XArray: Improve documentation of search marks

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>

Documentation/core-api/xarray.rst

@@ -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