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 different 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 calling 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 policy 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 associated with the policy will be stored in the location pointed to by this  argument.
       maxnode  specifies  the  number 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 allocated 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 specified 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 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/.