GET_MEMPOLICY(2)          (2017-09-15)           GET_MEMPOLICY(2)

     NAME
          get_mempolicy - retrieve NUMA memory policy for a thread

     SYNOPSIS
          #include <numaif.h>

          long get_mempolicy(int *mode, unsigned long *nodemask,
                            unsigned long maxnode, void *addr,
                            unsigned long flags);

          Link with -lnuma.

     DESCRIPTION
          get_mempolicy() retrieves the NUMA policy of the calling
          thread or of a memory address, depending on the setting of
          flags.

          A NUMA machine has different memory controllers with differ-
          ent distances to specific CPUs.  The memory policy defines
          from which node memory is allocated for the thread.

          If flags is specified as 0, then information about the call-
          ing thread's default policy (as set by set_mempolicy(2)) is
          returned, in the buffers pointed to by mode and nodemask.
          The value returned in these arguments may be used to restore
          the thread's policy to its state at the time of the call to
          get_mempolicy() using set_mempolicy(2).  When flags is 0,
          addr must be specified as NULL.

          If flags specifies MPOL_F_MEMS_ALLOWED (available since
          Linux 2.6.24), the mode argument is ignored and the set of
          nodes (memories) that the thread is allowed to specify in
          subsequent calls to mbind(2) or set_mempolicy(2) (in the
          absence of any mode flags) is returned in nodemask. It is
          not permitted to combine MPOL_F_MEMS_ALLOWED with either
          MPOL_F_ADDR or MPOL_F_NODE.

          If flags specifies MPOL_F_ADDR, then information is returned
          about the policy governing the memory address given in addr.
          This policy may be different from the thread's default pol-
          icy if mbind(2) or one of the helper functions described in
          numa(3) has been used to establish a policy for the memory
          range containing addr.

          If the mode argument is not NULL, then get_mempolicy() will
          store the policy mode and any optional mode flags of the
          requested NUMA policy in the location pointed to by this
          argument.  If nodemask is not NULL, then the nodemask asso-
          ciated with the policy will be stored in the location
          pointed to by this argument.  maxnode specifies the number

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          of node IDs that can be stored into nodemask-that is, the
          maximum node ID plus one.  The value specified by maxnode is
          always rounded to a multiple of sizeof(unsigned long)*8.

          If flags specifies both MPOL_F_NODE and MPOL_F_ADDR,
          get_mempolicy() will return the node ID of the node on which
          the address addr is allocated into the location pointed to
          by mode. If no page has yet been allocated for the specified
          address, get_mempolicy() will allocate a page as if the
          thread had performed a read (load) access to that address,
          and return the ID of the node where that page was allocated.

          If flags specifies MPOL_F_NODE, but not MPOL_F_ADDR, and the
          thread's current policy is MPOL_INTERLEAVE, then
          get_mempolicy() will return in the location pointed to by a
          non-NULL mode argument, the node ID of the next node that
          will be used for interleaving of internal kernel pages allo-
          cated on behalf of the thread.  These allocations include
          pages for memory-mapped files in process memory ranges
          mapped using the mmap(2) call with the MAP_PRIVATE flag for
          read accesses, and in memory ranges mapped with the
          MAP_SHARED flag for all accesses.

          Other flag values are reserved.

          For an overview of the possible policies see
          set_mempolicy(2).

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

     ERRORS
          EFAULT
               Part of all of the memory range specified by nodemask
               and maxnode points outside your accessible address
               space.

          EINVAL
               The value specified by maxnode is less than the number
               of node IDs supported by the system.  Or flags speci-
               fied values other than MPOL_F_NODE or MPOL_F_ADDR; or
               flags specified MPOL_F_ADDR and addr is NULL, or flags
               did not specify MPOL_F_ADDR and addr is not NULL.  Or,
               flags specified MPOL_F_NODE but not MPOL_F_ADDR and the
               current thread policy is not MPOL_INTERLEAVE.  Or,
               flags specified MPOL_F_MEMS_ALLOWED with either
               MPOL_F_ADDR or MPOL_F_NODE.  (And there are other
               EINVAL cases.)

     VERSIONS
          The get_mempolicy() system call was added to the Linux

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          kernel in version 2.6.7.

     CONFORMING TO
          This system call is Linux-specific.

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

     SEE ALSO
          getcpu(2), mbind(2), mmap(2), set_mempolicy(2), numa(3),
          numa(7), numactl(8)

     COLOPHON
          This page is part of release 5.10 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/.

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