Re: [PATCH 06/30] mm: kmem_alloc_estimate()

Peter Zijlstra wrote:

quoted

quoted

+/*
+ * Calculate the upper bound of pages requires to sequentially allocate @bytes
+ * from kmalloc in an unspecified number of allocations of nonuniform size.
+ */
+unsigned kmalloc_estimate_variable(gfp_t flags, size_t bytes)
+{
+	int i;
+	unsigned long pages;
+
+	/*
+	 * multiply by two, in order to account the worst case slack space
+	 * due to the power-of-two allocation sizes.
+	 */
+	pages = DIV_ROUND_UP(2 * bytes, PAGE_SIZE);

For bytes > PAGE_SIZE this doesn't look right (for SLUB). We do page
allocator pass-through which means that we'll be grabbing high order
pages which can be bigger than what 'pages' is here.

Satisfying allocations from a bucket distribution with power-of-two
(which page alloc order satisfies) has a worst case slack space of:

S(x) = 2^n - (2^(n-1)) - 1, n = ceil(log2(x))

This can be seen for the cases where x = 2^i + 1.

The needed bytes for a kmalloc allocation with size > PAGE_SIZE is

get_order(size) << PAGE_SHIFT bytes.

See kmalloc_large().