1 .. SPDX-License-Identifier: GPL-2.0+ 2 3 4 ========== 5 Maple Tree 6 ========== 7 8 :Author: Liam R. Howlett 9 10 Overview 11 ======== 12 13 The Maple Tree is a B-Tree data type which is optimized for storing 14 non-overlapping ranges, including ranges of size 1. The tree was designed to 15 be simple to use and does not require a user written search method. It 16 supports iterating over a range of entries and going to the previous or next 17 entry in a cache-efficient manner. The tree can also be put into an RCU-safe 18 mode of operation which allows reading and writing concurrently. Writers must 19 synchronize on a lock, which can be the default spinlock, or the user can set 20 the lock to an external lock of a different type. 21 22 The Maple Tree maintains a small memory footprint and was designed to use 23 modern processor cache efficiently. The majority of the users will be able to 24 use the normal API. An :ref:`maple-tree-advanced-api` exists for more complex 25 scenarios. The most important usage of the Maple Tree is the tracking of the 26 virtual memory areas. 27 28 The Maple Tree can store values between ``0`` and ``ULONG_MAX``. The Maple 29 Tree reserves values with the bottom two bits set to '10' which are below 4096 30 (ie 2, 6, 10 .. 4094) for internal use. If the entries may use reserved 31 entries then the users can convert the entries using xa_mk_value() and convert 32 them back by calling xa_to_value(). If the user needs to use a reserved 33 value, then the user can convert the value when using the 34 :ref:`maple-tree-advanced-api`, but are blocked by the normal API. 35 36 The Maple Tree can also be configured to support searching for a gap of a given 37 size (or larger). 38 39 Pre-allocating of nodes is also supported using the 40 :ref:`maple-tree-advanced-api`. This is useful for users who must guarantee a 41 successful store operation within a given 42 code segment when allocating cannot be done. Allocations of nodes are 43 relatively small at around 256 bytes. 44 45 .. _maple-tree-normal-api: 46 47 Normal API 48 ========== 49 50 Start by initialising a maple tree, either with DEFINE_MTREE() for statically 51 allocated maple trees or mt_init() for dynamically allocated ones. A 52 freshly-initialised maple tree contains a ``NULL`` pointer for the range ``0`` 53 - ``ULONG_MAX``. There are currently two types of maple trees supported: the 54 allocation tree and the regular tree. The regular tree has a higher branching 55 factor for internal nodes. The allocation tree has a lower branching factor 56 but allows the user to search for a gap of a given size or larger from either 57 ``0`` upwards or ``ULONG_MAX`` down. An allocation tree can be used by 58 passing in the ``MT_FLAGS_ALLOC_RANGE`` flag when initialising the tree. 59 60 You can then set entries using mtree_store() or mtree_store_range(). 61 mtree_store() will overwrite any entry with the new entry and return 0 on 62 success or an error code otherwise. mtree_store_range() works in the same way 63 but takes a range. mtree_load() is used to retrieve the entry stored at a 64 given index. You can use mtree_erase() to erase an entire range by only 65 knowing one value within that range, or mtree_store() call with an entry of 66 NULL may be used to partially erase a range or many ranges at once. 67 68 If you want to only store a new entry to a range (or index) if that range is 69 currently ``NULL``, you can use mtree_insert_range() or mtree_insert() which 70 return -EEXIST if the range is not empty. 71 72 You can search for an entry from an index upwards by using mt_find(). 73 74 You can walk each entry within a range by calling mt_for_each(). You must 75 provide a temporary variable to store a cursor. If you want to walk each 76 element of the tree then ``0`` and ``ULONG_MAX`` may be used as the range. If 77 the caller is going to hold the lock for the duration of the walk then it is 78 worth looking at the mas_for_each() API in the :ref:`maple-tree-advanced-api` 79 section. 80 81 Sometimes it is necessary to ensure the next call to store to a maple tree does 82 not allocate memory, please see :ref:`maple-tree-advanced-api` for this use case. 83 84 You can use mtree_dup() to duplicate an entire maple tree. It is a more 85 efficient way than inserting all elements one by one into a new tree. 86 87 Finally, you can remove all entries from a maple tree by calling 88 mtree_destroy(). If the maple tree entries are pointers, you may wish to free 89 the entries first. 90 91 Allocating Nodes 92 ---------------- 93 94 The allocations are handled by the internal tree code. See 95 :ref:`maple-tree-advanced-alloc` for other options. 96 97 Locking 98 ------- 99 100 You do not have to worry about locking. See :ref:`maple-tree-advanced-locks` 101 for other options. 102 103 The Maple Tree uses RCU and an internal spinlock to synchronise access: 104 105 Takes RCU read lock: 106 * mtree_load() 107 * mt_find() 108 * mt_for_each() 109 * mt_next() 110 * mt_prev() 111 112 Takes ma_lock internally: 113 * mtree_store() 114 * mtree_store_range() 115 * mtree_insert() 116 * mtree_insert_range() 117 * mtree_erase() 118 * mtree_dup() 119 * mtree_destroy() 120 * mt_set_in_rcu() 121 * mt_clear_in_rcu() 122 123 If you want to take advantage of the internal lock to protect the data 124 structures that you are storing in the Maple Tree, you can call mtree_lock() 125 before calling mtree_load(), then take a reference count on the object you 126 have found before calling mtree_unlock(). This will prevent stores from 127 removing the object from the tree between looking up the object and 128 incrementing the refcount. You can also use RCU to avoid dereferencing 129 freed memory, but an explanation of that is beyond the scope of this 130 document. 131 132 .. _maple-tree-advanced-api: 133 134 Advanced API 135 ============ 136 137 The advanced API offers more flexibility and better performance at the 138 cost of an interface which can be harder to use and has fewer safeguards. 139 You must take care of your own locking while using the advanced API. 140 You can use the ma_lock, RCU or an external lock for protection. 141 You can mix advanced and normal operations on the same array, as long 142 as the locking is compatible. The :ref:`maple-tree-normal-api` is implemented 143 in terms of the advanced API. 144 145 The advanced API is based around the ma_state, this is where the 'mas' 146 prefix originates. The ma_state struct keeps track of tree operations to make 147 life easier for both internal and external tree users. 148 149 Initialising the maple tree is the same as in the :ref:`maple-tree-normal-api`. 150 Please see above. 151 152 The maple state keeps track of the range start and end in mas->index and 153 mas->last, respectively. 154 155 mas_walk() will walk the tree to the location of mas->index and set the 156 mas->index and mas->last according to the range for the entry. 157 158 You can set entries using mas_store(). mas_store() will overwrite any entry 159 with the new entry and return the first existing entry that is overwritten. 160 The range is passed in as members of the maple state: index and last. 161 162 You can use mas_erase() to erase an entire range by setting index and 163 last of the maple state to the desired range to erase. This will erase 164 the first range that is found in that range, set the maple state index 165 and last as the range that was erased and return the entry that existed 166 at that location. 167 168 You can walk each entry within a range by using mas_for_each(). If you want 169 to walk each element of the tree then ``0`` and ``ULONG_MAX`` may be used as 170 the range. If the lock needs to be periodically dropped, see the locking 171 section mas_pause(). 172 173 Using a maple state allows mas_next() and mas_prev() to function as if the 174 tree was a linked list. With such a high branching factor the amortized 175 performance penalty is outweighed by cache optimization. mas_next() will 176 return the next entry which occurs after the entry at index. mas_prev() 177 will return the previous entry which occurs before the entry at index. 178 179 mas_find() will find the first entry which exists at or above index on 180 the first call, and the next entry from every subsequent calls. 181 182 mas_find_rev() will find the first entry which exists at or below the last on 183 the first call, and the previous entry from every subsequent calls. 184 185 If the user needs to yield the lock during an operation, then the maple state 186 must be paused using mas_pause(). 187 188 There are a few extra interfaces provided when using an allocation tree. 189 If you wish to search for a gap within a range, then mas_empty_area() 190 or mas_empty_area_rev() can be used. mas_empty_area() searches for a gap 191 starting at the lowest index given up to the maximum of the range. 192 mas_empty_area_rev() searches for a gap starting at the highest index given 193 and continues downward to the lower bound of the range. 194 195 .. _maple-tree-advanced-alloc: 196 197 Advanced Allocating Nodes 198 ------------------------- 199 200 Allocations are usually handled internally to the tree, however if allocations 201 need to occur before a write occurs then calling mas_expected_entries() will 202 allocate the worst-case number of needed nodes to insert the provided number of 203 ranges. This also causes the tree to enter mass insertion mode. Once 204 insertions are complete calling mas_destroy() on the maple state will free the 205 unused allocations. 206 207 .. _maple-tree-advanced-locks: 208 209 Advanced Locking 210 ---------------- 211 212 The maple tree uses a spinlock by default, but external locks can be used for 213 tree updates as well. To use an external lock, the tree must be initialized 214 with the ``MT_FLAGS_LOCK_EXTERN flag``, this is usually done with the 215 MTREE_INIT_EXT() #define, which takes an external lock as an argument. 216 217 Functions and structures 218 ======================== 219 220 .. kernel-doc:: include/linux/maple_tree.h 221 .. kernel-doc:: lib/maple_tree.c
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.