small: improve real_size calculation in small_alloc_info

Currently `real_size` is calculated as the size of a memory block that
would be allocated for the requested number of bytes. However, this
calculation is non-idempotent. For details see commit df7ba1f
("small: implement new allocator strategy").

This patch switches `small_alloc_info()` to `slab_from_ptr()` that
returns the actual slab that contains the requested `ptr`, so now
`real_size` is always correct.

Needed for tarantool/tarantool#10217

Author: Gumix

include/small/small.h

@@ -225,8 +225,6 @@ small_alloc_check(struct small_alloc *alloc)
 /**
  * Fill `info' with the information about allocation `ptr' of size `size'.
  * See `struct small_alloc_info' for the description of each field.
- * Note that this function can return different `info->real_size' for the same
- * input, depending on the current `small_mempool->used_pool'.
  */
 void
 small_alloc_info(struct small_alloc *alloc, void *ptr, size_t size,

small/small.c

@@ -468,12 +468,14 @@ void
 small_alloc_info(struct small_alloc *alloc, void *ptr, size_t size,
 		 struct small_alloc_info *info)
 {
-	(void)ptr;
 	struct small_mempool *small_mempool = small_mempool_search(alloc, size);
 	info->is_large = small_mempool == NULL;
-	if (info->is_large)
+	if (info->is_large) {
 		info->real_size = size;
-	else
-		info->real_size = small_mempool->used_pool->pool.objsize;
+	} else {
+		struct mslab *mslab = (struct mslab *)
+			slab_from_ptr(ptr, small_mempool->pool.slab_ptr_mask);
+		info->real_size = mslab->mempool->objsize;
+	}
 	assert(info->real_size >= size);
 }

test/small_alloc.c

@@ -141,8 +141,10 @@ static inline void
 check_small_alloc_info(struct small_alloc *alloc, size_t size, bool is_large,
 		       size_t real_size)
 {
+	void *ptr = smalloc(alloc, size);
 	struct small_alloc_info info;
-	small_alloc_info(alloc, NULL, size, &info);
+	small_alloc_info(alloc, ptr, size, &info);
+	smfree(alloc, ptr, size);
 	fail_unless(info.is_large == is_large);
 	fail_unless(info.real_size == real_size);
 }
@@ -185,6 +187,48 @@ test_small_alloc_info(void)
 	check_plan();
 }
 
+/**
+ * Make sure `info.real_size' is calculated correctly.
+ * See https://github.com/tarantool/tarantool/issues/10217
+ */
+static void
+test_small_alloc_info_gh_10217(void)
+{
+	plan(1);
+	header();
+	/*
+	 * One of the mempool groups consists of two mempools with objsizes
+	 * 3072 and 4096. The largest objsize in the previous group is 2048.
+	 */
+	float actual_alloc_factor;
+	small_alloc_create(&alloc, &cache, 64, 64, 1.5f, &actual_alloc_factor);
+
+	const size_t size = 2240;
+	const size_t count = 200;
+	struct small_alloc_info info;
+
+	for (size_t i = 0; i < count; i++) {
+		ptrs[i] = smalloc(&alloc, size);
+		small_alloc_info(&alloc, ptrs[i], size, &info);
+		fail_unless(info.real_size == (i > 127 ? 3072 : 4096));
+	}
+	/*
+	 * Check that all "real sizes" are still correct after `used_pool' was
+	 * switched.
+	 */
+	for (size_t i = 0; i < count; i++) {
+		small_alloc_info(&alloc, ptrs[i], size, &info);
+		fail_unless(info.real_size == (i > 127 ? 3072 : 4096));
+	}
+	ok(true);
+
+	for (size_t i = 0; i < count; i++)
+		smfree(&alloc, ptrs[i], size);
+	small_alloc_destroy(&alloc);
+	footer();
+	check_plan();
+}
+
 /**
  * Make sure allocator works with low alloc_factor and high memory
  * pressure.
@@ -334,6 +378,7 @@ int main()
 #ifndef ENABLE_ASAN
 	small_alloc_large();
 	test_small_alloc_info();
+	test_small_alloc_info_gh_10217();
 	small_alloc_low_alloc_factor();
 #else
 	small_wrong_size_in_free();