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>

2 files changed, 103 insertions, 0 deletions

diff --git a/lib/test_xarray.c b/lib/test_xarray.c
index b5a6b981454d..eaf53f742c72 100644
--- a/lib/test_xarray.c
+++ b/lib/test_xarray.c
@@ -715,6 +715,57 @@ static noinline void check_xa_alloc_2(struct xarray *xa, unsigned int base)
 	xa_destroy(xa);
 }
 
+static noinline void check_xa_alloc_3(struct xarray *xa, unsigned int base)
+{
+	struct xa_limit limit = XA_LIMIT(1, 0x3fff);
+	u32 next = 0;
+	unsigned int i, id;
+	unsigned long index;
+	void *entry;
+
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(1), limit,
+				&next, GFP_KERNEL) != 0);
+	XA_BUG_ON(xa, id != 1);
+
+	next = 0x3ffd;
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(0x3ffd), limit,
+				&next, GFP_KERNEL) != 0);
+	XA_BUG_ON(xa, id != 0x3ffd);
+	xa_erase_index(xa, 0x3ffd);
+	xa_erase_index(xa, 1);
+	XA_BUG_ON(xa, !xa_empty(xa));
+
+	for (i = 0x3ffe; i < 0x4003; i++) {
+		if (i < 0x4000)
+			entry = xa_mk_index(i);
+		else
+			entry = xa_mk_index(i - 0x3fff);
+		XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, entry, limit,
+					&next, GFP_KERNEL) != (id == 1));
+		XA_BUG_ON(xa, xa_mk_index(id) != entry);
+	}
+
+	/* Check wrap-around is handled correctly */
+	if (base != 0)
+		xa_erase_index(xa, base);
+	xa_erase_index(xa, base + 1);
+	next = UINT_MAX;
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(UINT_MAX),
+				xa_limit_32b, &next, GFP_KERNEL) != 0);
+	XA_BUG_ON(xa, id != UINT_MAX);
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(base),
+				xa_limit_32b, &next, GFP_KERNEL) != 1);
+	XA_BUG_ON(xa, id != base);
+	XA_BUG_ON(xa, xa_alloc_cyclic(xa, &id, xa_mk_index(base + 1),
+				xa_limit_32b, &next, GFP_KERNEL) != 0);
+	XA_BUG_ON(xa, id != base + 1);
+
+	xa_for_each(xa, index, entry)
+		xa_erase_index(xa, index);
+
+	XA_BUG_ON(xa, !xa_empty(xa));
+}
+
 static DEFINE_XARRAY_ALLOC(xa0);
 static DEFINE_XARRAY_ALLOC1(xa1);
 
@@ -724,6 +775,8 @@ static noinline void check_xa_alloc(void)
 	check_xa_alloc_1(&xa1, 1);
 	check_xa_alloc_2(&xa0, 0);
 	check_xa_alloc_2(&xa1, 1);
+	check_xa_alloc_3(&xa0, 0);
+	check_xa_alloc_3(&xa1, 1);
 }
 
 static noinline void __check_store_iter(struct xarray *xa, unsigned long start,
diff --git a/lib/xarray.c b/lib/xarray.c
index c707388fb05e..89e37ac50850 100644
--- a/lib/xarray.c
+++ b/lib/xarray.c
@@ -1657,6 +1657,56 @@ int __xa_alloc(struct xarray *xa, u32 *id, void *entry,
 EXPORT_SYMBOL(__xa_alloc);
 
 /**
+ * __xa_alloc_cyclic() - Find somewhere to store this entry in the XArray.
+ * @xa: XArray.
+ * @id: Pointer to ID.
+ * @entry: New entry.
+ * @limit: Range of allocated ID.
+ * @next: Pointer to next ID to allocate.
+ * @gfp: Memory allocation flags.
+ *
+ * Finds an empty entry in @xa between @limit.min and @limit.max,
+ * stores the index into the @id pointer, then stores the entry at
+ * that index.  A concurrent lookup will not see an uninitialised @id.
+ * The search for an empty entry will start at @next and will wrap
+ * around if necessary.
+ *
+ * Context: Any context.  Expects xa_lock to be held on entry.  May
+ * release and reacquire xa_lock if @gfp flags permit.
+ * Return: 0 if the allocation succeeded without wrapping.  1 if the
+ * allocation succeeded after wrapping, -ENOMEM if memory could not be
+ * allocated or -EBUSY if there are no free entries in @limit.
+ */
+int __xa_alloc_cyclic(struct xarray *xa, u32 *id, void *entry,
+		struct xa_limit limit, u32 *next, gfp_t gfp)
+{
+	u32 min = limit.min;
+	int ret;
+
+	limit.min = max(min, *next);
+	ret = __xa_alloc(xa, id, entry, limit, gfp);
+	if ((xa->xa_flags & XA_FLAGS_ALLOC_WRAPPED) && ret == 0) {
+		xa->xa_flags &= ~XA_FLAGS_ALLOC_WRAPPED;
+		ret = 1;
+	}
+
+	if (ret < 0 && limit.min > min) {
+		limit.min = min;
+		ret = __xa_alloc(xa, id, entry, limit, gfp);
+		if (ret == 0)
+			ret = 1;
+	}
+
+	if (ret >= 0) {
+		*next = *id + 1;
+		if (*next == 0)
+			xa->xa_flags |= XA_FLAGS_ALLOC_WRAPPED;
+	}
+	return ret;
+}
+EXPORT_SYMBOL(__xa_alloc_cyclic);
+
+/**
  * __xa_set_mark() - Set this mark on this entry while locked.
  * @xa: XArray.
  * @index: Index of entry.