Re: [PATCHv3 1/2] mm/gup: fix omission of check on FOLL_LONGTERM in get_user_pages_fast()

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  5 Jun 2019 17:10:19 +0800 Pingfan Liu <kernelfans@gmail.com> wrote:

> As for FOLL_LONGTERM, it is checked in the slow path
> __gup_longterm_unlocked(). But it is not checked in the fast path, which
> means a possible leak of CMA page to longterm pinned requirement through
> this crack.
> 
> Place a check in the fast path.

I'm not actually seeing a description (in either the existing code or
this changelog or patch) an explanation of *why* we wish to exclude CMA
pages from longterm pinning.

> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2196,6 +2196,26 @@ static int __gup_longterm_unlocked(unsigned long start, int nr_pages,
>  	return ret;
>  }
>  
> +#ifdef CONFIG_CMA
> +static inline int reject_cma_pages(int nr_pinned, struct page **pages)
> +{
> +	int i;
> +
> +	for (i = 0; i < nr_pinned; i++)
> +		if (is_migrate_cma_page(pages[i])) {
> +			put_user_pages(pages + i, nr_pinned - i);
> +			return i;
> +		}
> +
> +	return nr_pinned;
> +}

There's no point in inlining this.

The code seems inefficient.  If it encounters a single CMA page it can
end up discarding a possibly significant number of non-CMA pages.  I
guess that doesn't matter much, as get_user_pages(FOLL_LONGTERM) is
rare.  But could we avoid this (and the second pass across pages[]) by
checking for a CMA page within gup_pte_range()?

> +#else
> +static inline int reject_cma_pages(int nr_pinned, struct page **pages)
> +{
> +	return nr_pinned;
> +}
> +#endif
> +
>  /**
>   * get_user_pages_fast() - pin user pages in memory
>   * @start:	starting user address
> @@ -2236,6 +2256,9 @@ int get_user_pages_fast(unsigned long start, int nr_pages,
>  		ret = nr;
>  	}
>  
> +	if (unlikely(gup_flags & FOLL_LONGTERM) && nr)
> +		nr = reject_cma_pages(nr, pages);
> +

This would be a suitable place to add a comment explaining why we're
doing this...