MBIND(2)



MBIND(2)                   Linux Programmer's Manual                  MBIND(2)

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  allo-
       cated.  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 speci-
       fied 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 be-
       low.

       The mode argument must specify one of MPOL_DEFAULT, MPOL_BIND, MPOL_IN-
       TERLEAVE, 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 sup-
       ported 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  cur-
              rent 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  ig-
       nored.  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 de-
              fault policy will be used.  The system-wide default policy allo-
              cates 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 allo-
              cation to the nodes specified in nodemask.  If  nodemask  speci-
              fies  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 nu-
              meric 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  speci-
              fied 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 ker-
              nel 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 al-
              locate 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  re-
              verts  to  the memory policy of the thread (which may be set via
              set_mempolicy(2)); that policy may be something other than  "lo-
              cal allocation".

       If MPOL_MF_STRICT is passed in flags and mode is not MPOL_DEFAULT, then
       the call fails 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 fails 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  fails  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 maxn-
              ode 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 node-
              mask 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  speci-
              fied 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 lo-
       cal 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  effec-
       tively  removes  the explicit policy from the specified range, "falling
       back" to a possibly nondefault policy.  To select explicit "local allo-
       cation"  for  a memory range, specify a mode of MPOL_LOCAL or MPOL_PRE-
       FERRED with an empty set of nodes.  This method will work for  set_mem-
       policy(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.

       Before Linux 5.7.  MPOL_MF_STRICT was 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 5.07 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                             2020-06-09                          MBIND(2)

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