docs/vm: add documentation of memory models

Describe what {FLAT,DISCONTIG,SPARSE}MEM are and how they manage to
maintain pfn <-> struct page correspondence.

Signed-off-by: Mike Rapoport <[e-mail]>
Acked-by: Randy Dunlap <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/vm/index.rst

···
    hwpoison
    hugetlbfs_reserv
    ksm
+   memory-model
    mmu_notifier
    numa
    overcommit-accounting

Documentation/vm/memory-model.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _physical_memory_model:
+
+=====================
+Physical Memory Model
+=====================
+
+Physical memory in a system may be addressed in different ways. The
+simplest case is when the physical memory starts at address 0 and
+spans a contiguous range up to the maximal address. It could be,
+however, that this range contains small holes that are not accessible
+for the CPU. Then there could be several contiguous ranges at
+completely distinct addresses. And, don't forget about NUMA, where
+different memory banks are attached to different CPUs.
+
+Linux abstracts this diversity using one of the three memory models:
+FLATMEM, DISCONTIGMEM and SPARSEMEM. Each architecture defines what
+memory models it supports, what the default memory model is and
+whether it is possible to manually override that default.
+
+.. note::
+   At time of this writing, DISCONTIGMEM is considered deprecated,
+   although it is still in use by several architectures.
+
+All the memory models track the status of physical page frames using
+:c:type:`struct page` arranged in one or more arrays.
+
+Regardless of the selected memory model, there exists one-to-one
+mapping between the physical page frame number (PFN) and the
+corresponding `struct page`.
+
+Each memory model defines :c:func:`pfn_to_page` and :c:func:`page_to_pfn`
+helpers that allow the conversion from PFN to `struct page` and vice
+versa.
+
+FLATMEM
+=======
+
+The simplest memory model is FLATMEM. This model is suitable for
+non-NUMA systems with contiguous, or mostly contiguous, physical
+memory.
+
+In the FLATMEM memory model, there is a global `mem_map` array that
+maps the entire physical memory. For most architectures, the holes
+have entries in the `mem_map` array. The `struct page` objects
+corresponding to the holes are never fully initialized.
+
+To allocate the `mem_map` array, architecture specific setup code
+should call :c:func:`free_area_init_node` function or its convenience
+wrapper :c:func:`free_area_init`. Yet, the mappings array is not
+usable until the call to :c:func:`memblock_free_all` that hands all
+the memory to the page allocator.
+
+If an architecture enables `CONFIG_ARCH_HAS_HOLES_MEMORYMODEL` option,
+it may free parts of the `mem_map` array that do not cover the
+actual physical pages. In such case, the architecture specific
+:c:func:`pfn_valid` implementation should take the holes in the
+`mem_map` into account.
+
+With FLATMEM, the conversion between a PFN and the `struct page` is
+straightforward: `PFN - ARCH_PFN_OFFSET` is an index to the
+`mem_map` array.
+
+The `ARCH_PFN_OFFSET` defines the first page frame number for
+systems with physical memory starting at address different from 0.
+
+DISCONTIGMEM
+============
+
+The DISCONTIGMEM model treats the physical memory as a collection of
+`nodes` similarly to how Linux NUMA support does. For each node Linux
+constructs an independent memory management subsystem represented by
+`struct pglist_data` (or `pg_data_t` for short). Among other
+things, `pg_data_t` holds the `node_mem_map` array that maps
+physical pages belonging to that node. The `node_start_pfn` field of
+`pg_data_t` is the number of the first page frame belonging to that
+node.
+
+The architecture setup code should call :c:func:`free_area_init_node` for
+each node in the system to initialize the `pg_data_t` object and its
+`node_mem_map`.
+
+Every `node_mem_map` behaves exactly as FLATMEM's `mem_map` -
+every physical page frame in a node has a `struct page` entry in the
+`node_mem_map` array. When DISCONTIGMEM is enabled, a portion of the
+`flags` field of the `struct page` encodes the node number of the
+node hosting that page.
+
+The conversion between a PFN and the `struct page` in the
+DISCONTIGMEM model became slightly more complex as it has to determine
+which node hosts the physical page and which `pg_data_t` object
+holds the `struct page`.
+
+Architectures that support DISCONTIGMEM provide :c:func:`pfn_to_nid`
+to convert PFN to the node number. The opposite conversion helper
+:c:func:`page_to_nid` is generic as it uses the node number encoded in
+page->flags.
+
+Once the node number is known, the PFN can be used to index
+appropriate `node_mem_map` array to access the `struct page` and
+the offset of the `struct page` from the `node_mem_map` plus
+`node_start_pfn` is the PFN of that page.
+
+SPARSEMEM
+=========
+
+SPARSEMEM is the most versatile memory model available in Linux and it
+is the only memory model that supports several advanced features such
+as hot-plug and hot-remove of the physical memory, alternative memory
+maps for non-volatile memory devices and deferred initialization of
+the memory map for larger systems.
+
+The SPARSEMEM model presents the physical memory as a collection of
+sections. A section is represented with :c:type:`struct mem_section`
+that contains `section_mem_map` that is, logically, a pointer to an
+array of struct pages. However, it is stored with some other magic
+that aids the sections management. The section size and maximal number
+of section is specified using `SECTION_SIZE_BITS` and
+`MAX_PHYSMEM_BITS` constants defined by each architecture that
+supports SPARSEMEM. While `MAX_PHYSMEM_BITS` is an actual width of a
+physical address that an architecture supports, the
+`SECTION_SIZE_BITS` is an arbitrary value.
+
+The maximal number of sections is denoted `NR_MEM_SECTIONS` and
+defined as
+
+.. math::
+
+   NR\_MEM\_SECTIONS = 2 ^ {(MAX\_PHYSMEM\_BITS - SECTION\_SIZE\_BITS)}
+
+The `mem_section` objects are arranged in a two-dimensional array
+called `mem_sections`. The size and placement of this array depend
+on `CONFIG_SPARSEMEM_EXTREME` and the maximal possible number of
+sections:
+
+* When `CONFIG_SPARSEMEM_EXTREME` is disabled, the `mem_sections`
+  array is static and has `NR_MEM_SECTIONS` rows. Each row holds a
+  single `mem_section` object.
+* When `CONFIG_SPARSEMEM_EXTREME` is enabled, the `mem_sections`
+  array is dynamically allocated. Each row contains PAGE_SIZE worth of
+  `mem_section` objects and the number of rows is calculated to fit
+  all the memory sections.
+
+The architecture setup code should call :c:func:`memory_present` for
+each active memory range or use :c:func:`memblocks_present` or
+:c:func:`sparse_memory_present_with_active_regions` wrappers to
+initialize the memory sections. Next, the actual memory maps should be
+set up using :c:func:`sparse_init`.
+
+With SPARSEMEM there are two possible ways to convert a PFN to the
+corresponding `struct page` - a "classic sparse" and "sparse
+vmemmap". The selection is made at build time and it is determined by
+the value of `CONFIG_SPARSEMEM_VMEMMAP`.
+
+The classic sparse encodes the section number of a page in page->flags
+and uses high bits of a PFN to access the section that maps that page
+frame. Inside a section, the PFN is the index to the array of pages.
+
+The sparse vmemmap uses a virtually mapped memory map to optimize
+pfn_to_page and page_to_pfn operations. There is a global `struct
+page *vmemmap` pointer that points to a virtually contiguous array of
+`struct page` objects. A PFN is an index to that array and the the
+offset of the `struct page` from `vmemmap` is the PFN of that
+page.
+
+To use vmemmap, an architecture has to reserve a range of virtual
+addresses that will map the physical pages containing the memory
+map and make sure that `vmemmap` points to that range. In addition,
+the architecture should implement :c:func:`vmemmap_populate` method
+that will allocate the physical memory and create page tables for the
+virtual memory map. If an architecture does not have any special
+requirements for the vmemmap mappings, it can use default
+:c:func:`vmemmap_populate_basepages` provided by the generic memory
+management.
+
+The virtually mapped memory map allows storing `struct page` objects
+for persistent memory devices in pre-allocated storage on those
+devices. This storage is represented with :c:type:`struct vmem_altmap`
+that is eventually passed to vmemmap_populate() through a long chain
+of function calls. The vmemmap_populate() implementation may use the
+`vmem_altmap` along with :c:func:`altmap_alloc_block_buf` helper to
+allocate memory map on the persistent memory device.