Briefly describe what zones are and the fields of struct zone. Signed-off-by: Jiwen Qi <jiwen7.qi@xxxxxxxxx> --- Documentation/mm/physical_memory.rst | 259 ++++++++++++++++++++++++++- 1 file changed, 257 insertions(+), 2 deletions(-) diff --git a/Documentation/mm/physical_memory.rst b/Documentation/mm/physical_memory.rst index 71fd4a6acf42..227997694851 100644 --- a/Documentation/mm/physical_memory.rst +++ b/Documentation/mm/physical_memory.rst @@ -338,10 +338,265 @@ Statistics Zones ===== +As we have mentioned, each zone in memory is described by a ``struct zone`` +which is an element of the ``node_zones`` field of the node it belongs to. A +zone represents a range of physical memory. A zone may have holes. The +``spanned_pages`` field represents the total pages spanned by the zone, +the ``present_pages`` field represents the physical pages existing within the +zone and the managed_page field represents the pages managed by the buddy system. + +Linux uses the GFP flags, see ``include/linux/gfp_types.h``, specified by +a memory allocation to determine the highest zone in a node from which +the memory allocation can allocate memory. Linux first allocates memory from +that zone, if Linux 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 Linux +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 is pointed by the ``per_cpu_pageset`` filed. +The free areas is pointed by the ``free_area`` field. The Per-CPU Pagesets are +a vital mechanism in the Linux 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 Linux 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. Free areas in +a zone are represented by an array of ``free_area``, where each element +corresponds to a specific order which is a power of two." + +Architecture specific code calls free_area_init() to initializes zones. + +Zone structure +-------------- -.. admonition:: Stub +The zones structure ``struct zone`` is declared in ``include/linux/mmzone.h``. +Here we briefly describe fields of this structure: - This section is incomplete. Please list and describe the appropriate fields. +General +~~~~~~~ + +``_watermark`` + The watermarks for this zone. The min watermark is the point where boosting is + ignored and an allocation may trigger direct reclaim and direct compaction. + It is also used to throttle direct reclaim. The low watermark is the point + where kswapd is woken up. The high watermark is the point where 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. It is the point where 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 is caculated according + to ``vm.watermark_scale_factor`` sysctl. + +``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. The reason is that we don't know + if the memory that we're going to allocate will be freeable or/and it will be + released eventually, so to avoid totally wasting several GB of ram we must + reserve some of the lower zone memory (otherwise we risk to run OOM on the + lower zones despite there being tons of freeable ram on the higher 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 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, the Per-CPU Pagesets improve 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 system. See + ``include/linux/pageblock-flags.h``. 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. But, it 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`` + Free areas of different sizes. 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, + the other 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 (there is 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: base-commit: 0ad2507d5d93f39619fc42372c347d6006b64319 -- 2.34.1
