUARD_INSTALL** or **MADV_GUARD_REMOVE** , but the
                  specified address range contains an unsupported mapping.
    
           **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**(for **MADV_COLLAPSE**) Not enough memory: could not allocate
                  hugepage.
    
           **ENOMEM** Addresses in the specified range are not currently mapped,
                  or are outside the address space of the process.
    
           **ENOMEM** _advice_ is **MADV_POPULATE_READ** or **MADV_POPULATE_WRITE** , and
                  populating (prefaulting) page tables failed because there
                  was not enough memory.
    
           **EPERM** _advice_ is **MADV_HWPOISON** , but the caller does not have the
                  **CAP_SYS_ADMIN** capability.
    
           **EHWPOISON**
                  _advice_ is **MADV_POPULATE_READ** or **MADV_POPULATE_WRITE** , and
                  populating (prefaulting) page tables failed because a HW
                  poisoned page (HW poisoned pages can, for example, be
                  created using the **MADV_HWPOISON** flag described elsewhere in
                  this page) was encountered.
    


## VERSIONS



           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 describes posix_madvise(3) with constants
           **POSIX_MADV_NORMAL** , **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.
    
       **Linux**
           The Linux implementation requires that the address _addr_ be page-
           aligned, and allows _size_ 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).
    
           _madvise(0, 0, advice)_ will return zero iff _advice_ is supported by
           the kernel and can be relied on to probe for support.
    


## STANDARDS



           None.
    


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