.. SPDX-License-Identifier: GPL-2.0+ ======XArray====== :Author: Matthew Wilcox Overview======== The XArray is an abstract data type which behaves like a very large arrayof pointers.  It meets many of the same needs as a hash or a conventionalresizable array.  Unlike a hash, it allows you to sensibly go to thenext or previous entry in a cache-efficient manner.  In contrast to aresizable array, there is no need to copy data or change MMU mappings inorder to grow the array.  It is more memory-efficient, parallelisableand cache friendly than a doubly-linked list.  It takes advantage ofRCU to perform lookups without locking. The XArray implementation is efficient when the indices used are denselyclustered; hashing the object and using the hash as the index will notperform well.  The XArray is optimised for small indices, but still hasgood performance with large indices.  If your index can be larger than``ULONG_MAX`` then the XArray is not the data type for you.  The mostimportant user of the XArray is the page cache. Each non-``NULL`` entry in the array has three bits associated withit called marks.  Each mark may be set or cleared independently ofthe others.  You can iterate over entries which are marked. Normal pointers may be stored in the XArray directly.  They must be 4-bytealigned, which is true for any pointer returned from :c:func:`kmalloc` and:c:func:`alloc_page`.  It isn't true for arbitrary user-space pointers,nor for function pointers.  You can store pointers to statically allocatedobjects, 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 :c:func:`xa_mk_value`.When you retrieve an entry from the XArray, you can check whether it isa value entry by calling :c:func:`xa_is_value`, and convert it back toan integer by calling :c:func:`xa_to_value`. Some users want to store tagged pointers instead of using the marksdescribed above.  They can call :c:func:`xa_tag_pointer` to create anentry with a tag, :c:func:`xa_untag_pointer` to turn a tagged entryback into an untagged pointer and :c:func:`xa_pointer_tag` to retrievethe tag of an entry.  Tagged pointers use the same bits that are usedto distinguish value entries from normal pointers, so each user mustdecide whether they want to store value entries or tagged pointers inany particular XArray. The XArray does not support storing :c:func:`IS_ERR` pointers as someconflict with value entries or internal entries. An unusual feature of the XArray is the ability to create entries whichoccupy a range of indices.  Once stored to, looking up any index inthe range will return the same entry as looking up any other index inthe range.  Setting a mark on one index will set it on all of them.Storing to any index will store to all of them.  Multi-index entries canbe explicitly split into smaller entries, or storing ``NULL`` into anyentry will cause the XArray to forget about the range. Normal API========== Start by initialising an XArray, either with :c:func:`DEFINE_XARRAY`for statically allocated XArrays or :c:func:`xa_init` for dynamicallyallocated ones.  A freshly-initialised XArray contains a ``NULL``pointer at every index. You can then set entries using :c:func:`xa_store` and get entriesusing :c:func:`xa_load`.  xa_store will overwrite any entry with thenew entry and return the previous entry stored at that index.  You canuse :c:func:`xa_erase` instead of calling :c:func:`xa_store` with a``NULL`` entry.  There is no difference between an entry that has neverbeen stored to, one that has been erased and one that has most recentlyhad ``NULL`` stored to it. You can conditionally replace an entry at an index by using:c:func:`xa_cmpxchg`.  Like :c:func:`cmpxchg`, it will only succeed ifthe entry at that index has the 'old' value.  It also returns the entrywhich was at that index; if it returns the same entry which was passed as'old', then :c:func:`xa_cmpxchg` succeeded. If you want to only store a new entry to an index if the current entryat that index is ``NULL``, you can use :c:func:`xa_insert` whichreturns ``-EEXIST`` if the entry is not empty. You can enquire whether a mark is set on an entry by using:c:func:`xa_get_mark`.  If the entry is not ``NULL``, you can set a markon it by using :c:func:`xa_set_mark` and remove the mark from an entry bycalling :c:func:`xa_clear_mark`.  You can ask whether any entry in theXArray has a particular mark set by calling :c:func:`xa_marked`. You can copy entries out of the XArray into a plain array by calling:c:func:`xa_extract`.  Or you can iterate over the present entries inthe XArray by calling :c:func:`xa_for_each`.  You may prefer to use:c:func:`xa_find` or :c:func:`xa_find_after` to move to the next presententry in the XArray. Calling :c:func:`xa_store_range` stores the same entry in a rangeof indices.  If you do this, some of the other operations will behavein a slightly odd way.  For example, marking the entry at one indexmay result in the entry being marked at some, but not all of the otherindices.  Storing into one index may result in the entry retrieved bysome, but not all of the other indices changing. Sometimes you need to ensure that a subsequent call to :c:func:`xa_store`will not need to allocate memory.  The :c:func:`xa_reserve` functionwill store a reserved entry at the indicated index.  Users of the normalAPI will see this entry as containing ``NULL``.  If you do not need touse the reserved entry, you can call :c:func:`xa_release` to remove theunused entry.  If another user has stored to the entry in the meantime,:c:func:`xa_release` will do nothing; if instead you want the entry tobecome ``NULL``, you should use :c:func:`xa_erase`. If all entries in the array are ``NULL``, the :c:func:`xa_empty` functionwill return ``true``. Finally, you can remove all entries from an XArray by calling:c:func:`xa_destroy`.  If the XArray entries are pointers, you may wishto free the entries first.  You can do this by iterating over all presententries in the XArray using the :c:func:`xa_for_each` iterator. Allocating XArrays------------------ If you use :c:func:`DEFINE_XARRAY_ALLOC` to define the XArray, orinitialise it by passing ``XA_FLAGS_ALLOC`` to :c:func:`xa_init_flags`,the XArray changes to track whether entries are in use or not. You can call :c:func:`xa_alloc` to store the entry at any unused indexin the XArray.  If you need to modify the array from interrupt context,you can use :c:func:`xa_alloc_bh` or :c:func:`xa_alloc_irq` to disableinterrupts while allocating the ID. Using :c:func:`xa_store`, :c:func:`xa_cmpxchg` or :c:func:`xa_insert`will mark the entry as being allocated.  Unlike a normal XArray, storing``NULL`` will mark the entry as being in use, like :c:func:`xa_reserve`.To free an entry, use :c:func:`xa_erase` (or :c:func:`xa_release` ifyou only want to free the entry if it's ``NULL``). You cannot use ``XA_MARK_0`` with an allocating XArray as this markis used to track whether an entry is free or not.  The other marks areavailable for your use. Memory allocation----------------- The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_alloc`,:c:func:`xa_reserve` and :c:func:`xa_insert` functions take a gfp_tparameter 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 passa restrictive set of GFP flags.  In that case, the functions return aspecial value which can be turned into an errno using :c:func:`xa_err`.If you don't need to know exactly which error occurred, using:c:func:`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: * :c:func:`xa_empty` * :c:func:`xa_marked` Takes RCU read lock: * :c:func:`xa_load` * :c:func:`xa_for_each` * :c:func:`xa_find` * :c:func:`xa_find_after` * :c:func:`xa_extract` * :c:func:`xa_get_mark` Takes xa_lock internally: * :c:func:`xa_store` * :c:func:`xa_store_bh` * :c:func:`xa_store_irq` * :c:func:`xa_insert` * :c:func:`xa_erase` * :c:func:`xa_erase_bh` * :c:func:`xa_erase_irq` * :c:func:`xa_cmpxchg` * :c:func:`xa_cmpxchg_bh` * :c:func:`xa_cmpxchg_irq` * :c:func:`xa_store_range` * :c:func:`xa_alloc` * :c:func:`xa_alloc_bh` * :c:func:`xa_alloc_irq` * :c:func:`xa_reserve` * :c:func:`xa_reserve_bh` * :c:func:`xa_reserve_irq` * :c:func:`xa_destroy` * :c:func:`xa_set_mark` * :c:func:`xa_clear_mark` Assumes xa_lock held on entry: * :c:func:`__xa_store` * :c:func:`__xa_insert` * :c:func:`__xa_erase` * :c:func:`__xa_cmpxchg` * :c:func:`__xa_alloc` * :c:func:`__xa_reserve` * :c:func:`__xa_set_mark` * :c:func:`__xa_clear_mark` If you want to take advantage of the lock to protect the data structuresthat you are storing in the XArray, you can call :c:func:`xa_lock`before calling :c:func:`xa_load`, then take a reference count on theobject you have found before calling :c:func:`xa_unlock`.  This willprevent stores from removing the object from the array between lookingup the object and incrementing the refcount.  You can also use RCU toavoid dereferencing freed memory, but an explanation of that is beyondthe scope of this document. The XArray does not disable interrupts or softirqs while modifyingthe array.  It is safe to read the XArray from interrupt or softirqcontext as the RCU lock provides enough protection. If, for example, you want to store entries in the XArray in processcontext 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 :c:func:`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 thecoverage of the xa_lock on the store side to protect some statisticsassociated with the array. Sharing the XArray with interrupt context is also possible, eitherusing :c:func:`xa_lock_irqsave` in both the interrupt handler and processcontext, or :c:func:`xa_lock_irq` in process context and :c:func:`xa_lock`in the interrupt handler.  Some of the more common patterns have helperfunctions such as :c:func:`xa_store_bh`, :c:func:`xa_store_irq`,:c:func:`xa_erase_bh`, :c:func:`xa_erase_irq`, :c:func:`xa_cmpxchg_bh`and :c:func:`xa_cmpxchg_irq`. Sometimes you need to protect access to the XArray with a mutex becausethat lock sits above another mutex in the locking hierarchy.  That doesnot entitle you to use functions like :c:func:`__xa_erase` without takingthe xa_lock; the xa_lock is used for lockdep validation and will be usedfor other purposes in the future. The :c:func:`__xa_set_mark` and :c:func:`__xa_clear_mark` functions are alsoavailable for situations where you look up an entry and want to atomicallyset or clear a mark.  It may be more efficient to use the advanced APIin this case, as it will save you from walking the tree twice. Advanced API============ The advanced API offers more flexibility and better performance at thecost 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 requiredto use the xa_lock while modifying the array.  You can choose whetherto use the xa_lock or the RCU lock while doing read-only operations onthe array.  You can mix advanced and normal operations on the same array;indeed the normal API is implemented in terms of the advanced API.  Theadvanced API is only available to modules with a GPL-compatible license. The advanced API is based around the xa_state.  This is an opaque datastructure which you declare on the stack using the :c:func:`XA_STATE`macro.  This macro initialises the xa_state ready to start walkingaround the XArray.  It is used as a cursor to maintain the positionin the XArray and let you compose various operations together withouthaving to restart from the top every time. The xa_state is also used to store errors.  You can call:c:func:`xas_error` to retrieve the error.  All operations check whetherthe xa_state is in an error state before proceeding, so there's no needfor you to check for an error after each call; you can make multiplecalls in succession and only check at a convenient point.  The onlyerrors currently generated by the XArray code itself are ``ENOMEM`` and``EINVAL``, but it supports arbitrary errors in case you want to call:c:func:`xas_set_err` yourself. If the xa_state is holding an ``ENOMEM`` error, calling :c:func:`xas_nomem`will attempt to allocate more memory using the specified gfp flags andcache it in the xa_state for the next attempt.  The idea is that you takethe xa_lock, attempt the operation and drop the lock.  The operationattempts to allocate memory while holding the lock, but it is morelikely to fail.  Once you have dropped the lock, :c:func:`xas_nomem`can try harder to allocate more memory.  It will return ``true`` if itis worth retrying the operation (i.e. that there was a memory error *and*more memory was allocated).  If it has previously allocated memory, andthat 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 neverexposed through the normal API, but when using the advanced API, it'spossible to see them.  Usually the best way to handle them is to pass themto :c:func:`xas_retry`, and retry the operation if it returns ``true``. .. flat-table::   :widths: 1 1 6    * - Name     - Test     - Usage    * - Node     - :c:func:`xa_is_node`     - An XArray node.  May be visible when using a multi-index xa_state.    * - Sibling     - :c:func:`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     - :c:func:`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     - :c:func:`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, theywill be handled by :c:func:`xas_retry`. Additional functionality------------------------ The :c:func:`xas_create_range` function allocates all the necessary memoryto store every entry in a range.  It will set ENOMEM in the xa_state ifit cannot allocate memory. You can use :c:func:`xas_init_marks` to reset the marks on an entryto their default state.  This is usually all marks clear, unless theXArray is marked with ``XA_FLAGS_TRACK_FREE``, in which case mark 0 is setand all other marks are clear.  Replacing one entry with another using:c:func:`xas_store` will not reset the marks on that entry; if you wantthe marks reset, you should do that explicitly. The :c:func:`xas_load` will walk the xa_state as close to the entryas it can.  If you know the xa_state has already been walked to theentry and need to check that the entry hasn't changed, you can use:c:func:`xas_reload` to save a function call. If you need to move to a different index in the XArray, call:c:func:`xas_set`.  This resets the cursor to the top of the tree, whichwill generally make the next operation walk the cursor to the desiredspot in the tree.  If you want to move to the next or previous index,call :c:func:`xas_next` or :c:func:`xas_prev`.  Setting the index doesnot walk the cursor around the array so does not require a lock to beheld, while moving to the next or previous index does. You can search for the next present entry using :c:func:`xas_find`.  Thisis the equivalent of both :c:func:`xa_find` and :c:func:`xa_find_after`;if the cursor has been walked to an entry, then it will find the nextentry after the one currently referenced.  If not, it will return theentry at the index of the xa_state.  Using :c:func:`xas_next_entry` tomove to the next present entry instead of :c:func:`xas_find` will savea function call in the majority of cases at the expense of emitting moreinline code. The :c:func:`xas_find_marked` function is similar.  If the xa_state hasnot 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 afterthe entry referenced by the xa_state.  The :c:func:`xas_next_marked`function is the equivalent of :c:func:`xas_next_entry`. When iterating over a range of the XArray using :c:func:`xas_for_each`or :c:func:`xas_for_each_marked`, it may be necessary to temporarily stopthe iteration.  The :c:func:`xas_pause` function exists for this purpose.After you have done the necessary work and wish to resume, the xa_stateis in an appropriate state to continue the iteration after the entryyou last processed.  If you have interrupts disabled while iterating,then it is good manners to pause the iteration and reenable interruptsevery ``XA_CHECK_SCHED`` entries. The :c:func:`xas_get_mark`, :c:func:`xas_set_mark` and:c:func:`xas_clear_mark` functions require the xa_state cursor to havebeen moved to the appropriate location in the xarray; they will donothing if you have called :c:func:`xas_pause` or :c:func:`xas_set`immediately before. You can call :c:func:`xas_set_update` to have a callback functioncalled each time the XArray updates a node.  This is used by the pagecache workingset code to maintain its list of nodes which contain onlyshadow entries. Multi-Index Entries------------------- The XArray has the ability to tie multiple indices together so thatoperations on one index affect all indices.  For example, storing intoany 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 markon every index that is tied together.  The current implementationonly 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 maysave substantial quantities of memory; for example tying 512 entriestogether will save over 4kB. You can create a multi-index entry by using :c:func:`XA_STATE_ORDER`or :c:func:`xas_set_order` followed by a call to :c:func:`xas_store`.Calling :c:func:`xas_load` with a multi-index xa_state will walk thexa_state to the right location in the tree, but the return value is notmeaningful, potentially being an internal entry or ``NULL`` even when thereis an entry stored within the range.  Calling :c:func:`xas_find_conflict`will return the first entry within the range or ``NULL`` if there are noentries in the range.  The :c:func:`xas_for_each_conflict` iterator williterate over every entry which overlaps the specified range. If :c:func:`xas_load` encounters a multi-index entry, the xa_indexin the xa_state will not be changed.  When iterating over an XArrayor calling :c:func:`xas_find`, if the initial index is in the middleof a multi-index entry, it will not be altered.  Subsequent callsor iterations will move the index to the first index in the range.Each entry will only be returned once, no matter how many indices itoccupies. Using :c:func:`xas_next` or :c:func:`xas_prev` with a multi-index xa_stateis not supported.  Using either of these functions on a multi-index entrywill reveal sibling entries; these should be skipped over by the caller. Storing ``NULL`` into any index of a multi-index entry will set the entryat every index to ``NULL`` and dissolve the tie.  Splitting a multi-indexentry into entries occupying smaller ranges is not yet supported. Functions and structures======================== .. kernel-doc:: include/linux/xarray.h.. kernel-doc:: lib/xarray.c