matras: introduce the matras_alloc_reserve function

The function is meant to be used in order to reserve the amount of
extents possibly required to allocate the specified amount of blocks.

Needed for tarantool/tarantool#10831

Author: mkostoevr

include/small/matras.h

@@ -310,6 +310,13 @@ matras_dealloc_range(struct matras *m, matras_id_t range_count);
 int
 matras_touch_reserve(struct matras *m, int count);
 
+/**
+ * Reserve the max amount of extents required to successfully allocate @p count
+ * blocks. The extents are reserved in the allocator given on construction.
+ */
+int
+matras_alloc_reserve(struct matras *m, int count);
+
 /**
  * Convert block id into block address.
  */

include/small/util.h

@@ -88,6 +88,8 @@
 #  define lengthof(array) (sizeof (array) / sizeof ((array)[0]))
 #endif
 
+#define SMALL_DIV_ROUND_UP(a, b) ((a) + (b) - 1) / (b)
+
 #define small_xmalloc(size)							\
 	({									\
 		void *ret = malloc(size);					\

small/matras.c

@@ -12,6 +12,8 @@
 #pragma intrinsic (_BitScanReverse)
 #endif
 
+#include "util.h"
+
 /**
  * Dummy thread-local matras_stats struct used if matras_stats wasn't
  * passed to matras_create().
@@ -364,6 +366,30 @@ matras_touch_reserve(struct matras *m, int count)
 	return matras_allocator_reserve(m->allocator, max_extents_required);
 }
 
+/**
+ * Reserve the max amount of extents required to successfully allocate @p count
+ * blocks. The extents are reserved in the allocator given on construction.
+ */
+int
+matras_alloc_reserve(struct matras *m, int count)
+{
+	assert(count >= 0);
+
+	/* No allocations planned. */
+	if (count == 0)
+		return 0;
+
+	/*
+	 * This reserves up to 3 extents more than required, but it should be OK
+	 * since the reserved memory is generic for all matras_allocator users.
+	 */
+	int l3_count = SMALL_DIV_ROUND_UP(m->block_size * count,
+					  m->allocator->extent_size);
+	int l2_count = SMALL_DIV_ROUND_UP(l3_count * sizeof(void *),
+					  m->allocator->extent_size);
+	return matras_allocator_reserve(m->allocator, l3_count + l2_count + 1);
+}
+
 /**
  * Return the number of allocated extents (of size m->extent_size each)
  */