[PATCH v13 07/72] xarray: Add documentation
From: Matthew Wilcox
Date: Mon Jun 11 2018 - 10:31:46 EST
From: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
This is documentation on how to use the XArray, not details about its
Signed-off-by: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
Acked-by: Josef Bacik <jbacik@xxxxxx>
Documentation/core-api/index.rst | 1 +
Documentation/core-api/xarray.rst | 395 ++++++++++++++++++++++++++++++
2 files changed, 396 insertions(+)
create mode 100644 Documentation/core-api/xarray.rst
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index f5a66b72f984..e821cf78be3b 100644
@@ -21,6 +21,7 @@ Core utilities
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
new file mode 100644
@@ -0,0 +1,395 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+:Author: Matthew Wilcox
+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.
+Each non-``NULL`` entry in the array has three bits associated with it
+called tags. Each tag may be set or cleared independently of the others.
+You can iterate over entries which are tagged.
+Normal pointers may be stored in the XArray directly. They must be 4-byte
+aligned, 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 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 :c:func:`xa_mk_value`.
+When you retrieve an entry from the XArray, you can check whether it is
+a value entry by calling :c:func:`xa_is_value`, and convert it back to
+an integer by calling :c:func:`xa_to_value`.
+The XArray does not support storing :c:func:`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. Setting a tag on one index will set it on all of them.
+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.
+Start by initialising an XArray, either with :c:func:`DEFINE_XARRAY`
+for statically allocated XArrays or :c:func:`xa_init` for dynamically
+allocated ones. A freshly-initialised XArray contains a ``NULL``
+pointer at every index.
+You can then set entries using :c:func:`xa_store` and get entries
+using :c:func:`xa_load`. xa_store will overwrite any entry with the
+new entry and return the previous entry stored at that index. You can
+use :c:func:`xa_erase` instead of calling :c:func:`xa_store` with a
+%NULL entry. There is no difference between an entry that has never
+been stored to and one that has most recently had ``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 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 :c:func:`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 :c:func:`xa_insert` which
+returns ``-EEXIST`` if the entry is not empty.
+Calling :c:func:`xa_reserve` ensures that there is enough memory allocated
+to store an entry at the specified index. This is not normally needed,
+but some users have a complicated locking scheme.
+You can enquire whether a tag is set on an entry by using
+:c:func:`xa_get_tag`. If the entry is not ``NULL``, you can set a tag
+on it by using :c:func:`xa_set_tag` and remove the tag from an entry by
+calling :c:func:`xa_clear_tag`. You can ask whether any entry in the
+XArray has a particular tag set by calling :c:func:`xa_tagged`.
+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 in
+the 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 present
+entry in the XArray.
+Finally, you can remove all entries from an XArray by calling
+:c:func:`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 :c:func:`xa_for_each` iterator.
+The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_reserve`
+and :c:func:`xa_insert` functions take a gfp_t parameter in case
+the XArray needs to allocate memory to store this entry. If the entry
+being stored is ``NULL``, 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 :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.
+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_tagged`
+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_tag`
+Takes xa_lock internally:
+ * :c:func:`xa_store`
+ * :c:func:`xa_insert`
+ * :c:func:`xa_erase`
+ * :c:func:`xa_cmpxchg`
+ * :c:func:`xa_reserve`
+ * :c:func:`xa_destroy`
+ * :c:func:`xa_set_tag`
+ * :c:func:`xa_clear_tag`
+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_set_tag`
+ * :c:func:`__xa_clear_tag`
+If you want to take advantage of the lock to protect the data structures
+that 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 the
+object you have found before calling :c:func:`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;
+ err = xa_err(__xa_store(&foo->array, index, entry, GFP_KERNEL));
+ if (!err)
+ return err;
+ /* foo_erase() is only called from softirq context */
+ void foo_erase(struct foo *foo, unsigned long index)
+ __xa_erase(&foo->array, index);
+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 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 :c:func:`xa_lock_irqsave` in both the interrupt handler and process
+context, or :c:func:`xa_lock_irq` in process context and :c:func:`xa_lock`
+in the interrupt handler.
+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 :c:func:`__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 :c:func:`__xa_set_tag` and :c:func:`__xa_clear_tag` functions are also
+available for situations where you look up an entry and want to atomically
+set or clear a tag. It may be more efficient to use the advanced API
+in this case, as it will save you from walking the tree twice.
+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 :c:func:`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 xa_state is also used to store errors. You can call
+:c:func:`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
+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 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, :c:func:`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.
+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 :c:func:`xas_retry`, and retry the operation if it returns ``true``.
+ :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.
+Other internal entries may be added in the future. As far as possible, they
+will be handled by :c:func:`xas_retry`.
+The :c:func:`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 :c:func:`xas_init_tags` to reset the tags on an entry
+to their default state. This is usually all tags clear, unless the
+XArray is marked with ``XA_FLAGS_TRACK_FREE``, in which case tag 0 is set
+and all other tags are clear. Replacing one entry with another using
+:c:func:`xas_store` will not reset the tags on that entry; if you want
+the tags reset, you should do that explicitly.
+The :c:func:`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
+: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, 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 :c:func:`xas_next` or :c:func:`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 :c:func:`xas_find`. This
+is 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 next
+entry after the one currently referenced. If not, it will return the
+entry at the index of the xa_state. Using :c:func:`xas_next_entry` to
+move to the next present entry instead of :c:func:`xas_find` will save
+a function call in the majority of cases at the expense of emitting more
+The :c:func:`xas_find_tagged` 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 tagged. Otherwise, it will return the first tagged entry after
+the entry referenced by the xa_state. The :c:func:`xas_next_tagged`
+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_tagged`, it may be necessary to temporarily stop
+the iteration. The :c:func:`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 :c:func:`xas_get_tag`, :c:func:`xas_set_tag` and
+:c:func:`xas_clear_tag` 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 :c:func:`xas_pause` or :c:func:`xas_set`
+You can call :c:func:`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
+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 tag on any index will set or clear the tag
+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 :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
+the xa_state to the right location in the tree, but the return
+value is not meaningful, potentially being an internal entry.
+The :c:func:`xas_for_each_conflict` iterator will iterate over every
+entry which overlaps the specified range.
+If :c:func:`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 :c:func:`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
+Using :c:func:`xas_next` or :c:func:`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. Splitting a multi-index
+entry into entries occupying smaller ranges is not yet supported.
+Functions and structures
+.. kernel-doc:: include/linux/xarray.h
+.. kernel-doc:: lib/xarray.c