Re: [RFC PATCH v1] devres: align devres.data strictly only for devm_kmalloc()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Dec 20, 2019 at 11:19:27AM +0100, Marc Gonzalez wrote:
> Would anyone else have any suggestions, comments, insights, recommendations,
> improvements, guidance, or wisdom? :-)

Flip devres upside down!

**WARNING, wear protective glasses when reading the below**


struct devres {
	struct devres_node	node;
	void			*data;
};

/*
 * We place struct devres at the tail of the memory allocation
 * such that data retains the ARCH_KMALLOC_MINALIGN alignment.
 * struct devres itself is just 4 pointers and should therefore
 * only require trivial alignment.
 */
static inline struct devres *data2devres(void *data)
{
	return (struct devres *)(data + ksize(data) - sizeof(struct devres));
}

void *alloc_dr(...)
{
	struct devres *dr;
	void *data;

	data = kmalloc(size + sizeof(struct devres), GFP_KERNEL);
	dr = data2devres(data);
	WARN_ON((unsigned long)dr & __alignof(*dr)-1);
	INIT_LIST_HEAD(&dr->node.entry);
	dr->node.release = release;
	dr->data = data;

	return dr;
}

void devres_free(void *data)
{
	if (data) {
		struct devres *dr = data2devres(data);
		BUG_ON(!list_empty(dr->node.entry));
		kfree(data);
	}
}

static int release_nodes(...)
{
	...
	list_for_each_entry_safe_reverse(dr, ...) {
		...
		kfree(dr->data);
	}
}