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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). 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 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. 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 (Dynamic DMA mapping using the generic device), 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 Page migration and Memory Hot(Un)Plug 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 struct page 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 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

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 NUMA Memory Policy.

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 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 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

As we have mentioned, each zone in memory is described by a struct zone which is an element of the node_zones array of the node it belongs to. struct zone is the core data structure of the page allocator. A zone represents a range of physical memory and may have holes.

The page allocator uses the GFP flags, see Memory Allocation Controls, specified by a memory allocation to determine the highest zone in a node from which the memory allocation can allocate memory. The page allocator first allocates memory from that zone, if the page allocator can’t allocate the requested amount of memory from the zone, it will allocate memory from the next lower zone in the node, the process continues up to and including the lowest zone. For example, if a node contains ZONE_DMA32, ZONE_NORMAL and ZONE_MOVABLE and the highest zone of a memory allocation is ZONE_MOVABLE, the order of the zones from which the page allocator allocates memory is ZONE_MOVABLE > ZONE_NORMAL > ZONE_DMA32.

At runtime, free pages in a zone are in the Per-CPU Pagesets (PCP) or free areas of the zone. The Per-CPU Pagesets are a vital mechanism in the kernel’s memory management system. By handling most frequent allocations and frees locally on each CPU, the Per-CPU Pagesets improve performance and scalability, especially on systems with many cores. The page allocator in the kernel employs a two-step strategy for memory allocation, starting with the Per-CPU Pagesets before falling back to the buddy allocator. Pages are transferred between the Per-CPU Pagesets and the global free areas (managed by the buddy allocator) in batches. This minimizes the overhead of frequent interactions with the global buddy allocator.

Architecture specific code calls free_area_init() to initializes zones.

Zone structure

The zones structure struct zone is defined in include/linux/mmzone.h. Here we briefly describe fields of this structure:

General

_watermark

The watermarks for this zone. When the amount of free pages in a zone is below the min watermark, boosting is ignored, an allocation may trigger direct reclaim and direct compaction, it is also used to throttle direct reclaim. When the amount of free pages in a zone is below the low watermark, kswapd is woken up. When the amount of free pages in a zone is above the high watermark, kswapd stops reclaiming (a zone is balanced) when the NUMA_BALANCING_MEMORY_TIERING bit of sysctl_numa_balancing_mode is not set. The promo watermark is used for memory tiering and NUMA balancing. When the amount of free pages in a zone is above the promo watermark, kswapd stops reclaiming when the NUMA_BALANCING_MEMORY_TIERING bit of sysctl_numa_balancing_mode is set. The watermarks are set by __setup_per_zone_wmarks(). The min watermark is calculated according to vm.min_free_kbytes sysctl. The other three watermarks are set according to the distance between two watermarks. The distance itself is calculated taking vm.watermark_scale_factor sysctl into account.

watermark_boost

The number of pages which are used to boost watermarks to increase reclaim pressure to reduce the likelihood of future fallbacks and wake kswapd now as the node may be balanced overall and kswapd will not wake naturally.

nr_reserved_highatomic

The number of pages which are reserved for high-order atomic allocations.

nr_free_highatomic

The number of free pages in reserved highatomic pageblocks

lowmem_reserve

The array of the amounts of the memory reserved in this zone for memory allocations. For example, if the highest zone a memory allocation can allocate memory from is ZONE_MOVABLE, the amount of memory reserved in this zone for this allocation is lowmem_reserve[ZONE_MOVABLE] when attempting to allocate memory from this zone. This is a mechanism the page allocator uses to prevent allocations which could use highmem from using too much lowmem. For some specialised workloads on highmem machines, it is dangerous for the kernel to allow process memory to be allocated from the lowmem zone. This is because that memory could then be pinned via the mlock() system call, or by unavailability of swapspace. vm.lowmem_reserve_ratio sysctl determines how aggressive the kernel is in defending these lower zones. This array is recalculated by setup_per_zone_lowmem_reserve() at runtime if vm.lowmem_reserve_ratio sysctl changes.

node

The index of the node this zone belongs to. Available only when CONFIG_NUMA is enabled because there is only one zone in a UMA system.

zone_pgdat

Pointer to the struct pglist_data of the node this zone belongs to.

per_cpu_pageset

Pointer to the Per-CPU Pagesets (PCP) allocated and initialized by setup_zone_pageset(). By handling most frequent allocations and frees locally on each CPU, PCP improves performance and scalability on systems with many cores.

pageset_high_min

Copied to the high_min of the Per-CPU Pagesets for faster access.

pageset_high_max

Copied to the high_max of the Per-CPU Pagesets for faster access.

pageset_batch

Copied to the batch of the Per-CPU Pagesets for faster access. The batch, high_min and high_max of the Per-CPU Pagesets are used to calculate the number of elements the Per-CPU Pagesets obtain from the buddy allocator under a single hold of the lock for efficiency. They are also used to decide if the Per-CPU Pagesets return pages to the buddy allocator in page free process.

pageblock_flags

The pointer to the flags for the pageblocks in the zone (see include/linux/pageblock-flags.h for flags list). The memory is allocated in setup_usemap(). Each pageblock occupies NR_PAGEBLOCK_BITS bits. Defined only when CONFIG_FLATMEM is enabled. The flags is stored in mem_section when CONFIG_SPARSEMEM is enabled.

zone_start_pfn

The start pfn of the zone. It is initialized by calculate_node_totalpages().

managed_pages

The present pages managed by the buddy system, which is calculated as: managed_pages = present_pages - reserved_pages, reserved_pages includes pages allocated by the memblock allocator. It should be used by page allocator and vm scanner to calculate all kinds of watermarks and thresholds. It is accessed using atomic_long_xxx() functions. It is initialized in free_area_init_core() and then is reinitialized when memblock allocator frees pages into buddy system.

spanned_pages

The total pages spanned by the zone, including holes, which is calculated as: spanned_pages = zone_end_pfn - zone_start_pfn. It is initialized by calculate_node_totalpages().

present_pages

The physical pages existing within the zone, which is calculated as: present_pages = spanned_pages - absent_pages (pages in holes). It may be used by memory hotplug or memory power management logic to figure out unmanaged pages by checking (present_pages - managed_pages). Write access to present_pages at runtime should be protected by mem_hotplug_begin/done(). Any reader who can’t tolerant drift of present_pages should use get_online_mems() to get a stable value. It is initialized by calculate_node_totalpages().

present_early_pages

The present pages existing within the zone located on memory available since early boot, excluding hotplugged memory. Defined only when CONFIG_MEMORY_HOTPLUG is enabled and initialized by calculate_node_totalpages().

cma_pages

The pages reserved for CMA use. These pages behave like ZONE_MOVABLE when they are not used for CMA. Defined only when CONFIG_CMA is enabled.

name

The name of the zone. It is a pointer to the corresponding element of the zone_names array.

nr_isolate_pageblock

Number of isolated pageblocks. It is used to solve incorrect freepage counting problem due to racy retrieving migratetype of pageblock. Protected by zone->lock. Defined only when CONFIG_MEMORY_ISOLATION is enabled.

span_seqlock

The seqlock to protect zone_start_pfn and spanned_pages. It is a seqlock because it has to be read outside of zone->lock, and it is done in the main allocator path. However, the seqlock is written quite infrequently. Defined only when CONFIG_MEMORY_HOTPLUG is enabled.

initialized

The flag indicating if the zone is initialized. Set by init_currently_empty_zone() during boot.

free_area

The array of free areas, where each element corresponds to a specific order which is a power of two. The buddy allocator uses this structure to manage free memory efficiently. When allocating, it tries to find the smallest sufficient block, if the smallest sufficient block is larger than the requested size, it will be recursively split into the next smaller blocks until the required size is reached. When a page is freed, it may be merged with its buddy to form a larger block. It is initialized by zone_init_free_lists().

unaccepted_pages

The list of pages to be accepted. All pages on the list are MAX_PAGE_ORDER. Defined only when CONFIG_UNACCEPTED_MEMORY is enabled.

flags

The zone flags. The least three bits are used and defined by enum zone_flags. ZONE_BOOSTED_WATERMARK (bit 0): zone recently boosted watermarks. Cleared when kswapd is woken. ZONE_RECLAIM_ACTIVE (bit 1): kswapd may be scanning the zone. ZONE_BELOW_HIGH (bit 2): zone is below high watermark.

lock

The main lock that protects the internal data structures of the page allocator specific to the zone, especially protects free_area.

percpu_drift_mark

When free pages are below this point, additional steps are taken when reading the number of free pages to avoid per-cpu counter drift allowing watermarks to be breached. It is updated in refresh_zone_stat_thresholds().

Compaction control

compact_cached_free_pfn

The PFN where compaction free scanner should start in the next scan.

compact_cached_migrate_pfn

The PFNs where compaction migration scanner should start in the next scan. This array has two elements: the first one is used in MIGRATE_ASYNC mode, and the other one is used in MIGRATE_SYNC mode.

compact_init_migrate_pfn

The initial migration PFN which is initialized to 0 at boot time, and to the first pageblock with migratable pages in the zone after a full compaction finishes. It is used to check if a scan is a whole zone scan or not.

compact_init_free_pfn

The initial free PFN which is initialized to 0 at boot time and to the last pageblock with free MIGRATE_MOVABLE pages in the zone. It is used to check if it is the start of a scan.

compact_considered

The number of compactions attempted since last failure. It is reset in defer_compaction() when a compaction fails to result in a page allocation success. It is increased by 1 in compaction_deferred() when a compaction should be skipped. compaction_deferred() is called before compact_zone() is called, compaction_defer_reset() is called when compact_zone() returns COMPACT_SUCCESS, defer_compaction() is called when compact_zone() returns COMPACT_PARTIAL_SKIPPED or COMPACT_COMPLETE.

compact_defer_shift

The number of compactions skipped before trying again is 1<<compact_defer_shift. It is increased by 1 in defer_compaction(). It is reset in compaction_defer_reset() when a direct compaction results in a page allocation success. Its maximum value is COMPACT_MAX_DEFER_SHIFT.

compact_order_failed

The minimum compaction failed order. It is set in compaction_defer_reset() when a compaction succeeds and in defer_compaction() when a compaction fails to result in a page allocation success.

compact_blockskip_flush

Set to true when compaction migration scanner and free scanner meet, which means the PB_migrate_skip bits should be cleared.

contiguous

Set to true when the zone is contiguous (in other words, no hole).

Statistics

vm_stat

VM statistics for the zone. The items tracked are defined by enum zone_stat_item.

vm_numa_event

VM NUMA event statistics for the zone. The items tracked are defined by enum numa_stat_item.

per_cpu_zonestats

Per-CPU VM statistics for the zone. It records VM statistics and VM NUMA event statistics on a per-CPU basis. It reduces updates to the global vm_stat and vm_numa_event fields of the zone to improve performance.

Pages

Stub

This section is incomplete. Please list and describe the appropriate fields.

Folios

Stub

This section is incomplete. Please list and describe the appropriate fields.

Initialization

Stub

This section is incomplete. Please list and describe the appropriate fields.