Re: [PATCH 3/4] mm/gup: Remove enfornced COW mechanism

Oleg Nesterov
2020-09-14T14:29:22Z

On 08/21, Peter Xu wrote:
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -381,22 +381,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
> }
>
> /*
> - * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
> - * but only after we've gone through a COW cycle and they are dirty.
> + * FOLL_FORCE can write to even unwritable pte's, but only
> + * after we've gone through a COW cycle and they are dirty.
> */
> static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> {
> - return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
> -}
> -
> -/*
> - * A (separate) COW fault might break the page the other way and
> - * get_user_pages() would return the page from what is now the wrong
> - * VM. So we need to force a COW break at GUP time even for reads.
> - */
> -static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
> -{
> - return is_cow_mapping(vma->vm_flags) && (flags & (FOLL_GET | FOLL_PIN));
> + return pte_write(pte) ||
> + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));

Do we really need to add the FOLL_FORCE check back?

Afaics, FOLL_COW is only possible if FOLL_FORCE was set.

Oleg.