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NAME

       mbind - set memory policy for a memory range

SYNOPSIS

       #include <numaif.h>

       long mbind(void *addr, unsigned long len, int mode,
                  const unsigned long *nodemask, unsigned long maxnode,
                  unsigned flags);

       Link with -lnuma.

DESCRIPTION

       mbind()  sets  the  NUMA  memory  policy, which consists of a policy mode and zero or more
       nodes, for the memory range starting with addr and continuing for len bytes.   The  memory
       policy defines from which node memory is allocated.

       If the memory range specified by the addr and len arguments includes an "anonymous" region
       of memory—that is a region of memory created  using  the  mmap(2)  system  call  with  the
       MAP_ANONYMOUS—or  a  memory-mapped  file,  mapped  using  the mmap(2) system call with the
       MAP_PRIVATE flag, pages will be allocated only according to the specified policy when  the
       application  writes  (stores)  to the page.  For anonymous regions, an initial read access
       will use a shared page in the kernel  containing  all  zeros.   For  a  file  mapped  with
       MAP_PRIVATE,  an initial read access will allocate pages according to the memory policy of
       the thread that causes the page to be allocated.  This may not be the thread  that  called
       mbind().

       The  specified  policy will be ignored for any MAP_SHARED mappings in the specified memory
       range.  Rather the pages will be allocated according to the memory policy  of  the  thread
       that  caused  the  page  to  be  allocated.  Again, this may not be the thread that called
       mbind().

       If the specified memory range includes a shared memory region created using the  shmget(2)
       system call and attached using the shmat(2) system call, pages allocated for the anonymous
       or shared memory region will be allocated according to the policy specified, regardless of
       which  process  attached to the shared memory segment causes the allocation.  If, however,
       the shared memory region was created with the SHM_HUGETLB flag, the  huge  pages  will  be
       allocated  according  to the policy specified only if the page allocation is caused by the
       process that calls mbind() for that region.

       By default, mbind() has an effect only for new allocations; if the pages inside the  range
       have  been already touched before setting the policy, then the policy has no effect.  This
       default behavior  may  be  overridden  by  the  MPOL_MF_MOVE  and  MPOL_MF_MOVE_ALL  flags
       described below.

       The   mode   argument  must  specify  one  of  MPOL_DEFAULT,  MPOL_BIND,  MPOL_INTERLEAVE,
       MPOL_PREFERRED, or MPOL_LOCAL (which are described in detail  below).   All  policy  modes
       except  MPOL_DEFAULT  require  the  caller  to specify the node or nodes to which the mode
       applies, via the nodemask argument.

       The mode argument may also include an optional mode flag.  The supported mode flags are:

       MPOL_F_STATIC_NODES (since Linux-2.6.26)
              A nonempty nodemask specifies physical node IDs.  Linux does not remap the nodemask
              when  the  thread  moves  to  a different cpuset context, nor when the set of nodes
              allowed by the thread's current cpuset context changes.

       MPOL_F_RELATIVE_NODES (since Linux-2.6.26)
              A nonempty nodemask specifies node IDs that are relative to the  set  of  node  IDs
              allowed by the thread's current cpuset.

       nodemask  points  to a bit mask of nodes containing up to maxnode bits.  The bit mask size
       is rounded to the next multiple of sizeof(unsigned long), but the  kernel  will  use  bits
       only  up  to  maxnode.   A NULL value of nodemask or a maxnode value of zero specifies the
       empty set of nodes.  If the value of maxnode is zero, the nodemask  argument  is  ignored.
       Where  a  nodemask is required, it must contain at least one node that is on-line, allowed
       by the thread's current cpuset  context  (unless  the  MPOL_F_STATIC_NODES  mode  flag  is
       specified), and contains memory.

       The mode argument must include one of the following values:

       MPOL_DEFAULT
              This  mode  requests  that  any  nondefault  policy  be  removed, restoring default
              behavior.  When applied to a range of memory via mbind(), this  means  to  use  the
              thread  memory  policy, which may have been set with set_mempolicy(2).  If the mode
              of the thread memory policy is also MPOL_DEFAULT, the  system-wide  default  policy
              will  be  used.   The system-wide default policy allocates pages on the node of the
              CPU that triggers the allocation.   For  MPOL_DEFAULT,  the  nodemask  and  maxnode
              arguments must be specify the empty set of nodes.

       MPOL_BIND
              This  mode  specifies a strict policy that restricts memory allocation to the nodes
              specified in nodemask.  If nodemask specifies more than one node, page  allocations
              will  come  from  the  node with sufficient free memory that is closest to the node
              where the allocation takes place.  Pages will not be allocated from  any  node  not
              specified  in  the  IR nodemask .  (Before Linux 2.6.26, page allocations came from
              the node with the lowest numeric node ID first, until that node contained  no  free
              memory.   Allocations  then  came  from  the  node  with  the  next highest node ID
              specified in nodemask and so forth, until none of  the  specified  nodes  contained
              free memory.)

       MPOL_INTERLEAVE
              This  mode  specifies  that page allocations be interleaved across the set of nodes
              specified in  nodemask.   This  optimizes  for  bandwidth  instead  of  latency  by
              spreading  out  pages and memory accesses to those pages across multiple nodes.  To
              be effective the memory area should be fairly large, at least 1 MB or bigger with a
              fairly uniform access pattern.  Accesses to a single page of the area will still be
              limited to the memory bandwidth of a single node.

       MPOL_PREFERRED
              This mode sets the preferred node for allocation.  The kernel will try to  allocate
              pages  from  this node first and fall back to other nodes if the preferred nodes is
              low on free memory.  If nodemask specifies more than one node ID, the first node in
              the  mask  will  be  selected  as  the preferred node.  If the nodemask and maxnode
              arguments specify the empty set, then the memory is allocated on the  node  of  the
              CPU that triggered the allocation.

       MPOL_LOCAL (since Linux 3.8)
              This  mode specifies "local allocation"; the memory is allocated on the node of the
              CPU that triggered the allocation (the "local node").   The  nodemask  and  maxnode
              arguments  must  specify the empty set.  If the "local node" is low on free memory,
              the kernel will try to allocate memory from other nodes.  The kernel will  allocate
              memory  from  the  "local node" whenever memory for this node is available.  If the
              "local node" is not allowed by the thread's current cpuset context, the kernel will
              try  to allocate memory from other nodes.  The kernel will allocate memory from the
              "local node" whenever it becomes allowed by the thread's  current  cpuset  context.
              By  contrast, MPOL_DEFAULT reverts to the memory policy of the thread (which may be
              set  via  set_mempolicy(2));  that  policy  may  be  something  other  than  "local
              allocation".

       If MPOL_MF_STRICT is passed in flags and mode is not MPOL_DEFAULT, then the call will fail
       with the error EIO if the existing pages in the memory range don't follow the policy.

       If MPOL_MF_MOVE is specified in flags, then the  kernel  will  attempt  to  move  all  the
       existing  pages in the memory range so that they follow the policy.  Pages that are shared
       with other processes will not be moved.  If MPOL_MF_STRICT is  also  specified,  then  the
       call will fail with the error EIO if some pages could not be moved.

       If  MPOL_MF_MOVE_ALL is passed in flags, then the kernel will attempt to move all existing
       pages in the memory range regardless of  whether  other  processes  use  the  pages.   The
       calling  thread  must be privileged (CAP_SYS_NICE) to use this flag.  If MPOL_MF_STRICT is
       also specified, then the call will fail with the error EIO if  some  pages  could  not  be
       moved.

RETURN VALUE

       On  success,  mbind() returns 0; on error, -1 is returned and errno is set to indicate the
       error.

ERRORS

       EFAULT Part or all of the memory range specified by nodemask and  maxnode  points  outside
              your  accessible  address  space.   Or, there was an unmapped hole in the specified
              memory range specified by addr and len.

       EINVAL An invalid value was specified for flags or mode; or addr + len was less than addr;
              or  addr  is  not a multiple of the system page size.  Or, mode is MPOL_DEFAULT and
              nodemask specified a nonempty set; or mode  is  MPOL_BIND  or  MPOL_INTERLEAVE  and
              nodemask  is  empty.   Or,  maxnode  exceeds  a kernel-imposed limit.  Or, nodemask
              specifies one or more node IDs that are greater than the maximum supported node ID.
              Or,  none  of  the  node  IDs  specified by nodemask are on-line and allowed by the
              thread's current cpuset context, or none of the  specified  nodes  contain  memory.
              Or, the mode argument specified both MPOL_F_STATIC_NODES and MPOL_F_RELATIVE_NODES.

       EIO    MPOL_MF_STRICT  was  specified and an existing page was already on a node that does
              not follow the policy; or MPOL_MF_MOVE or MPOL_MF_MOVE_ALL was  specified  and  the
              kernel was unable to move all existing pages in the range.

       ENOMEM Insufficient kernel memory was available.

       EPERM  The  flags argument included the MPOL_MF_MOVE_ALL flag and the caller does not have
              the CAP_SYS_NICE privilege.

VERSIONS

       The mbind() system call was added to the Linux kernel in version 2.6.7.

CONFORMING TO

       This system call is Linux-specific.

NOTES

       For information on library support, see numa(7).

       NUMA policy is not supported on a memory-mapped  file  range  that  was  mapped  with  the
       MAP_SHARED flag.

       The  MPOL_DEFAULT  mode can have different effects for mbind() and set_mempolicy(2).  When
       MPOL_DEFAULT is specified for set_mempolicy(2), the thread's memory policy reverts to  the
       system  default policy or local allocation.  When MPOL_DEFAULT is specified for a range of
       memory using mbind(), any pages  subsequently  allocated  for  that  range  will  use  the
       thread's memory policy, as set by set_mempolicy(2).  This effectively removes the explicit
       policy from the specified range, "falling back"  to  a  possibly  nondefault  policy.   To
       select  explicit  "local  allocation"  for a memory range, specify a mode of MPOL_LOCAL or
       MPOL_PREFERRED with an empty set of nodes.  This method will work for set_mempolicy(2), as
       well.

       Support for huge page policy was added with 2.6.16.  For interleave policy to be effective
       on huge page mappings the policied memory needs to be tens of megabytes or larger.

       MPOL_MF_STRICT is ignored on huge page mappings.

       MPOL_MF_MOVE and MPOL_MF_MOVE_ALL are available only on Linux 2.6.16 and later.

SEE ALSO

       get_mempolicy(2), getcpu(2),  mmap(2),  set_mempolicy(2),  shmat(2),  shmget(2),  numa(3),
       cpuset(7), numa(7), numactl(8)

COLOPHON

       This  page  is  part of release 4.13 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/.