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2024-12-19 9370bb92b2d16684ee45cf24e879c93c509162da

 kernel/Documentation/admin-guide/mm/concepts.rst
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 Concepts overview
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 =================
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-The memory management in Linux is complex system that evolved over the
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-years and included more and more functionality to support variety of
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+The memory management in Linux is a complex system that evolved over the
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+years and included more and more functionality to support a variety of
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 systems from MMU-less microcontrollers to supercomputers. The memory
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-management for systems without MMU is called ``nommu`` and it
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+management for systems without an MMU is called ``nommu`` and it
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 definitely deserves a dedicated document, which hopefully will be
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 eventually written. Yet, although some of the concepts are the same,
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-here we assume that MMU is available and CPU can translate a virtual
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+here we assume that an MMU is available and a CPU can translate a virtual
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 address to a physical address.
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 .. contents:: :local:
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 The physical memory in a computer system is a limited resource and
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 even for systems that support memory hotplug there is a hard limit on
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 the amount of memory that can be installed. The physical memory is not
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-necessary contiguous, it might be accessible as a set of distinct
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+necessarily contiguous; it might be accessible as a set of distinct
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 address ranges. Besides, different CPU architectures, and even
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-different implementations of the same architecture have different view
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-how these address ranges defined.
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+different implementations of the same architecture have different views
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+of how these address ranges are defined.
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 All this makes dealing directly with physical memory quite complex and
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 to avoid this complexity a concept of virtual memory was developed.
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 protection and controlled sharing of data between processes.
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 With virtual memory, each and every memory access uses a virtual
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-address. When the CPU decodes the an instruction that reads (or
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+address. When the CPU decodes an instruction that reads (or
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 writes) from (or to) the system memory, it translates the `virtual`
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 address encoded in that instruction to a `physical` address that the
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 memory controller can understand.
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 Each physical memory page can be mapped as one or more virtual
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 pages. These mappings are described by page tables that allow
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-translation from virtual address used by programs to real address in
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-the physical memory. The page tables organized hierarchically.
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+translation from a virtual address used by programs to the physical
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+memory address. The page tables are organized hierarchically.
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 The tables at the lowest level of the hierarchy contain physical
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 addresses of actual pages used by the software. The tables at higher
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 Many multi-processor machines are NUMA - Non-Uniform Memory Access -
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 systems. In such systems the memory is arranged into banks that have
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 different access latency depending on the "distance" from the
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-processor. Each bank is referred as `node` and for each node Linux
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-constructs an independent memory management subsystem. A node has it's
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+processor. Each bank is referred to as a `node` and for each node Linux
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+constructs an independent memory management subsystem. A node has its
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 own set of zones, lists of free and used pages and various statistics
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 counters. You can find more details about NUMA in
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 :ref:`Documentation/vm/numa.rst <numa>` and in
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 call. Usually, the anonymous mappings only define virtual memory areas
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 that the program is allowed to access. The read accesses will result
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 in creation of a page table entry that references a special physical
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-page filled with zeroes. When the program performs a write, regular
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+page filled with zeroes. When the program performs a write, a regular
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 physical page will be allocated to hold the written data. The page
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-will be marked dirty and if the kernel will decide to repurpose it,
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+will be marked dirty and if the kernel decides to repurpose it,
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 the dirty page will be swapped out.
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 Reclaim
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 The process of freeing the reclaimable physical memory pages and
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 repurposing them is called (surprise!) `reclaim`. Linux can reclaim
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 pages either asynchronously or synchronously, depending on the state
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-of the system. When system is not loaded, most of the memory is free
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-and allocation request will be satisfied immediately from the free
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+of the system. When the system is not loaded, most of the memory is free
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+and allocation requests will be satisfied immediately from the free
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 pages supply. As the load increases, the amount of the free pages goes
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 down and when it reaches a certain threshold (high watermark), an
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 allocation request will awaken the ``kswapd`` daemon. It will
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 they contain is available elsewhere, or evict to the backing storage
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 device (remember those dirty pages?). As memory usage increases even
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 more and reaches another threshold - min watermark - an allocation
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-will trigger the `direct reclaim`. In this case allocation is stalled
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+will trigger `direct reclaim`. In this case allocation is stalled
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 until enough memory pages are reclaimed to satisfy the request.
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 Compaction
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 fragmented. Although with virtual memory it is possible to present
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 scattered physical pages as virtually contiguous range, sometimes it is
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 necessary to allocate large physically contiguous memory areas. Such
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-need may arise, for instance, when a device driver requires large
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+need may arise, for instance, when a device driver requires a large
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 buffer for DMA, or when THP allocates a huge page. Memory `compaction`
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 addresses the fragmentation issue. This mechanism moves occupied pages
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 from the lower part of a memory zone to free pages in the upper part
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 together at the beginning of the zone and allocations of large
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 physically contiguous areas become possible.
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-Like reclaim, the compaction may happen asynchronously in ``kcompactd``
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-daemon or synchronously as a result of memory allocation request.
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+Like reclaim, the compaction may happen asynchronously in the ``kcompactd``
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+daemon or synchronously as a result of a memory allocation request.
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 OOM killer
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 ==========
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-It may happen, that on a loaded machine memory will be exhausted. When
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-the kernel detects that the system runs out of memory (OOM) it invokes
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-`OOM killer`. Its mission is simple: all it has to do is to select a
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-task to sacrifice for the sake of the overall system health. The
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-selected task is killed in a hope that after it exits enough memory
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-will be freed to continue normal operation.
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+It is possible that on a loaded machine memory will be exhausted and the
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+kernel will be unable to reclaim enough memory to continue to operate. In
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+order to save the rest of the system, it invokes the `OOM killer`.
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+The `OOM killer` selects a task to sacrifice for the sake of the overall
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+system health. The selected task is killed in a hope that after it exits
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+enough memory will be freed to continue normal operation.