linux-kernel - Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

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Message-ID: <CAHk-=wjGzOjsfmX1Dc=yz6o_+62w4wcTVXX_hia9sHLfsCoxjg@mail.gmail.com>
Date:   Thu, 20 Aug 2020 15:01:00 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marty Mcfadden <mcfadden8@...l.gov>,
        "Maya B . Gokhale" <gokhale2@...l.gov>,
        Jann Horn <jannh@...gle.com>, Christoph Hellwig <hch@....de>,
        Oleg Nesterov <oleg@...hat.com>,
        Kirill Shutemov <kirill@...temov.name>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3] mm/gup: Allow real explicit breaking of COW

On Thu, Aug 20, 2020 at 2:54 PM Peter Xu <peterx@...hat.com> wrote:
>
> I kind of prefer the new suggestion to remove code rather than adding new
> codes.  I definitely don't know enough on the side effect of it, especially
> performance-wise on either ksm or swap, but... IIUC the worst case is we'll get
> some perf report later on, and it seems also not hard to revert the patch later
> if we want.

Well, would you be willing to try this patch out?

After you apply that patch, you should be able to remove the
should_force_cow_break() games entirely, because a write to the page
should always break cow towards the writer if there are any GUP users
around (put another way: away from the GUP).

However, to make the test meaningful, it really should do some swap testing too.

            Linus

View attachment "0001-Trial-do_wp_page-simplification.patch" of type "text/x-patch" (2775 bytes)