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