diff options
Diffstat (limited to 'Documentation')
| -rw-r--r-- | Documentation/admin-guide/mm/memory-hotplug.rst | 42 | ||||
| -rw-r--r-- | Documentation/core-api/boot-time-mm.rst | 69 |
2 files changed, 50 insertions, 61 deletions
diff --git a/Documentation/admin-guide/mm/memory-hotplug.rst b/Documentation/admin-guide/mm/memory-hotplug.rst index 25157aec5b31..5c4432c96c4b 100644 --- a/Documentation/admin-guide/mm/memory-hotplug.rst +++ b/Documentation/admin-guide/mm/memory-hotplug.rst @@ -5,7 +5,7 @@ Memory Hotplug ============== :Created: Jul 28 2007 -:Updated: Add description of notifier of memory hotplug: Oct 11 2007 +:Updated: Add some details about locking internals: Aug 20 2018 This document is about memory hotplug including how-to-use and current status. Because Memory Hotplug is still under development, contents of this text will @@ -392,6 +392,46 @@ Need more implementation yet.... - Notification completion of remove works by OS to firmware. - Guard from remove if not yet. + +Locking Internals +================= + +When adding/removing memory that uses memory block devices (i.e. ordinary RAM), +the device_hotplug_lock should be held to: + +- synchronize against online/offline requests (e.g. via sysfs). This way, memory + block devices can only be accessed (.online/.state attributes) by user + space once memory has been fully added. And when removing memory, we + know nobody is in critical sections. +- synchronize against CPU hotplug and similar (e.g. relevant for ACPI and PPC) + +Especially, there is a possible lock inversion that is avoided using +device_hotplug_lock when adding memory and user space tries to online that +memory faster than expected: + +- device_online() will first take the device_lock(), followed by + mem_hotplug_lock +- add_memory_resource() will first take the mem_hotplug_lock, followed by + the device_lock() (while creating the devices, during bus_add_device()). + +As the device is visible to user space before taking the device_lock(), this +can result in a lock inversion. + +onlining/offlining of memory should be done via device_online()/ +device_offline() - to make sure it is properly synchronized to actions +via sysfs. Holding device_hotplug_lock is advised (to e.g. protect online_type) + +When adding/removing/onlining/offlining memory or adding/removing +heterogeneous/device memory, we should always hold the mem_hotplug_lock in +write mode to serialise memory hotplug (e.g. access to global/zone +variables). + +In addition, mem_hotplug_lock (in contrast to device_hotplug_lock) in read +mode allows for a quite efficient get_online_mems/put_online_mems +implementation, so code accessing memory can protect from that memory +vanishing. + + Future Work =========== diff --git a/Documentation/core-api/boot-time-mm.rst b/Documentation/core-api/boot-time-mm.rst index 6e12e89a03e0..e5ec9f1a563d 100644 --- a/Documentation/core-api/boot-time-mm.rst +++ b/Documentation/core-api/boot-time-mm.rst @@ -5,54 +5,23 @@ 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. +physical page allocator. -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. +A specialized allocator called ``memblock`` performs the +boot time memory management. The architecture specific initialization +must 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 -======= +when an allocation fails and those that don't. -(mostly stolen from Mel Gorman's "Understanding the Linux Virtual -Memory Manager" `book`_) +Memblock also offers a variety of APIs that control its own behaviour. -.. _book: https://www.kernel.org/doc/gorman/ - -.. kernel-doc:: mm/bootmem.c - :doc: bootmem overview - -.. _memblock: - -Memblock -======== +Memblock Overview +================= .. kernel-doc:: mm/memblock.c :doc: memblock overview @@ -61,26 +30,6 @@ Memblock 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 - :functions: - -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 |