mm: generic adaptive large memory allocation APIs

kvmalloc() will try to allocate physically contiguous memory first, and try 
vmalloc to allocate virtually contiguous memory when the former allocation
fails.

kvfree() is used to free the memory allocated by kvmalloc(). It can't be used
in atomic context. If the callers are in atomic contex, they can use
kvfree_inatomic() instead.

There is much duplicate code to do such things in kernel, so I generate the
above APIs.

Thank Eric Dumazet for the "kv" prefix. :)

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/mm.h>
#include <linux/init.h>
#include <linux/slab.h>
#include <linux/vmalloc.h>
#include <linux/interrupt.h>

void *kvmalloc(size_t size)
{
	void *ptr;

	if (size < PAGE_SIZE)
		return kmalloc(PAGE_SIZE, GFP_KERNEL);
	ptr = alloc_pages_exact(size, GFP_KERNEL | __GFP_NOWARN);
	if (ptr != NULL)
		return ptr;

	return vmalloc(size);
}
EXPORT_SYMBOL(kvmalloc);

void kvfree(void *ptr, size_t size)
{
	if (size < PAGE_SIZE)
		kfree(ptr);
	else if (is_vmalloc_addr(ptr))
		vfree(ptr);
	else
		free_pages_exact(ptr, size);
}
EXPORT_SYMBOL(kvfree);

struct kvfree_work_struct {
	struct work_struct	work;
	void			*head;
	void			**ptail;
};

DEFINE_PER_CPU(struct kvfree_work_struct, kvfree_work_struct);

static void kvfree_work(struct work_struct *_work)
{
	struct kvfree_work_struct *work;
	void *head, *tmp;
	
	work = container_of(_work, struct kvfree_work_struct, work);
	local_bh_disable();
	head = work->head;
	work->head = NULL;
	work->ptail = &work->head;
	local_bh_enable();

	while (head) {
		tmp = head;
		head = *(void **)head;
		vfree(tmp);
	}
}

void kvfree_inatomic(void *ptr, size_t size)
{
	if (size < PAGE_SIZE) {
		kfree(ptr);
	} else if (is_vmalloc_addr(ptr)) {
		struct kvfree_work_struct *work;

		*(void **)ptr = NULL;
		local_irq_disable();
		work = this_cpu_ptr(&kvfree_work_struct);
		*(work->ptail) = ptr;
		work->ptail = (void**)ptr;
		schedule_work(&work->work);
		local_irq_enable();
	} else {
		free_pages_exact(ptr, size);
	}
}
EXPORT_SYMBOL(kvfree_inatomic);

static int kvfree_work_struct_init(void)
{
	int cpu;
	struct kvfree_work_struct *work;

	for_each_possible_cpu(cpu) {
		work = per_cpu_ptr(&kvfree_work_struct, cpu);
		INIT_WORK(&work->work, kvfree_work);
		work->head = NULL;
		work->ptail = &work->head;
	}

	return 0;
}
//pure_initcall(kvfree_work_struct_init);

//--------------------
// for testing
static int test_init(void)
{
	int size;
	void *ptr;

	kvfree_work_struct_init();
	for (size = 1; size < (1<<30); size <<= 1) {
		ptr = kvmalloc(size);
		if (is_vmalloc_addr(ptr)) {
			printk("%d\n", size);
			break;
		}
		kvfree(ptr, size);
	}

	return 0;
}
module_init(test_init);

static void test_exit(void)
{
	int cpu;
	struct kvfree_work_struct *work;

	for_each_possible_cpu(cpu) {
		work = per_cpu_ptr(&kvfree_work_struct, cpu);
		cancel_work_sync(&work->work);
	}
}
module_exit(test_exit);

MODULE_LICENSE("GPL");
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