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XArray: Improve documentation of search marks

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Move most of the mark-related documentation to its own section to make
it easier to understand.  Add clarification that you can't search for
an unset mark, and you can't yet search for combinations of marks.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2019-11-08 23:45:56 -05:00
parent 82a22311b7
commit 6b81141deb
1 changed files with 36 additions and 24 deletions
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60
Documentation/core-api/xarray.rst
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@ -25,10 +25,6 @@ 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 marks. Each mark may be set or cleared independently of
the others. You can iterate over entries which are marked.
Normal pointers may be stored in the XArray directly. They must be 4-byte
aligned, which is true for any pointer returned from kmalloc() and
alloc_page(). It isn't true for arbitrary user-space pointers,
@ -41,12 +37,11 @@ When you retrieve an entry from the XArray, you can check whether it is
a value entry by calling xa_is_value(), and convert it back to
an integer by calling xa_to_value().
Some users want to store tagged pointers instead of using the marks
described above. They can call xa_tag_pointer() to create an
entry with a tag, xa_untag_pointer() to turn a tagged entry
back into an untagged pointer and xa_pointer_tag() to retrieve
the tag of an entry. Tagged pointers use the same bits that are used
to distinguish value entries from normal pointers, so each user must
Some users want to tag the pointers they store in the XArray. You can
call xa_tag_pointer() to create an entry with a tag, xa_untag_pointer()
to turn a tagged entry back into an untagged pointer and xa_pointer_tag()
to retrieve the tag of an entry. Tagged pointers use the same bits that
are used to distinguish value entries from normal pointers, so you must
decide whether they want to store value entries or tagged pointers in
any particular XArray.
@ -56,10 +51,9 @@ 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 mark 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.
the range. Storing to any index will store to all of them. Multi-index
entries can be explicitly split into smaller entries, or storing ``NULL``
into any entry will cause the XArray to forget about the range.
Normal API
==========
@ -87,12 +81,6 @@ If you want to only store a new entry to an index if the current entry
at that index is ``NULL``, you can use xa_insert() which
returns ``-EBUSY`` if the entry is not empty.
You can enquire whether a mark is set on an entry by using
xa_get_mark(). If the entry is not ``NULL``, you can set a mark
on it by using xa_set_mark() and remove the mark from an entry by
calling xa_clear_mark(). You can ask whether any entry in the
XArray has a particular mark set by calling xa_marked().
You can copy entries out of the XArray into a plain array by calling
xa_extract(). Or you can iterate over the present entries in
the XArray by calling xa_for_each(). You may prefer to use
@ -124,6 +112,31 @@ xa_destroy(). If the XArray entries are pointers, you may wish
to free the entries first. You can do this by iterating over all present
entries in the XArray using the xa_for_each() iterator.
Search Marks
------------
Each entry in the array has three bits associated with it called marks.
Each mark may be set or cleared independently of the others. You can
iterate over marked entries by using the xa_for_each_marked() iterator.
You can enquire whether a mark is set on an entry by using
xa_get_mark(). If the entry is not ``NULL``, you can set a mark on it
by using xa_set_mark() and remove the mark from an entry by calling
xa_clear_mark(). You can ask whether any entry in the XArray has a
particular mark set by calling xa_marked(). Erasing an entry from the
XArray causes all marks associated with that entry to be cleared.
Setting or clearing a mark on any index of a multi-index entry will
affect all indices covered by that entry. Querying the mark on any
index will return the same result.
There is no way to iterate over entries which are not marked; the data
structure does not allow this to be implemented efficiently. There are
not currently iterators to search for logical combinations of bits (eg
iterate over all entries which have both ``XA_MARK_1`` and ``XA_MARK_2``
set, or iterate over all entries which have ``XA_MARK_0`` or ``XA_MARK_2``
set). It would be possible to add these if a user arises.
Allocating XArrays
------------------
@ -419,10 +432,9 @@ you last processed. If you have interrupts disabled while iterating,
then it is good manners to pause the iteration and reenable interrupts
every ``XA_CHECK_SCHED`` entries.
The xas_get_mark(), xas_set_mark() and
xas_clear_mark() functions require the xa_state cursor to have
been moved to the appropriate location in the xarray; they will do
nothing if you have called xas_pause() or xas_set()
The xas_get_mark(), xas_set_mark() and xas_clear_mark() functions require
the xa_state cursor to have been moved to the appropriate location in the
XArray; they will do nothing if you have called xas_pause() or xas_set()
immediately before.
You can call xas_set_update() to have a callback function