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

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Date:   Mon, 14 Sep 2020 16:27:24 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        Kirill Shutemov <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>, Jan Kara <jack@...e.cz>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/4] mm/gup: Remove enfornced COW mechanism

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.