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)