mbind(2) — Linux manual page

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

NAME         top

       mbind - set memory policy for a memory range

SYNOPSIS         top

       #include <numaif.h>

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

       Link with -lnuma.

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION         top

       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 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         top

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

ERRORS         top

       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         top

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

CONFORMING TO         top

       This system call is Linux-specific.

NOTES         top

       Glibc does not provide a wrapper for this system call.  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.

       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         top

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                       MBIND(2)

Pages that refer to this page: getcpu(2)get_mempolicy(2)migrate_pages(2)move_pages(2)set_mempolicy(2)swapon(2)syscalls(2)numa(3)proc(5)capabilities(7)cpuset(7)numa(7)migratepages(8)numactl(8)