Re: [PATCH 5/8] trace_uprobe: Support SDT markers having reference count (semaphore)

[Oleg Nesterov]

On 03/13, Ravi Bangoria wrote:
>
> +sdt_update_ref_ctr(struct mm_struct *mm, unsigned long vaddr, short d)
> +{
> +	void *kaddr;
> +	struct page *page;
> +	struct vm_area_struct *vma;
> +	int ret = 0;
> +	unsigned short orig = 0;
> +
> +	if (vaddr == 0)
> +		return -EINVAL;
> +
> +	ret = get_user_pages_remote(NULL, mm, vaddr, 1,
> +		FOLL_FORCE | FOLL_WRITE, &page, &vma, NULL);
> +	if (ret <= 0)
> +		return ret;
> +
> +	kaddr = kmap_atomic(page);
> +	memcpy(&orig, kaddr + (vaddr & ~PAGE_MASK), sizeof(orig));
> +	orig += d;
> +	memcpy(kaddr + (vaddr & ~PAGE_MASK), &orig, sizeof(orig));
> +	kunmap_atomic(kaddr);

Hmm. Why memcpy? You could simply do

	kaddr = kmap_atomic();
	unsigned short *ptr = kaddr + (vaddr & ~PAGE_MASK);
	*ptr += d;
	kunmap_atomic();

Oleg.