linux-kernel - Re: [PATCH 2/3] mm: gup: allow FOLL

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Message-ID: <CAHk-=wiNwNz33SEdZixX62XyEx4ZPPbkkNw+V4nn+9=YLc2Acw@mail.gmail.com>
Date:   Thu, 6 May 2021 19:34:07 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Xu <peterx@...hat.com>
Cc:     Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        Kirill Shutemov <kirill@...temov.name>,
        Jason Gunthorpe <jgg@...dia.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 2/3] mm: gup: allow FOLL_PIN to scale in SMP

On Thu, May 6, 2021 at 4:25 PM Peter Xu <peterx@...hat.com> wrote:
>
> +       if (flags & FOLL_PIN && !atomic_read(&mm->has_pinned))

Please add parentheses to clarify code like this and make the grouping
much more visually obvious.

Yes, yes, '&' has higher precedence than '&&'. This is not about the
compiler, this is about the humans reading it. So please write it as

        if ((flags & FOLL_PIN) && !atomic_read(&mm->has_pinned))

instead (this problem remains - and the fix is the same - in the 3/3 patch).

Otherwise the series looks fine to me (although admittedly I do find
the commit message to be ridiculously verbose for such a trivial patch
- at some point the actual _point_ if it all gets hidden in the long
commit message).

             Linus