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diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst new file mode 100644 index 0000000000..77e0ece2b1 --- /dev/null +++ b/Documentation/core-api/xarray.rst @@ -0,0 +1,496 @@ +.. SPDX-License-Identifier: GPL-2.0+ + +====== +XArray +====== + +:Author: Matthew Wilcox + +Overview +======== + +The XArray is an abstract data type which behaves like a very large array +of pointers. It meets many of the same needs as a hash or a conventional +resizable array. Unlike a hash, it allows you to sensibly go to the +next or previous entry in a cache-efficient manner. In contrast to a +resizable array, there is no need to copy data or change MMU mappings in +order to grow the array. It is more memory-efficient, parallelisable +and cache friendly than a doubly-linked list. It takes advantage of +RCU to perform lookups without locking. + +The XArray implementation is efficient when the indices used are densely +clustered; hashing the object and using the hash as the index will not +perform well. The XArray is optimised for small indices, but still has +good performance with large indices. If your index can be larger than +``ULONG_MAX`` then the XArray is not the data type for you. The most +important user of the XArray is the page cache. + +Normal pointers may be stored in the XArray directly. They must be 4-byte +aligned, which is true for any pointer returned from kmalloc() and +alloc_page(). It isn't true for arbitrary user-space pointers, +nor for function pointers. You can store pointers to statically allocated +objects, as long as those objects have an alignment of at least 4. + +You can also store integers between 0 and ``LONG_MAX`` in the XArray. +You must first convert it into an entry using xa_mk_value(). +When you retrieve an entry from the XArray, you can check whether it is +a value entry by calling xa_is_value(), and convert it back to +an integer by calling xa_to_value(). + +Some users want to tag the pointers they store in the XArray. You can +call xa_tag_pointer() to create an entry with a tag, xa_untag_pointer() +to turn a tagged entry back into an untagged pointer and xa_pointer_tag() +to retrieve the tag of an entry. Tagged pointers use the same bits that +are used to distinguish value entries from normal pointers, so you must +decide whether they want to store value entries or tagged pointers in +any particular XArray. + +The XArray does not support storing IS_ERR() pointers as some +conflict with value entries or internal entries. + +An unusual feature of the XArray is the ability to create entries which +occupy a range of indices. Once stored to, looking up any index in +the range will return the same entry as looking up any other index in +the range. Storing to any index will store to all of them. Multi-index +entries can be explicitly split into smaller entries, or storing ``NULL`` +into any entry will cause the XArray to forget about the range. + +Normal API +========== + +Start by initialising an XArray, either with DEFINE_XARRAY() +for statically allocated XArrays or xa_init() for dynamically +allocated ones. A freshly-initialised XArray contains a ``NULL`` +pointer at every index. + +You can then set entries using xa_store() and get entries +using xa_load(). xa_store will overwrite any entry with the +new entry and return the previous entry stored at that index. You can +use xa_erase() instead of calling xa_store() with a +``NULL`` entry. There is no difference between an entry that has never +been stored to, one that has been erased and one that has most recently +had ``NULL`` stored to it. + +You can conditionally replace an entry at an index by using +xa_cmpxchg(). Like cmpxchg(), it will only succeed if +the entry at that index has the 'old' value. It also returns the entry +which was at that index; if it returns the same entry which was passed as +'old', then xa_cmpxchg() succeeded. + +If you want to only store a new entry to an index if the current entry +at that index is ``NULL``, you can use xa_insert() which +returns ``-EBUSY`` if the entry is not empty. + +You can copy entries out of the XArray into a plain array by calling +xa_extract(). Or you can iterate over the present entries in the XArray +by calling xa_for_each(), xa_for_each_start() or xa_for_each_range(). +You may prefer to use xa_find() or xa_find_after() to move to the next +present entry in the XArray. + +Calling xa_store_range() stores the same entry in a range +of indices. If you do this, some of the other operations will behave +in a slightly odd way. For example, marking the entry at one index +may result in the entry being marked at some, but not all of the other +indices. Storing into one index may result in the entry retrieved by +some, but not all of the other indices changing. + +Sometimes you need to ensure that a subsequent call to xa_store() +will not need to allocate memory. The xa_reserve() function +will store a reserved entry at the indicated index. Users of the +normal API will see this entry as containing ``NULL``. If you do +not need to use the reserved entry, you can call xa_release() +to remove the unused entry. If another user has stored to the entry +in the meantime, xa_release() will do nothing; if instead you +want the entry to become ``NULL``, you should use xa_erase(). +Using xa_insert() on a reserved entry will fail. + +If all entries in the array are ``NULL``, the xa_empty() function +will return ``true``. + +Finally, you can remove all entries from an XArray by calling +xa_destroy(). If the XArray entries are pointers, you may wish +to free the entries first. You can do this by iterating over all present +entries in the XArray using the xa_for_each() iterator. + +Search Marks +------------ + +Each entry in the array has three bits associated with it called marks. +Each mark may be set or cleared independently of the others. You can +iterate over marked entries by using the xa_for_each_marked() iterator. + +You can enquire whether a mark is set on an entry by using +xa_get_mark(). If the entry is not ``NULL``, you can set a mark on it +by using xa_set_mark() and remove the mark from an entry by calling +xa_clear_mark(). You can ask whether any entry in the XArray has a +particular mark set by calling xa_marked(). Erasing an entry from the +XArray causes all marks associated with that entry to be cleared. + +Setting or clearing a mark on any index of a multi-index entry will +affect all indices covered by that entry. Querying the mark on any +index will return the same result. + +There is no way to iterate over entries which are not marked; the data +structure does not allow this to be implemented efficiently. There are +not currently iterators to search for logical combinations of bits (eg +iterate over all entries which have both ``XA_MARK_1`` and ``XA_MARK_2`` +set, or iterate over all entries which have ``XA_MARK_0`` or ``XA_MARK_2`` +set). It would be possible to add these if a user arises. + +Allocating XArrays +------------------ + +If you use DEFINE_XARRAY_ALLOC() to define the XArray, or +initialise it by passing ``XA_FLAGS_ALLOC`` to xa_init_flags(), +the XArray changes to track whether entries are in use or not. + +You can call xa_alloc() to store the entry at an unused index +in the XArray. If you need to modify the array from interrupt context, +you can use xa_alloc_bh() or xa_alloc_irq() to disable +interrupts while allocating the ID. + +Using xa_store(), xa_cmpxchg() or xa_insert() will +also mark the entry as being allocated. Unlike a normal XArray, storing +``NULL`` will mark the entry as being in use, like xa_reserve(). +To free an entry, use xa_erase() (or xa_release() if +you only want to free the entry if it's ``NULL``). + +By default, the lowest free entry is allocated starting from 0. If you +want to allocate entries starting at 1, it is more efficient to use +DEFINE_XARRAY_ALLOC1() or ``XA_FLAGS_ALLOC1``. If you want to +allocate IDs up to a maximum, then wrap back around to the lowest free +ID, you can use xa_alloc_cyclic(). + +You cannot use ``XA_MARK_0`` with an allocating XArray as this mark +is used to track whether an entry is free or not. The other marks are +available for your use. + +Memory allocation +----------------- + +The xa_store(), xa_cmpxchg(), xa_alloc(), +xa_reserve() and xa_insert() functions take a gfp_t +parameter in case the XArray needs to allocate memory to store this entry. +If the entry is being deleted, no memory allocation needs to be performed, +and the GFP flags specified will be ignored. + +It is possible for no memory to be allocatable, particularly if you pass +a restrictive set of GFP flags. In that case, the functions return a +special value which can be turned into an errno using xa_err(). +If you don't need to know exactly which error occurred, using +xa_is_err() is slightly more efficient. + +Locking +------- + +When using the Normal API, you do not have to worry about locking. +The XArray uses RCU and an internal spinlock to synchronise access: + +No lock needed: + * xa_empty() + * xa_marked() + +Takes RCU read lock: + * xa_load() + * xa_for_each() + * xa_for_each_start() + * xa_for_each_range() + * xa_find() + * xa_find_after() + * xa_extract() + * xa_get_mark() + +Takes xa_lock internally: + * xa_store() + * xa_store_bh() + * xa_store_irq() + * xa_insert() + * xa_insert_bh() + * xa_insert_irq() + * xa_erase() + * xa_erase_bh() + * xa_erase_irq() + * xa_cmpxchg() + * xa_cmpxchg_bh() + * xa_cmpxchg_irq() + * xa_store_range() + * xa_alloc() + * xa_alloc_bh() + * xa_alloc_irq() + * xa_reserve() + * xa_reserve_bh() + * xa_reserve_irq() + * xa_destroy() + * xa_set_mark() + * xa_clear_mark() + +Assumes xa_lock held on entry: + * __xa_store() + * __xa_insert() + * __xa_erase() + * __xa_cmpxchg() + * __xa_alloc() + * __xa_set_mark() + * __xa_clear_mark() + +If you want to take advantage of the lock to protect the data structures +that you are storing in the XArray, you can call xa_lock() +before calling xa_load(), then take a reference count on the +object you have found before calling xa_unlock(). This will +prevent stores from removing the object from the array between looking +up the object and incrementing the refcount. You can also use RCU to +avoid dereferencing freed memory, but an explanation of that is beyond +the scope of this document. + +The XArray does not disable interrupts or softirqs while modifying +the array. It is safe to read the XArray from interrupt or softirq +context as the RCU lock provides enough protection. + +If, for example, you want to store entries in the XArray in process +context and then erase them in softirq context, you can do that this way:: + + void foo_init(struct foo *foo) + { + xa_init_flags(&foo->array, XA_FLAGS_LOCK_BH); + } + + int foo_store(struct foo *foo, unsigned long index, void *entry) + { + int err; + + xa_lock_bh(&foo->array); + err = xa_err(__xa_store(&foo->array, index, entry, GFP_KERNEL)); + if (!err) + foo->count++; + xa_unlock_bh(&foo->array); + return err; + } + + /* foo_erase() is only called from softirq context */ + void foo_erase(struct foo *foo, unsigned long index) + { + xa_lock(&foo->array); + __xa_erase(&foo->array, index); + foo->count--; + xa_unlock(&foo->array); + } + +If you are going to modify the XArray from interrupt or softirq context, +you need to initialise the array using xa_init_flags(), passing +``XA_FLAGS_LOCK_IRQ`` or ``XA_FLAGS_LOCK_BH``. + +The above example also shows a common pattern of wanting to extend the +coverage of the xa_lock on the store side to protect some statistics +associated with the array. + +Sharing the XArray with interrupt context is also possible, either +using xa_lock_irqsave() in both the interrupt handler and process +context, or xa_lock_irq() in process context and xa_lock() +in the interrupt handler. Some of the more common patterns have helper +functions such as xa_store_bh(), xa_store_irq(), +xa_erase_bh(), xa_erase_irq(), xa_cmpxchg_bh() +and xa_cmpxchg_irq(). + +Sometimes you need to protect access to the XArray with a mutex because +that lock sits above another mutex in the locking hierarchy. That does +not entitle you to use functions like __xa_erase() without taking +the xa_lock; the xa_lock is used for lockdep validation and will be used +for other purposes in the future. + +The __xa_set_mark() and __xa_clear_mark() functions are also +available for situations where you look up an entry and want to atomically +set or clear a mark. It may be more efficient to use the advanced API +in this case, as it will save you from walking the tree twice. + +Advanced API +============ + +The advanced API offers more flexibility and better performance at the +cost of an interface which can be harder to use and has fewer safeguards. +No locking is done for you by the advanced API, and you are required +to use the xa_lock while modifying the array. You can choose whether +to use the xa_lock or the RCU lock while doing read-only operations on +the array. You can mix advanced and normal operations on the same array; +indeed the normal API is implemented in terms of the advanced API. The +advanced API is only available to modules with a GPL-compatible license. + +The advanced API is based around the xa_state. This is an opaque data +structure which you declare on the stack using the XA_STATE() macro. +This macro initialises the xa_state ready to start walking around the +XArray. It is used as a cursor to maintain the position in the XArray +and let you compose various operations together without having to restart +from the top every time. The contents of the xa_state are protected by +the rcu_read_lock() or the xas_lock(). If you need to drop whichever of +those locks is protecting your state and tree, you must call xas_pause() +so that future calls do not rely on the parts of the state which were +left unprotected. + +The xa_state is also used to store errors. You can call +xas_error() to retrieve the error. All operations check whether +the xa_state is in an error state before proceeding, so there's no need +for you to check for an error after each call; you can make multiple +calls in succession and only check at a convenient point. The only +errors currently generated by the XArray code itself are ``ENOMEM`` and +``EINVAL``, but it supports arbitrary errors in case you want to call +xas_set_err() yourself. + +If the xa_state is holding an ``ENOMEM`` error, calling xas_nomem() +will attempt to allocate more memory using the specified gfp flags and +cache it in the xa_state for the next attempt. The idea is that you take +the xa_lock, attempt the operation and drop the lock. The operation +attempts to allocate memory while holding the lock, but it is more +likely to fail. Once you have dropped the lock, xas_nomem() +can try harder to allocate more memory. It will return ``true`` if it +is worth retrying the operation (i.e. that there was a memory error *and* +more memory was allocated). If it has previously allocated memory, and +that memory wasn't used, and there is no error (or some error that isn't +``ENOMEM``), then it will free the memory previously allocated. + +Internal Entries +---------------- + +The XArray reserves some entries for its own purposes. These are never +exposed through the normal API, but when using the advanced API, it's +possible to see them. Usually the best way to handle them is to pass them +to xas_retry(), and retry the operation if it returns ``true``. + +.. flat-table:: + :widths: 1 1 6 + + * - Name + - Test + - Usage + + * - Node + - xa_is_node() + - An XArray node. May be visible when using a multi-index xa_state. + + * - Sibling + - xa_is_sibling() + - A non-canonical entry for a multi-index entry. The value indicates + which slot in this node has the canonical entry. + + * - Retry + - xa_is_retry() + - This entry is currently being modified by a thread which has the + xa_lock. The node containing this entry may be freed at the end + of this RCU period. You should restart the lookup from the head + of the array. + + * - Zero + - xa_is_zero() + - Zero entries appear as ``NULL`` through the Normal API, but occupy + an entry in the XArray which can be used to reserve the index for + future use. This is used by allocating XArrays for allocated entries + which are ``NULL``. + +Other internal entries may be added in the future. As far as possible, they +will be handled by xas_retry(). + +Additional functionality +------------------------ + +The xas_create_range() function allocates all the necessary memory +to store every entry in a range. It will set ENOMEM in the xa_state if +it cannot allocate memory. + +You can use xas_init_marks() to reset the marks on an entry +to their default state. This is usually all marks clear, unless the +XArray is marked with ``XA_FLAGS_TRACK_FREE``, in which case mark 0 is set +and all other marks are clear. Replacing one entry with another using +xas_store() will not reset the marks on that entry; if you want +the marks reset, you should do that explicitly. + +The xas_load() will walk the xa_state as close to the entry +as it can. If you know the xa_state has already been walked to the +entry and need to check that the entry hasn't changed, you can use +xas_reload() to save a function call. + +If you need to move to a different index in the XArray, call +xas_set(). This resets the cursor to the top of the tree, which +will generally make the next operation walk the cursor to the desired +spot in the tree. If you want to move to the next or previous index, +call xas_next() or xas_prev(). Setting the index does +not walk the cursor around the array so does not require a lock to be +held, while moving to the next or previous index does. + +You can search for the next present entry using xas_find(). This +is the equivalent of both xa_find() and xa_find_after(); +if the cursor has been walked to an entry, then it will find the next +entry after the one currently referenced. If not, it will return the +entry at the index of the xa_state. Using xas_next_entry() to +move to the next present entry instead of xas_find() will save +a function call in the majority of cases at the expense of emitting more +inline code. + +The xas_find_marked() function is similar. If the xa_state has +not been walked, it will return the entry at the index of the xa_state, +if it is marked. Otherwise, it will return the first marked entry after +the entry referenced by the xa_state. The xas_next_marked() +function is the equivalent of xas_next_entry(). + +When iterating over a range of the XArray using xas_for_each() +or xas_for_each_marked(), it may be necessary to temporarily stop +the iteration. The xas_pause() function exists for this purpose. +After you have done the necessary work and wish to resume, the xa_state +is in an appropriate state to continue the iteration after the entry +you last processed. If you have interrupts disabled while iterating, +then it is good manners to pause the iteration and reenable interrupts +every ``XA_CHECK_SCHED`` entries. + +The xas_get_mark(), xas_set_mark() and xas_clear_mark() functions require +the xa_state cursor to have been moved to the appropriate location in the +XArray; they will do nothing if you have called xas_pause() or xas_set() +immediately before. + +You can call xas_set_update() to have a callback function +called each time the XArray updates a node. This is used by the page +cache workingset code to maintain its list of nodes which contain only +shadow entries. + +Multi-Index Entries +------------------- + +The XArray has the ability to tie multiple indices together so that +operations on one index affect all indices. For example, storing into +any index will change the value of the entry retrieved from any index. +Setting or clearing a mark on any index will set or clear the mark +on every index that is tied together. The current implementation +only allows tying ranges which are aligned powers of two together; +eg indices 64-127 may be tied together, but 2-6 may not be. This may +save substantial quantities of memory; for example tying 512 entries +together will save over 4kB. + +You can create a multi-index entry by using XA_STATE_ORDER() +or xas_set_order() followed by a call to xas_store(). +Calling xas_load() with a multi-index xa_state will walk the +xa_state to the right location in the tree, but the return value is not +meaningful, potentially being an internal entry or ``NULL`` even when there +is an entry stored within the range. Calling xas_find_conflict() +will return the first entry within the range or ``NULL`` if there are no +entries in the range. The xas_for_each_conflict() iterator will +iterate over every entry which overlaps the specified range. + +If xas_load() encounters a multi-index entry, the xa_index +in the xa_state will not be changed. When iterating over an XArray +or calling xas_find(), if the initial index is in the middle +of a multi-index entry, it will not be altered. Subsequent calls +or iterations will move the index to the first index in the range. +Each entry will only be returned once, no matter how many indices it +occupies. + +Using xas_next() or xas_prev() with a multi-index xa_state is not +supported. Using either of these functions on a multi-index entry will +reveal sibling entries; these should be skipped over by the caller. + +Storing ``NULL`` into any index of a multi-index entry will set the +entry at every index to ``NULL`` and dissolve the tie. A multi-index +entry can be split into entries occupying smaller ranges by calling +xas_split_alloc() without the xa_lock held, followed by taking the lock +and calling xas_split(). + +Functions and structures +======================== + +.. kernel-doc:: include/linux/xarray.h +.. kernel-doc:: lib/xarray.c |