XArray: Add cyclic allocation

This differs slightly from the IDR equivalent in five ways.

1. It can allocate up to UINT_MAX instead of being limited to INT_MAX,
   like xa_alloc().  Also like xa_alloc(), it will write to the 'id'
   pointer before placing the entry in the XArray.
2. The 'next' cursor is allocated separately from the XArray instead
   of being part of the IDR.  This saves memory for all the users which
   do not use the cyclic allocation API and suits some users better.
3. It returns -EBUSY instead of -ENOSPC.
4. It will attempt to wrap back to the minimum value on memory allocation
   failure as well as on an -EBUSY error, assuming that a user would
   rather allocate a small ID than suffer an ID allocation failure.
5. It reports whether it has wrapped, which is important to some users.

Signed-off-by: Matthew Wilcox <willy@infradead.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/Documentation/core-api/xarray.rst b/Documentation/core-api/xarray.rst
index e90c4925cd37..c7436da5c4ad 100644
--- a/Documentation/core-api/xarray.rst
+++ b/Documentation/core-api/xarray.rst
@@ -144,7 +144,9 @@ you only want to free the entry if it's ``NULL``).
 
 By default, the lowest free entry is allocated starting from 0.  If you
 want to allocate entries starting at 1, it is more efficient to use
-:c:func:`DEFINE_XARRAY_ALLOC1` or ``XA_FLAGS_ALLOC1``.
+:c:func:`DEFINE_XARRAY_ALLOC1` or ``XA_FLAGS_ALLOC1``.  If you want to
+allocate IDs up to a maximum, then wrap back around to the lowest free
+ID, you can use :c:func:`xa_alloc_cyclic`.
 
 You cannot use ``XA_MARK_0`` with an allocating XArray as this mark
 is used to track whether an entry is free or not.  The other marks are