linux-kernel - Re: [PATCH 3/3] mm: gup: pack has_pinned in MMF_HAS

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Date:   Fri, 7 May 2021 00:39:35 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Peter Xu <peterx@...hat.com>, Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.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 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED

On Thu, May 6, 2021 at 11:43 PM John Hubbard <jhubbard@...dia.com> wrote:
>
> +static void set_mm_has_pinned_flag(unsigned long *mm_flags)
> +{
> +       /*
> +        * Avoid setting the bit unless necessary. This matters a lot with
> +        * large SMP machines.
> +        */
> +       if (!test_bit(MMF_HAS_PINNED, mm_flags))
> +               set_bit(MMF_HAS_PINNED, mm_flags);
> +}

Yes, please do split it up like this.

But please make it explicitly inline, and move the comment to above
the function.

And add the important key part to it: that the bit is never cleared.

That idempotent behavior of the "set_bit()" is what makes it safe to
do this non-atomic test-and-set (yes, the "set_bit()" itself is
atomic, but the sequence above is not).

Side note: we do have a few other places where this kind of thing
happens, so it *might* make sense to even make this a generic pattern
in case somebody can come up with a good descriptive name for that
("set_bit_if_not_set()" sounds descriptive, but the subtle
non-atomicity should probably be part of it).

> +       if (flags & FOLL_PIN)
> +               set_mm_has_pinned_flag(&mm->flags);
>
> ...which is now very readable, once again.

Yes, that does look much better.

Thanks,

              Linus