Boot time memory management

Early system initialization cannot use "normal" memory management simply because it is not set up yet. But there is still need to allocate memory for various data structures, for instance for the physical page allocator. To address this, a specialized allocator called the :ref:`Boot Memory Allocator <bootmem>`, or bootmem, was introduced. Several years later PowerPC developers added a "Logical Memory Blocks" allocator, which was later adopted by other architectures and renamed to :ref:`memblock <memblock>`. There is also a compatibility layer called nobootmem that translates bootmem allocation interfaces to memblock calls.

The selection of the early allocator is done using CONFIG_NO_BOOTMEM and CONFIG_HAVE_MEMBLOCK kernel configuration options. These options are enabled or disabled statically by the architectures' Kconfig files.

• Architectures that rely only on bootmem select CONFIG_NO_BOOTMEM=n && CONFIG_HAVE_MEMBLOCK=n.
• The users of memblock with the nobootmem compatibility layer set CONFIG_NO_BOOTMEM=y && CONFIG_HAVE_MEMBLOCK=y.
• And for those that use both memblock and bootmem the configuration includes CONFIG_NO_BOOTMEM=n && CONFIG_HAVE_MEMBLOCK=y.

Whichever allocator is used, it is the responsibility of the architecture specific initialization to set it up in :c:func:`setup_arch` and tear it down in :c:func:`mem_init` functions.

Once the early memory management is available it offers a variety of functions and macros for memory allocations. The allocation request may be directed to the first (and probably the only) node or to a particular node in a NUMA system. There are API variants that panic when an allocation fails and those that don't. And more recent and advanced memblock even allows controlling its own behaviour.

Bootmem

(mostly stolen from Mel Gorman's "Understanding the Linux Virtual Memory Manager" book)

Memblock

Functions and structures

Common API

The functions that are described in this section are available regardless of what early memory manager is enabled.

Bootmem specific API

These interfaces available only with bootmem, i.e when CONFIG_NO_BOOTMEM=n

Memblock specific API

Here is the description of memblock data structures, functions and macros. Some of them are actually internal, but since they are documented it would be silly to omit them. Besides, reading the descriptions for the internal functions can help to understand what really happens under the hood.