Document Linux Memory Policy

I couldn't find any memory policy documentation in the Documentation
directory, so here is my attempt to document it.

There's lots more that could be written about the internal design--including
data structures, functions, etc. However, if you agree that this is better
that the nothing that exists now, perhaps it could be merged. This will
provide a baseline for updates to document the many policy patches that are
currently being worked.

Signed-off-by: Lee Schermerhorn <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Michael Kerrisk <[e-mail]>
Acked-by: Rob Landley <[e-mail]>
Acked-by: Mel Gorman <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/vm/numa_memory_policy.txt

···
+
+What is Linux Memory Policy?
+
+In the Linux kernel, "memory policy" determines from which node the kernel will
+allocate memory in a NUMA system or in an emulated NUMA system.  Linux has
+supported platforms with Non-Uniform Memory Access architectures since 2.4.?.
+The current memory policy support was added to Linux 2.6 around May 2004.  This
+document attempts to describe the concepts and APIs of the 2.6 memory policy
+support.
+
+Memory policies should not be confused with cpusets (Documentation/cpusets.txt)
+which is an administrative mechanism for restricting the nodes from which
+memory may be allocated by a set of processes. Memory policies are a
+programming interface that a NUMA-aware application can take advantage of.  When
+both cpusets and policies are applied to a task, the restrictions of the cpuset
+takes priority.  See "MEMORY POLICIES AND CPUSETS" below for more details.
+
+MEMORY POLICY CONCEPTS
+
+Scope of Memory Policies
+
+The Linux kernel supports _scopes_ of memory policy, described here from
+most general to most specific:
+
+    System Default Policy:  this policy is "hard coded" into the kernel.  It
+    is the policy that governs all page allocations that aren't controlled
+    by one of the more specific policy scopes discussed below.  When the
+    system is "up and running", the system default policy will use "local
+    allocation" described below.  However, during boot up, the system
+    default policy will be set to interleave allocations across all nodes
+    with "sufficient" memory, so as not to overload the initial boot node
+    with boot-time allocations.
+
+    Task/Process Policy:  this is an optional, per-task policy.  When defined
+    for a specific task, this policy controls all page allocations made by or
+    on behalf of the task that aren't controlled by a more specific scope.
+    If a task does not define a task policy, then all page allocations that
+    would have been controlled by the task policy "fall back" to the System
+    Default Policy.
+
+	The task policy applies to the entire address space of a task. Thus,
+	it is inheritable, and indeed is inherited, across both fork()
+	[clone() w/o the CLONE_VM flag] and exec*().  This allows a parent task
+	to establish the task policy for a child task exec()'d from an
+	executable image that has no awareness of memory policy.  See the
+	MEMORY POLICY APIS section, below, for an overview of the system call
+	that a task may use to set/change it's task/process policy.
+
+	In a multi-threaded task, task policies apply only to the thread
+	[Linux kernel task] that installs the policy and any threads
+	subsequently created by that thread.  Any sibling threads existing
+	at the time a new task policy is installed retain their current
+	policy.
+
+	A task policy applies only to pages allocated after the policy is
+	installed.  Any pages already faulted in by the task when the task
+	changes its task policy remain where they were allocated based on
+	the policy at the time they were allocated.
+
+    VMA Policy:  A "VMA" or "Virtual Memory Area" refers to a range of a task's
+    virtual adddress space.  A task may define a specific policy for a range
+    of its virtual address space.   See the MEMORY POLICIES APIS section,
+    below, for an overview of the mbind() system call used to set a VMA
+    policy.
+
+    A VMA policy will govern the allocation of pages that back this region of
+    the address space.  Any regions of the task's address space that don't
+    have an explicit VMA policy will fall back to the task policy, which may
+    itself fall back to the System Default Policy.
+
+    VMA policies have a few complicating details:
+
+	VMA policy applies ONLY to anonymous pages.  These include pages
+	allocated for anonymous segments, such as the task stack and heap, and
+	any regions of the address space mmap()ed with the MAP_ANONYMOUS flag.
+	If a VMA policy is applied to a file mapping, it will be ignored if
+	the mapping used the MAP_SHARED flag.  If the file mapping used the
+	MAP_PRIVATE flag, the VMA policy will only be applied when an
+	anonymous page is allocated on an attempt to write to the mapping--
+	i.e., at Copy-On-Write.
+
+	VMA policies are shared between all tasks that share a virtual address
+	space--a.k.a. threads--independent of when the policy is installed; and
+	they are inherited across fork().  However, because VMA policies refer
+	to a specific region of a task's address space, and because the address
+	space is discarded and recreated on exec*(), VMA policies are NOT
+	inheritable across exec().  Thus, only NUMA-aware applications may
+	use VMA policies.
+
+	A task may install a new VMA policy on a sub-range of a previously
+	mmap()ed region.  When this happens, Linux splits the existing virtual
+	memory area into 2 or 3 VMAs, each with it's own policy.
+
+	By default, VMA policy applies only to pages allocated after the policy
+	is installed.  Any pages already faulted into the VMA range remain
+	where they were allocated based on the policy at the time they were
+	allocated.  However, since 2.6.16, Linux supports page migration via
+	the mbind() system call, so that page contents can be moved to match
+	a newly installed policy.
+
+    Shared Policy:  Conceptually, shared policies apply to "memory objects"
+    mapped shared into one or more tasks' distinct address spaces.  An
+    application installs a shared policies the same way as VMA policies--using
+    the mbind() system call specifying a range of virtual addresses that map
+    the shared object.  However, unlike VMA policies, which can be considered
+    to be an attribute of a range of a task's address space, shared policies
+    apply directly to the shared object.  Thus, all tasks that attach to the
+    object share the policy, and all pages allocated for the shared object,
+    by any task, will obey the shared policy.
+
+	As of 2.6.22, only shared memory segments, created by shmget() or
+	mmap(MAP_ANONYMOUS|MAP_SHARED), support shared policy.  When shared
+	policy support was added to Linux, the associated data structures were
+	added to hugetlbfs shmem segments.  At the time, hugetlbfs did not
+	support allocation at fault time--a.k.a lazy allocation--so hugetlbfs
+	shmem segments were never "hooked up" to the shared policy support.
+	Although hugetlbfs segments now support lazy allocation, their support
+	for shared policy has not been completed.
+
+	As mentioned above [re: VMA policies], allocations of page cache
+	pages for regular files mmap()ed with MAP_SHARED ignore any VMA
+	policy installed on the virtual address range backed by the shared
+	file mapping.  Rather, shared page cache pages, including pages backing
+	private mappings that have not yet been written by the task, follow
+	task policy, if any, else System Default Policy.
+
+	The shared policy infrastructure supports different policies on subset
+	ranges of the shared object.  However, Linux still splits the VMA of
+	the task that installs the policy for each range of distinct policy.
+	Thus, different tasks that attach to a shared memory segment can have
+	different VMA configurations mapping that one shared object.  This
+	can be seen by examining the /proc/<pid>/numa_maps of tasks sharing
+	a shared memory region, when one task has installed shared policy on
+	one or more ranges of the region.
+
+Components of Memory Policies
+
+    A Linux memory policy is a tuple consisting of a "mode" and an optional set
+    of nodes.  The mode determine the behavior of the policy, while the
+    optional set of nodes can be viewed as the arguments to the behavior.
+
+   Internally, memory policies are implemented by a reference counted
+   structure, struct mempolicy.  Details of this structure will be discussed
+   in context, below, as required to explain the behavior.
+
+	Note:  in some functions AND in the struct mempolicy itself, the mode
+	is called "policy".  However, to avoid confusion with the policy tuple,
+	this document will continue to use the term "mode".
+
+   Linux memory policy supports the following 4 behavioral modes:
+
+	Default Mode--MPOL_DEFAULT:  The behavior specified by this mode is
+	context or scope dependent.
+
+	    As mentioned in the Policy Scope section above, during normal
+	    system operation, the System Default Policy is hard coded to
+	    contain the Default mode.
+
+	    In this context, default mode means "local" allocation--that is
+	    attempt to allocate the page from the node associated with the cpu
+	    where the fault occurs.  If the "local" node has no memory, or the
+	    node's memory can be exhausted [no free pages available], local
+	    allocation will "fallback to"--attempt to allocate pages from--
+	    "nearby" nodes, in order of increasing "distance".
+
+		Implementation detail -- subject to change:  "Fallback" uses
+		a per node list of sibling nodes--called zonelists--built at
+		boot time, or when nodes or memory are added or removed from
+		the system [memory hotplug].  These per node zonelist are
+		constructed with nodes in order of increasing distance based
+		on information provided by the platform firmware.
+
+	    When a task/process policy or a shared policy contains the Default
+	    mode, this also means "local allocation", as described above.
+
+	    In the context of a VMA, Default mode means "fall back to task
+	    policy"--which may or may not specify Default mode.  Thus, Default
+	    mode can not be counted on to mean local allocation when used
+	    on a non-shared region of the address space.  However, see
+	    MPOL_PREFERRED below.
+
+	    The Default mode does not use the optional set of nodes.
+
+	MPOL_BIND:  This mode specifies that memory must come from the
+	set of nodes specified by the policy.
+
+	    The memory policy APIs do not specify an order in which the nodes
+	    will be searched.  However, unlike "local allocation", the Bind
+	    policy does not consider the distance between the nodes.  Rather,
+	    allocations will fallback to the nodes specified by the policy in
+	    order of numeric node id.  Like everything in Linux, this is subject
+	    to change.
+
+	MPOL_PREFERRED:  This mode specifies that the allocation should be
+	attempted from the single node specified in the policy.  If that
+	allocation fails, the kernel will search other nodes, exactly as
+	it would for a local allocation that started at the preferred node
+	in increasing distance from the preferred node.  "Local" allocation
+	policy can be viewed as a Preferred policy that starts at the node
+	containing the cpu where the allocation takes place.
+
+	    Internally, the Preferred policy uses a single node--the
+	    preferred_node member of struct mempolicy.  A "distinguished
+	    value of this preferred_node, currently '-1', is interpreted
+	    as "the node containing the cpu where the allocation takes
+	    place"--local allocation.  This is the way to specify
+	    local allocation for a specific range of addresses--i.e. for
+	    VMA policies.
+
+	MPOL_INTERLEAVED:  This mode specifies that page allocations be
+	interleaved, on a page granularity, across the nodes specified in
+	the policy.  This mode also behaves slightly differently, based on
+	the context where it is used:
+
+	    For allocation of anonymous pages and shared memory pages,
+	    Interleave mode indexes the set of nodes specified by the policy
+	    using the page offset of the faulting address into the segment
+	    [VMA] containing the address modulo the number of nodes specified
+	    by the policy.  It then attempts to allocate a page, starting at
+	    the selected node, as if the node had been specified by a Preferred
+	    policy or had been selected by a local allocation.  That is,
+	    allocation will follow the per node zonelist.
+
+	    For allocation of page cache pages, Interleave mode indexes the set
+	    of nodes specified by the policy using a node counter maintained
+	    per task.  This counter wraps around to the lowest specified node
+	    after it reaches the highest specified node.  This will tend to
+	    spread the pages out over the nodes specified by the policy based
+	    on the order in which they are allocated, rather than based on any
+	    page offset into an address range or file.  During system boot up,
+	    the temporary interleaved system default policy works in this
+	    mode.
+
+MEMORY POLICY APIs
+
+Linux supports 3 system calls for controlling memory policy.  These APIS
+always affect only the calling task, the calling task's address space, or
+some shared object mapped into the calling task's address space.
+
+	Note:  the headers that define these APIs and the parameter data types
+	for user space applications reside in a package that is not part of
+	the Linux kernel.  The kernel system call interfaces, with the 'sys_'
+	prefix, are defined in <linux/syscalls.h>; the mode and flag
+	definitions are defined in <linux/mempolicy.h>.
+
+Set [Task] Memory Policy:
+
+	long set_mempolicy(int mode, const unsigned long *nmask,
+					unsigned long maxnode);
+
+	Set's the calling task's "task/process memory policy" to mode
+	specified by the 'mode' argument and the set of nodes defined
+	by 'nmask'.  'nmask' points to a bit mask of node ids containing
+	at least 'maxnode' ids.
+
+	See the set_mempolicy(2) man page for more details
+
+
+Get [Task] Memory Policy or Related Information
+
+	long get_mempolicy(int *mode,
+			   const unsigned long *nmask, unsigned long maxnode,
+			   void *addr, int flags);
+
+	Queries the "task/process memory policy" of the calling task, or
+	the policy or location of a specified virtual address, depending
+	on the 'flags' argument.
+
+	See the get_mempolicy(2) man page for more details
+
+
+Install VMA/Shared Policy for a Range of Task's Address Space
+
+	long mbind(void *start, unsigned long len, int mode,
+		   const unsigned long *nmask, unsigned long maxnode,
+		   unsigned flags);
+
+	mbind() installs the policy specified by (mode, nmask, maxnodes) as
+	a VMA policy for the range of the calling task's address space
+	specified by the 'start' and 'len' arguments.  Additional actions
+	may be requested via the 'flags' argument.
+
+	See the mbind(2) man page for more details.
+
+MEMORY POLICY COMMAND LINE INTERFACE
+
+Although not strictly part of the Linux implementation of memory policy,
+a command line tool, numactl(8), exists that allows one to:
+
++ set the task policy for a specified program via set_mempolicy(2), fork(2) and
+  exec(2)
+
++ set the shared policy for a shared memory segment via mbind(2)
+
+The numactl(8) tool is packages with the run-time version of the library
+containing the memory policy system call wrappers.  Some distributions
+package the headers and compile-time libraries in a separate development
+package.
+
+
+MEMORY POLICIES AND CPUSETS
+
+Memory policies work within cpusets as described above.  For memory policies
+that require a node or set of nodes, the nodes are restricted to the set of
+nodes whose memories are allowed by the cpuset constraints.  If the
+intersection of the set of nodes specified for the policy and the set of nodes
+allowed by the cpuset is the empty set, the policy is considered invalid and
+cannot be installed.
+
+The interaction of memory policies and cpusets can be problematic for a
+couple of reasons:
+
+1) the memory policy APIs take physical node id's as arguments.  However, the
+   memory policy APIs do not provide a way to determine what nodes are valid
+   in the context where the application is running.  An application MAY consult
+   the cpuset file system [directly or via an out of tree, and not generally
+   available, libcpuset API] to obtain this information, but then the
+   application must be aware that it is running in a cpuset and use what are
+   intended primarily as administrative APIs.
+
+   However, as long as the policy specifies at least one node that is valid
+   in the controlling cpuset, the policy can be used.
+
+2) when tasks in two cpusets share access to a memory region, such as shared
+   memory segments created by shmget() of mmap() with the MAP_ANONYMOUS and
+   MAP_SHARED flags, and any of the tasks install shared policy on the region,
+   only nodes whose memories are allowed in both cpusets may be used in the
+   policies.  Again, obtaining this information requires "stepping outside"
+   the memory policy APIs, as well as knowing in what cpusets other task might
+   be attaching to the shared region, to use the cpuset information.
+   Furthermore, if the cpusets' allowed memory sets are disjoint, "local"
+   allocation is the only valid policy.