224 lines
10 KiB
ReStructuredText
224 lines
10 KiB
ReStructuredText
.. 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.
|
|
|
|
ZONE_DEVICE
|
|
===========
|
|
The `ZONE_DEVICE` facility builds upon `SPARSEMEM_VMEMMAP` to offer
|
|
`struct page` `mem_map` services for device driver identified physical
|
|
address ranges. The "device" aspect of `ZONE_DEVICE` relates to the fact
|
|
that the page objects for these address ranges are never marked online,
|
|
and that a reference must be taken against the device, not just the page
|
|
to keep the memory pinned for active use. `ZONE_DEVICE`, via
|
|
:c:func:`devm_memremap_pages`, performs just enough memory hotplug to
|
|
turn on :c:func:`pfn_to_page`, :c:func:`page_to_pfn`, and
|
|
:c:func:`get_user_pages` service for the given range of pfns. Since the
|
|
page reference count never drops below 1 the page is never tracked as
|
|
free memory and the page's `struct list_head lru` space is repurposed
|
|
for back referencing to the host device / driver that mapped the memory.
|
|
|
|
While `SPARSEMEM` presents memory as a collection of sections,
|
|
optionally collected into memory blocks, `ZONE_DEVICE` users have a need
|
|
for smaller granularity of populating the `mem_map`. Given that
|
|
`ZONE_DEVICE` memory is never marked online it is subsequently never
|
|
subject to its memory ranges being exposed through the sysfs memory
|
|
hotplug api on memory block boundaries. The implementation relies on
|
|
this lack of user-api constraint to allow sub-section sized memory
|
|
ranges to be specified to :c:func:`arch_add_memory`, the top-half of
|
|
memory hotplug. Sub-section support allows for 2MB as the cross-arch
|
|
common alignment granularity for :c:func:`devm_memremap_pages`.
|
|
|
|
The users of `ZONE_DEVICE` are:
|
|
|
|
* pmem: Map platform persistent memory to be used as a direct-I/O target
|
|
via DAX mappings.
|
|
|
|
* hmm: Extend `ZONE_DEVICE` with `->page_fault()` and `->page_free()`
|
|
event callbacks to allow a device-driver to coordinate memory management
|
|
events related to device-memory, typically GPU memory. See
|
|
Documentation/vm/hmm.rst.
|
|
|
|
* p2pdma: Create `struct page` objects to allow peer devices in a
|
|
PCI/-E topology to coordinate direct-DMA operations between themselves,
|
|
i.e. bypass host memory.
|