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

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Date:   Mon, 14 Sep 2020 12:03:34 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Oleg Nesterov <oleg@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        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 Mon, Sep 14, 2020 at 10:59 AM Peter Xu <peterx@...hat.com> wrote:
>
> However now I'm a bit confused on why FOLL_COW must be with FOLL_FORCE even
> without the enforced COW... Shouldn't FOLL_COW be able to happen even without
> FOLL_FORCE (as long as when a page is shared, and the gup is with WRITE
> permission)?  Not sure what I've missed, though.

Afaik, the FOLL_FORCE test was (and is) unnecessary.

If FOLL_COW is set, we're going through this the second time, and we
either had that pte_write() or we had FOLL_FORCE originally.

So can_follow_write_pte() doesn't need the FOLL_FORCE test - it's
redundant - but it isn't technically wrong either.

                 Linus