On Tue, Jan 10, 2023 at 05:23:58PM +0200, Mike Rapoport wrote: > diff --git a/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst > index 2ab7b8c1c863..9ad42ff22d88 100644 > --- a/Documentation/mm/physical_memory.rst > +++ b/Documentation/mm/physical_memory.rst > @@ -3,3 +3,343 @@ > =============== > 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 to by it's 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`` represent memory suitable for DMA by > + peripheral devices that cannot access all of the addressable memory. > + 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 most pages in ``ZONE_MOVABLE`` are movable. That means > + that while virtual addresses of these pages do not change, their content may > + move between different physical pages. ``ZONE_MOVABLE`` is only enabled when > + one of ``kernelcore``, ``movablecore`` and ``movable_node`` parameters is > + present in the kernel command line. See :ref:`Page migration > + <page_migration>` 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 | > + +---------+----------+-----------+ +------------+-------------+ > + > +.. _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_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_EXTENTION`` 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 :ref:`Page Reclaim <page_reclaim>`. > + > +``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. The doc LGTM, thanks. I leave the actual content review to mm people. Reviewed-by: Bagas Sanjaya <bagasdotme@xxxxxxxxx> -- An old man doll... just what I always wanted! - Clara
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