Documentation/mm/physical_memory.rst

1 .. SPDX-License-Identifier: GPL-2.0
8 architecture-independent abstraction to represent the physical memory. This
13 `Non-Uniform Memory Access (NUMA)
14 <https://en.wikipedia.org/wiki/Non-uniform_memory_access>`_.
15 With multi-core and multi-socket machines, memory may be arranged into banks
42   memory with DMA specific requirements (Documentation/core-api/dma-api.rst),
47   ``CONFIG_ZONE_DMA32`` configuration options. Some 64-bit platforms may need
59   only on some 32-bit architectures and is enabled with ``CONFIG_HIGHMEM``.
69   Documentation/admin-guide/mm/memory-hotplug.rst for additional details.
85 For example, with 32-bit kernel on an x86 UMA machine with 2 Gbytes of RAM the
90   +-------------------------------------------------------------+
92   +-------------------------------------------------------------+
95   +----------+-----------------------+--------------------------+
97   +----------+-----------------------+--------------------------+
108   +--------------------------------+ +--------------------------+
110   +--------------------------------+ +--------------------------+
113   +---------+----------+-----------+ +------------+-------------+
115   +---------+----------+-----------+ +------------+-------------+
124   +-------------+ +-------------+ +-------------+ +-------------+
126   +-------------+ +-------------+ +-------------+ +-------------+
129   +-----+-------+ +-------------+ +-------------+ +-------------+
131   +-----+-------+ +-------------+ +-------------+ +-------------+
143 Linux uses a node-local allocation policy to allocate memory from the node
147 Documentation/admin-guide/mm/numa_memory_policy.rst.
193 For instance, to call a function foo() for each online node::
198 		foo(pgdat);
202 --------------
255   This is a per-node reserve of pages that are not available to userspace
264 …Per-node queue of huge pages that their split was deferred. Defined only when ``CONFIG_TRANSPARENT…
267   Per-node lruvec holding LRU lists and related parameters. Used only when
277   Per-node instance of kswapd kernel thread.
322   Per-node instance of kcompactd kernel thread.
332   Per-CPU VM statistics for the node
346 The page allocator uses the GFP flags, see :ref:`mm-api-gfp-flags`, specified by
357 At runtime, free pages in a zone are in the Per-CPU Pagesets (PCP) or free areas
358 of the zone. The Per-CPU Pagesets are a vital mechanism in the kernel's memory
360 each CPU, the Per-CPU Pagesets improve performance and scalability, especially
361 on systems with many cores. The page allocator in the kernel employs a two-step
362 strategy for memory allocation, starting with the Per-CPU Pagesets before
363 falling back to the buddy allocator. Pages are transferred between the Per-CPU
371 --------------
401   The number of pages which are reserved for high-order atomic allocations.
430   Pointer to the Per-CPU Pagesets (PCP) allocated and initialized by
436   Copied to the ``high_min`` of the Per-CPU Pagesets for faster access.
439   Copied to the ``high_max`` of the Per-CPU Pagesets for faster access.
442   Copied to the ``batch`` of the Per-CPU Pagesets for faster access. The
443   ``batch``, ``high_min`` and ``high_max`` of the Per-CPU Pagesets are used to
444   calculate the number of elements the Per-CPU Pagesets obtain from the buddy
446   to decide if the Per-CPU Pagesets return pages to the buddy allocator in page
451   ``include/linux/pageblock-flags.h`` for flags list). The memory is allocated
462   ``managed_pages`` = ``present_pages`` - ``reserved_pages``, ``reserved_pages``
471   ``spanned_pages`` = ``zone_end_pfn`` - ``zone_start_pfn``. It is initialized
476   ``present_pages`` = ``spanned_pages`` - ``absent_pages`` (pages in holes). It
478   unmanaged pages by checking (``present_pages`` - ``managed_pages``). Write
501   ``zone->lock``. Defined only when ``CONFIG_MEMORY_ISOLATION`` is enabled.
505   seqlock because it has to be read outside of ``zone->lock``, and it is done in
540   the number of free pages to avoid per-cpu counter drift allowing watermarks
604   Per-CPU VM statistics for the zone. It records VM statistics and VM NUMA event
605   statistics on a per-CPU basis. It reduces updates to the global ``vm_stat``