@@ -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:
Briefly describe what zones are and the fields of struct zone. Signed-off-by: Jiwen Qi <jiwen7.qi@gmail.com> --- Documentation/mm/physical_memory.rst | 259 ++++++++++++++++++++++++++- 1 file changed, 257 insertions(+), 2 deletions(-) base-commit: 0ad2507d5d93f39619fc42372c347d6006b64319