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diff --git a/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst new file mode 100644 index 000000000000..531e73b003dd --- /dev/null +++ b/Documentation/mm/physical_memory.rst @@ -0,0 +1,371 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=============== +Physical Memory +=============== + +Linux is available for a wide range of architectures so there is a need for an +architecture-independent abstraction to represent the physical memory. This +chapter describes the structures used to manage physical memory in a running +system. + +The first principal concept prevalent in the memory management is +`Non-Uniform Memory Access (NUMA) +<https://en.wikipedia.org/wiki/Non-uniform_memory_access>`_. +With multi-core and multi-socket machines, memory may be arranged into banks +that incur a different cost to access depending on the “distance” from the +processor. For example, there might be a bank of memory assigned to each CPU or +a bank of memory very suitable for DMA near peripheral devices. + +Each bank is called a node and the concept is represented under Linux by a +``struct pglist_data`` even if the architecture is UMA. This structure is +always referenced by its typedef ``pg_data_t``. A ``pg_data_t`` structure +for a particular node can be referenced by ``NODE_DATA(nid)`` macro where +``nid`` is the ID of that node. + +For NUMA architectures, the node structures are allocated by the architecture +specific code early during boot. Usually, these structures are allocated +locally on the memory bank they represent. For UMA architectures, only one +static ``pg_data_t`` structure called ``contig_page_data`` is used. Nodes will +be discussed further in Section :ref:`Nodes <nodes>` + +The entire physical address space is partitioned into one or more blocks +called zones which represent ranges within memory. These ranges are usually +determined by architectural constraints for accessing the physical memory. +The memory range within a node that corresponds to a particular zone is +described by a ``struct zone``, typedeffed to ``zone_t``. Each zone has +one of the types described below. + +* ``ZONE_DMA`` and ``ZONE_DMA32`` historically represented memory suitable for + DMA by peripheral devices that cannot access all of the addressable + memory. For many years there are better more and robust interfaces to get + memory with DMA specific requirements (Documentation/core-api/dma-api.rst), + but ``ZONE_DMA`` and ``ZONE_DMA32`` still represent memory ranges that have + restrictions on how they can be accessed. + Depending on the architecture, either of these zone types or even they both + can be disabled at build time using ``CONFIG_ZONE_DMA`` and + ``CONFIG_ZONE_DMA32`` configuration options. Some 64-bit platforms may need + both zones as they support peripherals with different DMA addressing + limitations. + +* ``ZONE_NORMAL`` is for normal memory that can be accessed by the kernel all + the time. DMA operations can be performed on pages in this zone if the DMA + devices support transfers to all addressable memory. ``ZONE_NORMAL`` is + always enabled. + +* ``ZONE_HIGHMEM`` is the part of the physical memory that is not covered by a + permanent mapping in the kernel page tables. The memory in this zone is only + accessible to the kernel using temporary mappings. This zone is available + only on some 32-bit architectures and is enabled with ``CONFIG_HIGHMEM``. + +* ``ZONE_MOVABLE`` is for normal accessible memory, just like ``ZONE_NORMAL``. + The difference is that the contents of most pages in ``ZONE_MOVABLE`` is + movable. That means that while virtual addresses of these pages do not + change, their content may move between different physical pages. Often + ``ZONE_MOVABLE`` is populated during memory hotplug, but it may be + also populated on boot using one of ``kernelcore``, ``movablecore`` and + ``movable_node`` kernel command line parameters. See + Documentation/mm/page_migration.rst and + Documentation/admin-guide/mm/memory-hotplug.rst for additional details. + +* ``ZONE_DEVICE`` represents memory residing on devices such as PMEM and GPU. + It has different characteristics than RAM zone types and it exists to provide + :ref:`struct page <Pages>` and memory map services for device driver + identified physical address ranges. ``ZONE_DEVICE`` is enabled with + configuration option ``CONFIG_ZONE_DEVICE``. + +It is important to note that many kernel operations can only take place using +``ZONE_NORMAL`` so it is the most performance critical zone. Zones are +discussed further in Section :ref:`Zones <zones>`. + +The relation between node and zone extents is determined by the physical memory +map reported by the firmware, architectural constraints for memory addressing +and certain parameters in the kernel command line. + +For example, with 32-bit kernel on an x86 UMA machine with 2 Gbytes of RAM the +entire memory will be on node 0 and there will be three zones: ``ZONE_DMA``, +``ZONE_NORMAL`` and ``ZONE_HIGHMEM``:: + + 0 2G + +-------------------------------------------------------------+ + | node 0 | + +-------------------------------------------------------------+ + + 0 16M 896M 2G + +----------+-----------------------+--------------------------+ + | ZONE_DMA | ZONE_NORMAL | ZONE_HIGHMEM | + +----------+-----------------------+--------------------------+ + + +With a kernel built with ``ZONE_DMA`` disabled and ``ZONE_DMA32`` enabled and +booted with ``movablecore=80%`` parameter on an arm64 machine with 16 Gbytes of +RAM equally split between two nodes, there will be ``ZONE_DMA32``, +``ZONE_NORMAL`` and ``ZONE_MOVABLE`` on node 0, and ``ZONE_NORMAL`` and +``ZONE_MOVABLE`` on node 1:: + + + 1G 9G 17G + +--------------------------------+ +--------------------------+ + | node 0 | | node 1 | + +--------------------------------+ +--------------------------+ + + 1G 4G 4200M 9G 9320M 17G + +---------+----------+-----------+ +------------+-------------+ + | DMA32 | NORMAL | MOVABLE | | NORMAL | MOVABLE | + +---------+----------+-----------+ +------------+-------------+ + + +Memory banks may belong to interleaving nodes. In the example below an x86 +machine has 16 Gbytes of RAM in 4 memory banks, even banks belong to node 0 +and odd banks belong to node 1:: + + + 0 4G 8G 12G 16G + +-------------+ +-------------+ +-------------+ +-------------+ + | node 0 | | node 1 | | node 0 | | node 1 | + +-------------+ +-------------+ +-------------+ +-------------+ + + 0 16M 4G + +-----+-------+ +-------------+ +-------------+ +-------------+ + | DMA | DMA32 | | NORMAL | | NORMAL | | NORMAL | + +-----+-------+ +-------------+ +-------------+ +-------------+ + +In this case node 0 will span from 0 to 12 Gbytes and node 1 will span from +4 to 16 Gbytes. + +.. _nodes: + +Nodes +===== + +As we have mentioned, each node in memory is described by a ``pg_data_t`` which +is a typedef for a ``struct pglist_data``. When allocating a page, by default +Linux uses a node-local allocation policy to allocate memory from the node +closest to the running CPU. As processes tend to run on the same CPU, it is +likely the memory from the current node will be used. The allocation policy can +be controlled by users as described in +Documentation/admin-guide/mm/numa_memory_policy.rst. + +Most NUMA architectures maintain an array of pointers to the node +structures. The actual structures are allocated early during boot when +architecture specific code parses the physical memory map reported by the +firmware. The bulk of the node initialization happens slightly later in the +boot process by free_area_init() function, described later in Section +:ref:`Initialization <initialization>`. + + +Along with the node structures, kernel maintains an array of ``nodemask_t`` +bitmasks called ``node_states``. Each bitmask in this array represents a set of +nodes with particular properties as defined by ``enum node_states``: + +``N_POSSIBLE`` + The node could become online at some point. +``N_ONLINE`` + The node is online. +``N_NORMAL_MEMORY`` + The node has regular memory. +``N_HIGH_MEMORY`` + The node has regular or high memory. When ``CONFIG_HIGHMEM`` is disabled + aliased to ``N_NORMAL_MEMORY``. +``N_MEMORY`` + The node has memory(regular, high, movable) +``N_CPU`` + The node has one or more CPUs + +For each node that has a property described above, the bit corresponding to the +node ID in the ``node_states[<property>]`` bitmask is set. + +For example, for node 2 with normal memory and CPUs, bit 2 will be set in :: + + node_states[N_POSSIBLE] + node_states[N_ONLINE] + node_states[N_NORMAL_MEMORY] + node_states[N_HIGH_MEMORY] + node_states[N_MEMORY] + node_states[N_CPU] + +For various operations possible with nodemasks please refer to +``include/linux/nodemask.h``. + +Among other things, nodemasks are used to provide macros for node traversal, +namely ``for_each_node()`` and ``for_each_online_node()``. + +For instance, to call a function foo() for each online node:: + + for_each_online_node(nid) { + pg_data_t *pgdat = NODE_DATA(nid); + + foo(pgdat); + } + +Node structure +-------------- + +The nodes structure ``struct pglist_data`` is declared in +``include/linux/mmzone.h``. Here we briefly describe fields of this +structure: + +General +~~~~~~~ + +``node_zones`` + The zones for this node. Not all of the zones may be populated, but it is + the full list. It is referenced by this node's node_zonelists as well as + other node's node_zonelists. + +``node_zonelists`` + The list of all zones in all nodes. This list defines the order of zones + that allocations are preferred from. The ``node_zonelists`` is set up by + ``build_zonelists()`` in ``mm/page_alloc.c`` during the initialization of + core memory management structures. + +``nr_zones`` + Number of populated zones in this node. + +``node_mem_map`` + For UMA systems that use FLATMEM memory model the 0's node + ``node_mem_map`` is array of struct pages representing each physical frame. + +``node_page_ext`` + For UMA systems that use FLATMEM memory model the 0's node + ``node_page_ext`` is array of extensions of struct pages. Available only + in the kernels built with ``CONFIG_PAGE_EXTENSION`` enabled. + +``node_start_pfn`` + The page frame number of the starting page frame in this node. + +``node_present_pages`` + Total number of physical pages present in this node. + +``node_spanned_pages`` + Total size of physical page range, including holes. + +``node_size_lock`` + A lock that protects the fields defining the node extents. Only defined when + at least one of ``CONFIG_MEMORY_HOTPLUG`` or + ``CONFIG_DEFERRED_STRUCT_PAGE_INIT`` configuration options are enabled. + ``pgdat_resize_lock()`` and ``pgdat_resize_unlock()`` are provided to + manipulate ``node_size_lock`` without checking for ``CONFIG_MEMORY_HOTPLUG`` + or ``CONFIG_DEFERRED_STRUCT_PAGE_INIT``. + +``node_id`` + The Node ID (NID) of the node, starts at 0. + +``totalreserve_pages`` + This is a per-node reserve of pages that are not available to userspace + allocations. + +``first_deferred_pfn`` + If memory initialization on large machines is deferred then this is the first + PFN that needs to be initialized. Defined only when + ``CONFIG_DEFERRED_STRUCT_PAGE_INIT`` is enabled + +``deferred_split_queue`` + Per-node queue of huge pages that their split was deferred. Defined only when ``CONFIG_TRANSPARENT_HUGEPAGE`` is enabled. + +``__lruvec`` + Per-node lruvec holding LRU lists and related parameters. Used only when + memory cgroups are disabled. It should not be accessed directly, use + ``mem_cgroup_lruvec()`` to look up lruvecs instead. + +Reclaim control +~~~~~~~~~~~~~~~ + +See also Documentation/mm/page_reclaim.rst. + +``kswapd`` + Per-node instance of kswapd kernel thread. + +``kswapd_wait``, ``pfmemalloc_wait``, ``reclaim_wait`` + Workqueues used to synchronize memory reclaim tasks + +``nr_writeback_throttled`` + Number of tasks that are throttled waiting on dirty pages to clean. + +``nr_reclaim_start`` + Number of pages written while reclaim is throttled waiting for writeback. + +``kswapd_order`` + Controls the order kswapd tries to reclaim + +``kswapd_highest_zoneidx`` + The highest zone index to be reclaimed by kswapd + +``kswapd_failures`` + Number of runs kswapd was unable to reclaim any pages + +``min_unmapped_pages`` + Minimal number of unmapped file backed pages that cannot be reclaimed. + Determined by ``vm.min_unmapped_ratio`` sysctl. Only defined when + ``CONFIG_NUMA`` is enabled. + +``min_slab_pages`` + Minimal number of SLAB pages that cannot be reclaimed. Determined by + ``vm.min_slab_ratio sysctl``. Only defined when ``CONFIG_NUMA`` is enabled + +``flags`` + Flags controlling reclaim behavior. + +Compaction control +~~~~~~~~~~~~~~~~~~ + +``kcompactd_max_order`` + Page order that kcompactd should try to achieve. + +``kcompactd_highest_zoneidx`` + The highest zone index to be compacted by kcompactd. + +``kcompactd_wait`` + Workqueue used to synchronize memory compaction tasks. + +``kcompactd`` + Per-node instance of kcompactd kernel thread. + +``proactive_compact_trigger`` + Determines if proactive compaction is enabled. Controlled by + ``vm.compaction_proactiveness`` sysctl. + +Statistics +~~~~~~~~~~ + +``per_cpu_nodestats`` + Per-CPU VM statistics for the node + +``vm_stat`` + VM statistics for the node. + +.. _zones: + +Zones +===== + +.. admonition:: Stub + + This section is incomplete. Please list and describe the appropriate fields. + +.. _pages: + +Pages +===== + +.. admonition:: Stub + + This section is incomplete. Please list and describe the appropriate fields. + +.. _folios: + +Folios +====== + +.. admonition:: Stub + + This section is incomplete. Please list and describe the appropriate fields. + +.. _initialization: + +Initialization +============== + +.. admonition:: Stub + + This section is incomplete. Please list and describe the appropriate fields. |