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authorMike Rapoport <rppt@linux.vnet.ibm.com>2018-06-30 17:55:06 +0300
committerJonathan Corbet <corbet@lwn.net>2018-08-02 12:40:27 -0600
commitae9d88454446a314f9cda785a072a0c77a81591b (patch)
treefe298371b57855fcc66e6b0caac9470a5cb702fa /Documentation/core-api
parent3e039c5c0a070050172e0382f2d3f5396c759019 (diff)
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docs/mm: add description of boot time memory management
Both bootmem and memblock are have pretty good internal documentation coverage. With addition of some overview we get a nice description of the early memory management. Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Acked-by: Michal Hocko <mhocko@suse.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/core-api')
-rw-r--r--Documentation/core-api/boot-time-mm.rst92
-rw-r--r--Documentation/core-api/index.rst1
2 files changed, 93 insertions, 0 deletions
diff --git a/Documentation/core-api/boot-time-mm.rst b/Documentation/core-api/boot-time-mm.rst
new file mode 100644
index 000000000000..03cb1643f46f
--- /dev/null
+++ b/Documentation/core-api/boot-time-mm.rst
@@ -0,0 +1,92 @@
+===========================
+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:
+
+Bootmem
+=======
+
+(mostly stolen from Mel Gorman's "Understanding the Linux Virtual
+Memory Manager" `book`_)
+
+.. _book: https://www.kernel.org/doc/gorman/
+
+.. kernel-doc:: mm/bootmem.c
+ :doc: bootmem overview
+
+.. _memblock:
+
+Memblock
+========
+
+.. kernel-doc:: mm/memblock.c
+ :doc: memblock overview
+
+
+Functions and structures
+========================
+
+Common API
+----------
+
+The functions that are described in this section are available
+regardless of what early memory manager is enabled.
+
+.. kernel-doc:: mm/nobootmem.c
+
+Bootmem specific API
+--------------------
+
+These interfaces available only with bootmem, i.e when ``CONFIG_NO_BOOTMEM=n``
+
+.. kernel-doc:: include/linux/bootmem.h
+.. kernel-doc:: mm/bootmem.c
+ :nodocs:
+
+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.
+
+.. kernel-doc:: include/linux/memblock.h
+.. kernel-doc:: mm/memblock.c
+ :nodocs:
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f5a66b72f984..93d5a46122a1 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -28,6 +28,7 @@ Core utilities
printk-formats
circular-buffers
gfp_mask-from-fs-io
+ boot-time-mm
Interfaces for kernel debugging
===============================