From: "Mike Rapoport (IBM)" <rppt@xxxxxxxxxx> Signed-off-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> --- Documentation/mm/physical_memory.rst | 322 +++++++++++++++++++++++++++ 1 file changed, 322 insertions(+) diff --git a/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst index 2ab7b8c1c863..fcf52f1db16b 100644 --- a/Documentation/mm/physical_memory.rst +++ b/Documentation/mm/physical_memory.rst @@ -3,3 +3,325 @@ =============== 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>` + +Each node may be divided up into a number of blocks called zones which +represent ranges within memory. These ranges are usually determined by +architectural constraints for accessing the physical memory. A zone is +described by a ``struct zone_struct``, typedeffed to ``zone_t`` and 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 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 +<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/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 struct pglist_data is declared in `include/linux/mmzone.h +<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/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 (and the only) + `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 (and the only) node + `node_mem_map` 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. 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 synchronizes 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 +===== + +.. _pages: + +Pages +===== + +.. _folios: + +Folios +====== + +.. _initialization: + +Initialization +============== -- 2.35.1
