xarray: Add documentation
This is documentation on how to use the XArray, not details about its internal implementation. Signed-off-by: Matthew Wilcox <willy@infradead.org> Acked-by: Josef Bacik <jbacik@fb.com>
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Matthew Wilcox
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@ -21,6 +21,7 @@ Core utilities
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local_ops
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workqueue
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genericirq
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xarray
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flexible-arrays
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librs
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genalloc
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@ -0,0 +1,404 @@
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.. SPDX-License-Identifier: GPL-2.0+
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======
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XArray
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======
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:Author: Matthew Wilcox
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Overview
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========
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The XArray is an abstract data type which behaves like a very large array
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of pointers. It meets many of the same needs as a hash or a conventional
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resizable array. Unlike a hash, it allows you to sensibly go to the
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next or previous entry in a cache-efficient manner. In contrast to a
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resizable array, there is no need to copy data or change MMU mappings in
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order to grow the array. It is more memory-efficient, parallelisable
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and cache friendly than a doubly-linked list. It takes advantage of
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RCU to perform lookups without locking.
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The XArray implementation is efficient when the indices used are densely
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clustered; hashing the object and using the hash as the index will not
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perform well. The XArray is optimised for small indices, but still has
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good performance with large indices. If your index can be larger than
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``ULONG_MAX`` then the XArray is not the data type for you. The most
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important user of the XArray is the page cache.
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Each non-``NULL`` entry in the array has three bits associated with
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it called marks. Each mark may be set or cleared independently of
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the others. You can iterate over entries which are marked.
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Normal pointers may be stored in the XArray directly. They must be 4-byte
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aligned, which is true for any pointer returned from :c:func:`kmalloc` and
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:c:func:`alloc_page`. It isn't true for arbitrary user-space pointers,
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nor for function pointers. You can store pointers to statically allocated
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objects, as long as those objects have an alignment of at least 4.
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You can also store integers between 0 and ``LONG_MAX`` in the XArray.
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You must first convert it into an entry using :c:func:`xa_mk_value`.
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When you retrieve an entry from the XArray, you can check whether it is
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a value entry by calling :c:func:`xa_is_value`, and convert it back to
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an integer by calling :c:func:`xa_to_value`.
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Some users want to store tagged pointers instead of using the marks
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described above. They can call :c:func:`xa_tag_pointer` to create an
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entry with a tag, :c:func:`xa_untag_pointer` to turn a tagged entry
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back into an untagged pointer and :c:func:`xa_pointer_tag` to retrieve
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the tag of an entry. Tagged pointers use the same bits that are used
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to distinguish value entries from normal pointers, so each user must
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decide whether they want to store value entries or tagged pointers in
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any particular XArray.
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The XArray does not support storing :c:func:`IS_ERR` pointers as some
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conflict with value entries or internal entries.
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An unusual feature of the XArray is the ability to create entries which
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occupy a range of indices. Once stored to, looking up any index in
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the range will return the same entry as looking up any other index in
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the range. Setting a mark on one index will set it on all of them.
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Storing to any index will store to all of them. Multi-index entries can
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be explicitly split into smaller entries, or storing ``NULL`` into any
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entry will cause the XArray to forget about the range.
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Normal API
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==========
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Start by initialising an XArray, either with :c:func:`DEFINE_XARRAY`
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for statically allocated XArrays or :c:func:`xa_init` for dynamically
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allocated ones. A freshly-initialised XArray contains a ``NULL``
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pointer at every index.
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You can then set entries using :c:func:`xa_store` and get entries
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using :c:func:`xa_load`. xa_store will overwrite any entry with the
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new entry and return the previous entry stored at that index. You can
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use :c:func:`xa_erase` instead of calling :c:func:`xa_store` with a
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``NULL`` entry. There is no difference between an entry that has never
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been stored to and one that has most recently had ``NULL`` stored to it.
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You can conditionally replace an entry at an index by using
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:c:func:`xa_cmpxchg`. Like :c:func:`cmpxchg`, it will only succeed if
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the entry at that index has the 'old' value. It also returns the entry
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which was at that index; if it returns the same entry which was passed as
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'old', then :c:func:`xa_cmpxchg` succeeded.
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If you want to only store a new entry to an index if the current entry
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at that index is ``NULL``, you can use :c:func:`xa_insert` which
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returns ``-EEXIST`` if the entry is not empty.
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You can enquire whether a mark is set on an entry by using
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:c:func:`xa_get_mark`. If the entry is not ``NULL``, you can set a mark
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on it by using :c:func:`xa_set_mark` and remove the mark from an entry by
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calling :c:func:`xa_clear_mark`. You can ask whether any entry in the
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XArray has a particular mark set by calling :c:func:`xa_marked`.
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You can copy entries out of the XArray into a plain array by calling
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:c:func:`xa_extract`. Or you can iterate over the present entries in
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the XArray by calling :c:func:`xa_for_each`. You may prefer to use
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:c:func:`xa_find` or :c:func:`xa_find_after` to move to the next present
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entry in the XArray.
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Finally, you can remove all entries from an XArray by calling
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:c:func:`xa_destroy`. If the XArray entries are pointers, you may wish
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to free the entries first. You can do this by iterating over all present
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entries in the XArray using the :c:func:`xa_for_each` iterator.
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Memory allocation
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-----------------
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The :c:func:`xa_store`, :c:func:`xa_cmpxchg`, :c:func:`xa_reserve`
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and :c:func:`xa_insert` functions take a gfp_t parameter in case
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the XArray needs to allocate memory to store this entry.
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If the entry is being deleted, no memory allocation needs to be performed,
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and the GFP flags specified will be ignored.
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It is possible for no memory to be allocatable, particularly if you pass
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a restrictive set of GFP flags. In that case, the functions return a
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special value which can be turned into an errno using :c:func:`xa_err`.
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If you don't need to know exactly which error occurred, using
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:c:func:`xa_is_err` is slightly more efficient.
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Locking
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-------
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When using the Normal API, you do not have to worry about locking.
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The XArray uses RCU and an internal spinlock to synchronise access:
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No lock needed:
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* :c:func:`xa_empty`
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* :c:func:`xa_marked`
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Takes RCU read lock:
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* :c:func:`xa_load`
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* :c:func:`xa_for_each`
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* :c:func:`xa_find`
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* :c:func:`xa_find_after`
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* :c:func:`xa_extract`
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* :c:func:`xa_get_mark`
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Takes xa_lock internally:
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* :c:func:`xa_store`
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* :c:func:`xa_insert`
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* :c:func:`xa_erase`
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* :c:func:`xa_erase_bh`
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* :c:func:`xa_erase_irq`
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* :c:func:`xa_cmpxchg`
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* :c:func:`xa_destroy`
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* :c:func:`xa_set_mark`
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* :c:func:`xa_clear_mark`
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Assumes xa_lock held on entry:
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* :c:func:`__xa_store`
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* :c:func:`__xa_insert`
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* :c:func:`__xa_erase`
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* :c:func:`__xa_cmpxchg`
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* :c:func:`__xa_set_mark`
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* :c:func:`__xa_clear_mark`
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If you want to take advantage of the lock to protect the data structures
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that you are storing in the XArray, you can call :c:func:`xa_lock`
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before calling :c:func:`xa_load`, then take a reference count on the
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object you have found before calling :c:func:`xa_unlock`. This will
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prevent stores from removing the object from the array between looking
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up the object and incrementing the refcount. You can also use RCU to
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avoid dereferencing freed memory, but an explanation of that is beyond
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the scope of this document.
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The XArray does not disable interrupts or softirqs while modifying
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the array. It is safe to read the XArray from interrupt or softirq
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context as the RCU lock provides enough protection.
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If, for example, you want to store entries in the XArray in process
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context and then erase them in softirq context, you can do that this way::
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void foo_init(struct foo *foo)
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{
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xa_init_flags(&foo->array, XA_FLAGS_LOCK_BH);
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}
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int foo_store(struct foo *foo, unsigned long index, void *entry)
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{
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int err;
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xa_lock_bh(&foo->array);
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err = xa_err(__xa_store(&foo->array, index, entry, GFP_KERNEL));
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if (!err)
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foo->count++;
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xa_unlock_bh(&foo->array);
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return err;
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}
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/* foo_erase() is only called from softirq context */
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void foo_erase(struct foo *foo, unsigned long index)
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{
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xa_lock(&foo->array);
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__xa_erase(&foo->array, index);
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foo->count--;
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xa_unlock(&foo->array);
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}
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If you are going to modify the XArray from interrupt or softirq context,
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you need to initialise the array using :c:func:`xa_init_flags`, passing
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``XA_FLAGS_LOCK_IRQ`` or ``XA_FLAGS_LOCK_BH``.
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The above example also shows a common pattern of wanting to extend the
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coverage of the xa_lock on the store side to protect some statistics
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associated with the array.
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Sharing the XArray with interrupt context is also possible, either
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using :c:func:`xa_lock_irqsave` in both the interrupt handler and process
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context, or :c:func:`xa_lock_irq` in process context and :c:func:`xa_lock`
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in the interrupt handler. Some of the more common patterns have helper
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functions such as :c:func:`xa_erase_bh` and :c:func:`xa_erase_irq`.
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Sometimes you need to protect access to the XArray with a mutex because
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that lock sits above another mutex in the locking hierarchy. That does
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not entitle you to use functions like :c:func:`__xa_erase` without taking
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the xa_lock; the xa_lock is used for lockdep validation and will be used
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for other purposes in the future.
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The :c:func:`__xa_set_mark` and :c:func:`__xa_clear_mark` functions are also
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available for situations where you look up an entry and want to atomically
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set or clear a mark. It may be more efficient to use the advanced API
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in this case, as it will save you from walking the tree twice.
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Advanced API
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============
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The advanced API offers more flexibility and better performance at the
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cost of an interface which can be harder to use and has fewer safeguards.
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No locking is done for you by the advanced API, and you are required
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to use the xa_lock while modifying the array. You can choose whether
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to use the xa_lock or the RCU lock while doing read-only operations on
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the array. You can mix advanced and normal operations on the same array;
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indeed the normal API is implemented in terms of the advanced API. The
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advanced API is only available to modules with a GPL-compatible license.
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The advanced API is based around the xa_state. This is an opaque data
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structure which you declare on the stack using the :c:func:`XA_STATE`
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macro. This macro initialises the xa_state ready to start walking
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around the XArray. It is used as a cursor to maintain the position
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in the XArray and let you compose various operations together without
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having to restart from the top every time.
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The xa_state is also used to store errors. You can call
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:c:func:`xas_error` to retrieve the error. All operations check whether
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the xa_state is in an error state before proceeding, so there's no need
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for you to check for an error after each call; you can make multiple
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calls in succession and only check at a convenient point. The only
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errors currently generated by the XArray code itself are ``ENOMEM`` and
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``EINVAL``, but it supports arbitrary errors in case you want to call
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:c:func:`xas_set_err` yourself.
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If the xa_state is holding an ``ENOMEM`` error, calling :c:func:`xas_nomem`
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will attempt to allocate more memory using the specified gfp flags and
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cache it in the xa_state for the next attempt. The idea is that you take
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the xa_lock, attempt the operation and drop the lock. The operation
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attempts to allocate memory while holding the lock, but it is more
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likely to fail. Once you have dropped the lock, :c:func:`xas_nomem`
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can try harder to allocate more memory. It will return ``true`` if it
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is worth retrying the operation (i.e. that there was a memory error *and*
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more memory was allocated). If it has previously allocated memory, and
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that memory wasn't used, and there is no error (or some error that isn't
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``ENOMEM``), then it will free the memory previously allocated.
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Internal Entries
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----------------
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The XArray reserves some entries for its own purposes. These are never
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exposed through the normal API, but when using the advanced API, it's
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possible to see them. Usually the best way to handle them is to pass them
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to :c:func:`xas_retry`, and retry the operation if it returns ``true``.
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.. flat-table::
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:widths: 1 1 6
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* - Name
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- Test
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- Usage
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* - Node
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- :c:func:`xa_is_node`
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- An XArray node. May be visible when using a multi-index xa_state.
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* - Sibling
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- :c:func:`xa_is_sibling`
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- A non-canonical entry for a multi-index entry. The value indicates
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which slot in this node has the canonical entry.
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* - Retry
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- :c:func:`xa_is_retry`
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- This entry is currently being modified by a thread which has the
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xa_lock. The node containing this entry may be freed at the end
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of this RCU period. You should restart the lookup from the head
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of the array.
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Other internal entries may be added in the future. As far as possible, they
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will be handled by :c:func:`xas_retry`.
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Additional functionality
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------------------------
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The :c:func:`xas_create_range` function allocates all the necessary memory
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to store every entry in a range. It will set ENOMEM in the xa_state if
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it cannot allocate memory.
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You can use :c:func:`xas_init_marks` to reset the marks on an entry
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to their default state. This is usually all marks clear, unless the
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XArray is marked with ``XA_FLAGS_TRACK_FREE``, in which case mark 0 is set
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and all other marks are clear. Replacing one entry with another using
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:c:func:`xas_store` will not reset the marks on that entry; if you want
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the marks reset, you should do that explicitly.
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The :c:func:`xas_load` will walk the xa_state as close to the entry
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as it can. If you know the xa_state has already been walked to the
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entry and need to check that the entry hasn't changed, you can use
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:c:func:`xas_reload` to save a function call.
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If you need to move to a different index in the XArray, call
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:c:func:`xas_set`. This resets the cursor to the top of the tree, which
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will generally make the next operation walk the cursor to the desired
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spot in the tree. If you want to move to the next or previous index,
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call :c:func:`xas_next` or :c:func:`xas_prev`. Setting the index does
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not walk the cursor around the array so does not require a lock to be
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held, while moving to the next or previous index does.
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You can search for the next present entry using :c:func:`xas_find`. This
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is the equivalent of both :c:func:`xa_find` and :c:func:`xa_find_after`;
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if the cursor has been walked to an entry, then it will find the next
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entry after the one currently referenced. If not, it will return the
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entry at the index of the xa_state. Using :c:func:`xas_next_entry` to
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move to the next present entry instead of :c:func:`xas_find` will save
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a function call in the majority of cases at the expense of emitting more
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inline code.
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The :c:func:`xas_find_marked` function is similar. If the xa_state has
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not been walked, it will return the entry at the index of the xa_state,
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if it is marked. Otherwise, it will return the first marked entry after
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the entry referenced by the xa_state. The :c:func:`xas_next_marked`
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function is the equivalent of :c:func:`xas_next_entry`.
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When iterating over a range of the XArray using :c:func:`xas_for_each`
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or :c:func:`xas_for_each_marked`, it may be necessary to temporarily stop
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the iteration. The :c:func:`xas_pause` function exists for this purpose.
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After you have done the necessary work and wish to resume, the xa_state
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is in an appropriate state to continue the iteration after the entry
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you last processed. If you have interrupts disabled while iterating,
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then it is good manners to pause the iteration and reenable interrupts
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every ``XA_CHECK_SCHED`` entries.
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The :c:func:`xas_get_mark`, :c:func:`xas_set_mark` and
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:c:func:`xas_clear_mark` functions require the xa_state cursor to have
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been moved to the appropriate location in the xarray; they will do
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nothing if you have called :c:func:`xas_pause` or :c:func:`xas_set`
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immediately before.
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You can call :c:func:`xas_set_update` to have a callback function
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called each time the XArray updates a node. This is used by the page
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cache workingset code to maintain its list of nodes which contain only
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shadow entries.
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Multi-Index Entries
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-------------------
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The XArray has the ability to tie multiple indices together so that
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operations on one index affect all indices. For example, storing into
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any index will change the value of the entry retrieved from any index.
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Setting or clearing a mark on any index will set or clear the mark
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on every index that is tied together. The current implementation
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only allows tying ranges which are aligned powers of two together;
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eg indices 64-127 may be tied together, but 2-6 may not be. This may
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save substantial quantities of memory; for example tying 512 entries
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together will save over 4kB.
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You can create a multi-index entry by using :c:func:`XA_STATE_ORDER`
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or :c:func:`xas_set_order` followed by a call to :c:func:`xas_store`.
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Calling :c:func:`xas_load` with a multi-index xa_state will walk the
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xa_state to the right location in the tree, but the return value is not
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meaningful, potentially being an internal entry or ``NULL`` even when there
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is an entry stored within the range. Calling :c:func:`xas_find_conflict`
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will return the first entry within the range or ``NULL`` if there are no
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entries in the range. The :c:func:`xas_for_each_conflict` iterator will
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iterate over every entry which overlaps the specified range.
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If :c:func:`xas_load` encounters a multi-index entry, the xa_index
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in the xa_state will not be changed. When iterating over an XArray
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or calling :c:func:`xas_find`, if the initial index is in the middle
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of a multi-index entry, it will not be altered. Subsequent calls
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or iterations will move the index to the first index in the range.
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Each entry will only be returned once, no matter how many indices it
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occupies.
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||||
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
|
||||
Loading…
Reference in New Issue