Re: [PATH V2 1/6] mm: Introduce a general RCU get_user_pages_fast.

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Thu, Aug 21, 2014 at 04:43:27PM +0100, Steve Capper wrote:
> +int get_user_pages_fast(unsigned long start, int nr_pages, int write,
> +			struct page **pages)
> +{
> +	struct mm_struct *mm = current->mm;
> +	int nr, ret;
> +
> +	start &= PAGE_MASK;
> +	nr = __get_user_pages_fast(start, nr_pages, write, pages);
> +	ret = nr;
> +
> +	if (nr < nr_pages) {
> +		/* Try to get the remaining pages with get_user_pages */
> +		start += nr << PAGE_SHIFT;
> +		pages += nr;

When I read this, my first reaction was... what if nr is negative?  In
that case, if nr_pages is positive, we fall through into this if, and
start to wind things backwards - which isn't what we want.

It looks like that can't happen... right?  __get_user_pages_fast() only
returns greater-or-equal to zero right now, but what about the future?

> +
> +		down_read(&mm->mmap_sem);
> +		ret = get_user_pages(current, mm, start,
> +				     nr_pages - nr, write, 0, pages, NULL);
> +		up_read(&mm->mmap_sem);
> +
> +		/* Have to be a bit careful with return values */
> +		if (nr > 0) {

This kind'a makes it look like nr could be negative.

Other than that, I don't see anything obviously wrong with it.

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