1.. SPDX-License-Identifier: GPL-2.0+ 2 3====== 4XArray 5====== 6 7:Author: Matthew Wilcox 8 9Overview 10======== 11 12The XArray is an abstract data type which behaves like a very large array 13of pointers. It meets many of the same needs as a hash or a conventional 14resizable array. Unlike a hash, it allows you to sensibly go to the 15next or previous entry in a cache-efficient manner. In contrast to a 16resizable array, there is no need to copy data or change MMU mappings in 17order to grow the array. It is more memory-efficient, parallelisable 18and cache friendly than a doubly-linked list. It takes advantage of 19RCU to perform lookups without locking. 20 21The XArray implementation is efficient when the indices used are densely 22clustered; hashing the object and using the hash as the index will not 23perform well. The XArray is optimised for small indices, but still has 24good performance with large indices. If your index can be larger than 25``ULONG_MAX`` then the XArray is not the data type for you. The most 26important user of the XArray is the page cache. 27 28Each non-``NULL`` entry in the array has three bits associated with 29it called marks. Each mark may be set or cleared independently of 30the others. You can iterate over entries which are marked. 31 32Normal pointers may be stored in the XArray directly. They must be 4-byte 33aligned, which is true for any pointer returned from :c:func:`kmalloc` and 34:c:func:`alloc_page`. It isn't true for arbitrary user-space pointers, 35nor for function pointers. You can store pointers to statically allocated 36objects, as long as those objects have an alignment of at least 4. 37 38You can also store integers between 0 and ``LONG_MAX`` in the XArray. 39You must first convert it into an entry using :c:func:`xa_mk_value`. 40When you retrieve an entry from the XArray, you can check whether it is 41a value entry by calling :c:func:`xa_is_value`, and convert it back to 42an integer by calling :c:func:`xa_to_value`. 43 44Some users want to store tagged pointers instead of using the marks 45described above. They can call :c:func:`xa_tag_pointer` to create an 46entry with a tag, :c:func:`xa_untag_pointer` to turn a tagged entry 47back into an untagged pointer and :c:func:`xa_pointer_tag` to retrieve 48the tag of an entry. Tagged pointers use the same bits that are used 49to distinguish value entries from normal pointers, so each user must 50decide whether they want to store value entries or tagged pointers in 51any particular XArray. 52 53The XArray does not support storing :c:func:`IS_ERR` pointers as some 54conflict with value entries or internal entries. 55 56An unusual feature of the XArray is the ability to create entries which 57occupy a range of indices. Once stored to, looking up any index in 58the range will return the same entry as looking up any other index in 59the range. Setting a mark on one index will set it on all of them. 60Storing to any index will store to all of them. Multi-index entries can 61be explicitly split into smaller entries, or storing ``NULL`` into any 62entry will cause the XArray to forget about the range. 63 64Normal API 65========== 66 67Start by initialising an XArray, either with :c:func:`DEFINE_XARRAY` 68for statically allocated XArrays or :c:func:`xa_init` for dynamically 69allocated ones. A freshly-initialised XArray contains a ``NULL`` 70pointer at every index. 71 72You can then set entries using :c:func:`xa_store` and get entries 73using :c:func:`xa_load`. xa_store will overwrite any entry with the 74new entry and return the previous entry stored at that index. You can 75use :c:func:`xa_erase` instead of calling :c:func:`xa_store` with a 76``NULL`` entry. There is no difference between an entry that has never 77been stored to and one that has most recently had ``NULL`` stored to it. 78 79You can conditionally replace an entry at an index by using 80:c:func:`xa_cmpxchg`. Like :c:func:`cmpxchg`, it will only succeed if 81the entry at that index has the 'old' value. It also returns the entry 82which was at that index; if it returns the same entry which was passed as 83'old', then :c:func:`xa_cmpxchg` succeeded. 84 85If you want to only store a new entry to an index if the current entry 86at that index is ``NULL``, you can use :c:func:`xa_insert` which 87returns ``-EEXIST`` if the entry is not empty. 88 89You can enquire whether a mark is set on an entry by using 90:c:func:`xa_get_mark`. If the entry is not ``NULL``, you can set a mark 91on it by using :c:func:`xa_set_mark` and remove the mark from an entry by 92calling :c:func:`xa_clear_mark`. You can ask whether any entry in the 93XArray has a particular mark set by calling :c:func:`xa_marked`. 94 95You can copy entries out of the XArray into a plain array by calling 96:c:func:`xa_extract`. Or you can iterate over the present entries in 97the XArray by calling :c:func:`xa_for_each`. You may prefer to use 98:c:func:`xa_find` or :c:func:`xa_find_after` to move to the next present 99entry in the XArray. 100 101Finally, you can remove all entries from an XArray by calling 102:c:func:`xa_destroy`. If the XArray entries are pointers, you may wish 103to free the entries first. You can do this by iterating over all present 104entries in the XArray using the :c:func:`xa_for_each` iterator. 105 106Memory allocation 107----------------- 108 109The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_reserve` 110and :c:func:`xa_insert` functions take a gfp_t parameter in case 111the XArray needs to allocate memory to store this entry. 112If the entry is being deleted, no memory allocation needs to be performed, 113and the GFP flags specified will be ignored. 114 115It is possible for no memory to be allocatable, particularly if you pass 116a restrictive set of GFP flags. In that case, the functions return a 117special value which can be turned into an errno using :c:func:`xa_err`. 118If you don't need to know exactly which error occurred, using 119:c:func:`xa_is_err` is slightly more efficient. 120 121Locking 122------- 123 124When using the Normal API, you do not have to worry about locking. 125The XArray uses RCU and an internal spinlock to synchronise access: 126 127No lock needed: 128 * :c:func:`xa_empty` 129 * :c:func:`xa_marked` 130 131Takes RCU read lock: 132 * :c:func:`xa_load` 133 * :c:func:`xa_for_each` 134 * :c:func:`xa_find` 135 * :c:func:`xa_find_after` 136 * :c:func:`xa_extract` 137 * :c:func:`xa_get_mark` 138 139Takes xa_lock internally: 140 * :c:func:`xa_store` 141 * :c:func:`xa_insert` 142 * :c:func:`xa_erase` 143 * :c:func:`xa_erase_bh` 144 * :c:func:`xa_erase_irq` 145 * :c:func:`xa_cmpxchg` 146 * :c:func:`xa_destroy` 147 * :c:func:`xa_set_mark` 148 * :c:func:`xa_clear_mark` 149 150Assumes xa_lock held on entry: 151 * :c:func:`__xa_store` 152 * :c:func:`__xa_insert` 153 * :c:func:`__xa_erase` 154 * :c:func:`__xa_cmpxchg` 155 * :c:func:`__xa_set_mark` 156 * :c:func:`__xa_clear_mark` 157 158If you want to take advantage of the lock to protect the data structures 159that you are storing in the XArray, you can call :c:func:`xa_lock` 160before calling :c:func:`xa_load`, then take a reference count on the 161object you have found before calling :c:func:`xa_unlock`. This will 162prevent stores from removing the object from the array between looking 163up the object and incrementing the refcount. You can also use RCU to 164avoid dereferencing freed memory, but an explanation of that is beyond 165the scope of this document. 166 167The XArray does not disable interrupts or softirqs while modifying 168the array. It is safe to read the XArray from interrupt or softirq 169context as the RCU lock provides enough protection. 170 171If, for example, you want to store entries in the XArray in process 172context and then erase them in softirq context, you can do that this way:: 173 174 void foo_init(struct foo *foo) 175 { 176 xa_init_flags(&foo->array, XA_FLAGS_LOCK_BH); 177 } 178 179 int foo_store(struct foo *foo, unsigned long index, void *entry) 180 { 181 int err; 182 183 xa_lock_bh(&foo->array); 184 err = xa_err(__xa_store(&foo->array, index, entry, GFP_KERNEL)); 185 if (!err) 186 foo->count++; 187 xa_unlock_bh(&foo->array); 188 return err; 189 } 190 191 /* foo_erase() is only called from softirq context */ 192 void foo_erase(struct foo *foo, unsigned long index) 193 { 194 xa_lock(&foo->array); 195 __xa_erase(&foo->array, index); 196 foo->count--; 197 xa_unlock(&foo->array); 198 } 199 200If you are going to modify the XArray from interrupt or softirq context, 201you need to initialise the array using :c:func:`xa_init_flags`, passing 202``XA_FLAGS_LOCK_IRQ`` or ``XA_FLAGS_LOCK_BH``. 203 204The above example also shows a common pattern of wanting to extend the 205coverage of the xa_lock on the store side to protect some statistics 206associated with the array. 207 208Sharing the XArray with interrupt context is also possible, either 209using :c:func:`xa_lock_irqsave` in both the interrupt handler and process 210context, or :c:func:`xa_lock_irq` in process context and :c:func:`xa_lock` 211in the interrupt handler. Some of the more common patterns have helper 212functions such as :c:func:`xa_erase_bh` and :c:func:`xa_erase_irq`. 213 214Sometimes you need to protect access to the XArray with a mutex because 215that lock sits above another mutex in the locking hierarchy. That does 216not entitle you to use functions like :c:func:`__xa_erase` without taking 217the xa_lock; the xa_lock is used for lockdep validation and will be used 218for other purposes in the future. 219 220The :c:func:`__xa_set_mark` and :c:func:`__xa_clear_mark` functions are also 221available for situations where you look up an entry and want to atomically 222set or clear a mark. It may be more efficient to use the advanced API 223in this case, as it will save you from walking the tree twice. 224 225Advanced API 226============ 227 228The advanced API offers more flexibility and better performance at the 229cost of an interface which can be harder to use and has fewer safeguards. 230No locking is done for you by the advanced API, and you are required 231to use the xa_lock while modifying the array. You can choose whether 232to use the xa_lock or the RCU lock while doing read-only operations on 233the array. You can mix advanced and normal operations on the same array; 234indeed the normal API is implemented in terms of the advanced API. The 235advanced API is only available to modules with a GPL-compatible license. 236 237The advanced API is based around the xa_state. This is an opaque data 238structure which you declare on the stack using the :c:func:`XA_STATE` 239macro. This macro initialises the xa_state ready to start walking 240around the XArray. It is used as a cursor to maintain the position 241in the XArray and let you compose various operations together without 242having to restart from the top every time. 243 244The xa_state is also used to store errors. You can call 245:c:func:`xas_error` to retrieve the error. All operations check whether 246the xa_state is in an error state before proceeding, so there's no need 247for you to check for an error after each call; you can make multiple 248calls in succession and only check at a convenient point. The only 249errors currently generated by the XArray code itself are ``ENOMEM`` and 250``EINVAL``, but it supports arbitrary errors in case you want to call 251:c:func:`xas_set_err` yourself. 252 253If the xa_state is holding an ``ENOMEM`` error, calling :c:func:`xas_nomem` 254will attempt to allocate more memory using the specified gfp flags and 255cache it in the xa_state for the next attempt. The idea is that you take 256the xa_lock, attempt the operation and drop the lock. The operation 257attempts to allocate memory while holding the lock, but it is more 258likely to fail. Once you have dropped the lock, :c:func:`xas_nomem` 259can try harder to allocate more memory. It will return ``true`` if it 260is worth retrying the operation (i.e. that there was a memory error *and* 261more memory was allocated). If it has previously allocated memory, and 262that memory wasn't used, and there is no error (or some error that isn't 263``ENOMEM``), then it will free the memory previously allocated. 264 265Internal Entries 266---------------- 267 268The XArray reserves some entries for its own purposes. These are never 269exposed through the normal API, but when using the advanced API, it's 270possible to see them. Usually the best way to handle them is to pass them 271to :c:func:`xas_retry`, and retry the operation if it returns ``true``. 272 273.. flat-table:: 274 :widths: 1 1 6 275 276 * - Name 277 - Test 278 - Usage 279 280 * - Node 281 - :c:func:`xa_is_node` 282 - An XArray node. May be visible when using a multi-index xa_state. 283 284 * - Sibling 285 - :c:func:`xa_is_sibling` 286 - A non-canonical entry for a multi-index entry. The value indicates 287 which slot in this node has the canonical entry. 288 289 * - Retry 290 - :c:func:`xa_is_retry` 291 - This entry is currently being modified by a thread which has the 292 xa_lock. The node containing this entry may be freed at the end 293 of this RCU period. You should restart the lookup from the head 294 of the array. 295 296 * - Zero 297 - :c:func:`xa_is_zero` 298 - Zero entries appear as ``NULL`` through the Normal API, but occupy 299 an entry in the XArray which can be used to reserve the index for 300 future use. 301 302Other internal entries may be added in the future. As far as possible, they 303will be handled by :c:func:`xas_retry`. 304 305Additional functionality 306------------------------ 307 308The :c:func:`xas_create_range` function allocates all the necessary memory 309to store every entry in a range. It will set ENOMEM in the xa_state if 310it cannot allocate memory. 311 312You can use :c:func:`xas_init_marks` to reset the marks on an entry 313to their default state. This is usually all marks clear, unless the 314XArray is marked with ``XA_FLAGS_TRACK_FREE``, in which case mark 0 is set 315and all other marks are clear. Replacing one entry with another using 316:c:func:`xas_store` will not reset the marks on that entry; if you want 317the marks reset, you should do that explicitly. 318 319The :c:func:`xas_load` will walk the xa_state as close to the entry 320as it can. If you know the xa_state has already been walked to the 321entry and need to check that the entry hasn't changed, you can use 322:c:func:`xas_reload` to save a function call. 323 324If you need to move to a different index in the XArray, call 325:c:func:`xas_set`. This resets the cursor to the top of the tree, which 326will generally make the next operation walk the cursor to the desired 327spot in the tree. If you want to move to the next or previous index, 328call :c:func:`xas_next` or :c:func:`xas_prev`. Setting the index does 329not walk the cursor around the array so does not require a lock to be 330held, while moving to the next or previous index does. 331 332You can search for the next present entry using :c:func:`xas_find`. This 333is the equivalent of both :c:func:`xa_find` and :c:func:`xa_find_after`; 334if the cursor has been walked to an entry, then it will find the next 335entry after the one currently referenced. If not, it will return the 336entry at the index of the xa_state. Using :c:func:`xas_next_entry` to 337move to the next present entry instead of :c:func:`xas_find` will save 338a function call in the majority of cases at the expense of emitting more 339inline code. 340 341The :c:func:`xas_find_marked` function is similar. If the xa_state has 342not been walked, it will return the entry at the index of the xa_state, 343if it is marked. Otherwise, it will return the first marked entry after 344the entry referenced by the xa_state. The :c:func:`xas_next_marked` 345function is the equivalent of :c:func:`xas_next_entry`. 346 347When iterating over a range of the XArray using :c:func:`xas_for_each` 348or :c:func:`xas_for_each_marked`, it may be necessary to temporarily stop 349the iteration. The :c:func:`xas_pause` function exists for this purpose. 350After you have done the necessary work and wish to resume, the xa_state 351is in an appropriate state to continue the iteration after the entry 352you last processed. If you have interrupts disabled while iterating, 353then it is good manners to pause the iteration and reenable interrupts 354every ``XA_CHECK_SCHED`` entries. 355 356The :c:func:`xas_get_mark`, :c:func:`xas_set_mark` and 357:c:func:`xas_clear_mark` functions require the xa_state cursor to have 358been moved to the appropriate location in the xarray; they will do 359nothing if you have called :c:func:`xas_pause` or :c:func:`xas_set` 360immediately before. 361 362You can call :c:func:`xas_set_update` to have a callback function 363called each time the XArray updates a node. This is used by the page 364cache workingset code to maintain its list of nodes which contain only 365shadow entries. 366 367Multi-Index Entries 368------------------- 369 370The XArray has the ability to tie multiple indices together so that 371operations on one index affect all indices. For example, storing into 372any index will change the value of the entry retrieved from any index. 373Setting or clearing a mark on any index will set or clear the mark 374on every index that is tied together. The current implementation 375only allows tying ranges which are aligned powers of two together; 376eg indices 64-127 may be tied together, but 2-6 may not be. This may 377save substantial quantities of memory; for example tying 512 entries 378together will save over 4kB. 379 380You can create a multi-index entry by using :c:func:`XA_STATE_ORDER` 381or :c:func:`xas_set_order` followed by a call to :c:func:`xas_store`. 382Calling :c:func:`xas_load` with a multi-index xa_state will walk the 383xa_state to the right location in the tree, but the return value is not 384meaningful, potentially being an internal entry or ``NULL`` even when there 385is an entry stored within the range. Calling :c:func:`xas_find_conflict` 386will return the first entry within the range or ``NULL`` if there are no 387entries in the range. The :c:func:`xas_for_each_conflict` iterator will 388iterate over every entry which overlaps the specified range. 389 390If :c:func:`xas_load` encounters a multi-index entry, the xa_index 391in the xa_state will not be changed. When iterating over an XArray 392or calling :c:func:`xas_find`, if the initial index is in the middle 393of a multi-index entry, it will not be altered. Subsequent calls 394or iterations will move the index to the first index in the range. 395Each entry will only be returned once, no matter how many indices it 396occupies. 397 398Using :c:func:`xas_next` or :c:func:`xas_prev` with a multi-index xa_state 399is not supported. Using either of these functions on a multi-index entry 400will reveal sibling entries; these should be skipped over by the caller. 401 402Storing ``NULL`` into any index of a multi-index entry will set the entry 403at every index to ``NULL`` and dissolve the tie. Splitting a multi-index 404entry into entries occupying smaller ranges is not yet supported. 405 406Functions and structures 407======================== 408 409.. kernel-doc:: include/linux/xarray.h 410.. kernel-doc:: lib/xarray.c 411