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NAME

       mbind - Set memory policy for a memory range

SYNOPSIS

       #include <numaif.h>

       int mbind(void *addr, unsigned long len, int mode,
                 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 only be allocated 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 process policy of
       the process that causes the page to be allocated.  This may not be the process 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 process policy of the  process
       that  caused  the  page  to  be allocated.  Again, this may not be the process 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
       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() only has an effect 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 or
       MPOL_PREFERRED.  All policy modes except MPOL_DEFAULT require the caller  to  specify  via
       the nodemask argument, the node or nodes to which the mode applies.

       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 process moves to a different cpuset context, nor when  the  set  of  nodes
              allowed by the process'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 process's current cpuset.

       nodemask points to a bitmask 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 only use bits
       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
       process's  current cpuset context [unless the MPOL_F_STATIC_NODES mode flag is specified],
       and contains memory.

       The MPOL_DEFAULT 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 process
       policy, which may have been set with set_mempolicy(2).  If the mode of the process  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.

       The MPOL_BIND 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 the lowest numeric node ID first, until that node contains no free
       memory.  Allocations will then come from the node with the next highest node ID  specified
       in  nodemask  and  so forth, until none of the specified nodes contain free memory.  Pages
       will not be allocated from any node not specified in the nodemask.

       The MPOL_INTERLEAVE 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 1MB 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  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.  This is the only way to specify "local allocation" for a range of memory  via
       mbind().

       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  process 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.

       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
              process'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 process's  policy  reverts  to  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  process's
       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_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 only available 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 3.35 of the Linux man-pages project.  A  description  of  the
       project,  and information about reporting bugs, can be found at http://man7.org/linux/man-
       pages/.