[PATCH v6 07/99] xarray: Add documentation

[Matthew Wilcox <willy@infradead.org>]

From: Matthew Wilcox <mawilcox@microsoft.com>

This is documentation on how to use the XArray, not details about its
internal implementation.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
---
 Documentation/core-api/index.rst  |   1 +
 Documentation/core-api/xarray.rst | 361 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 362 insertions(+)
 create mode 100644 Documentation/core-api/xarray.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index d5bbe035316d..eb16ba30aeb6 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -18,6 +18,7 @@ Core utilities
    local_ops
    workqueue
    genericirq
+   xarray
    flexible-arrays
    librs
    genalloc
diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
new file mode 100644
index 000000000000..914999c0bf3f
--- /dev/null
+++ b/Documentation/core-api/xarray.rst
@@ -0,0 +1,361 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+======
+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 for copying data or changing 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.
+
+A freshly-initialised XArray contains a ``NULL`` pointer at every index.
+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.
+
+Normal API
+==========
+
+Start by initialising an XArray, either with :c:func:`DEFINE_XARRAY`
+for statically allocated XArrays or :c:func:`xa_init` for dynamically
+allocated ones.
+
+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.
+
+Memory allocation
+-----------------
+
+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.
+
+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_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::
+
+    foo_init(struct foo *foo)
+    {
+        xa_init_flags(&foo->array, XA_FLAGS_LOCK_BH);
+    }
+
+    foo_store(struct foo *foo, unsigned long index, void *entry)
+    {
+        xa_lock_bh(&foo->array);
+        __xa_store(&foo->array, index, entry, GFP_KERNEL);
+        foo->count++;
+        xa_unlock_bh(&foo->array);
+    }
+
+    /* foo_erase() is only called from softirq context */
+    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.
+
+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 :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
+: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 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.
+
+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 :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.  Should never be visible; all functions should recurse
+       into an XArray node.
+
+   * - 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`.
+
+Additional functionality
+------------------------
+
+The :c:func:`xas_create` function ensures that there is somewhere in the
+XArray to store an entry.  It will store ENOMEM in the xa_state if it
+cannot allocate memory.  You do not normally need to call this function
+yourself as it is called by :c:func:`xas_store`.
+
+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 reinitialises the cursor, which will generally
+have the effect of making 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 create a multi-index entry by using :c:func:`xas_set_order`.
+If a load or find operation finds a multi-index entry, the index in the
+xa_state will be the one searched for, and not necessarily the
+lowest or highest index used by the entry.
+Currently the only supported multi-index entries supported are powers
+of two, but there are two potential users of arbitrary ranges, so that
+functionality may be added soon.
+
+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
+inline code.
+
+The :c:func:`xas_find_tag` function is similar, returning the first tagged
+entry after the entry referenced by the xa_state if it has already been
+walked, and returning the entry at the index of the xa_state if it is
+tagged, and the xa_state has not been walked.  The :c:func:`xas_next_tag`
+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_tag`, 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`
+immediately before.
+
+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
+shadow entries.
+
+Functions and structures
+========================
+
+.. kernel-doc:: include/linux/xarray.h
+.. kernel-doc:: lib/xarray.c
-- 
2.15.1

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