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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 7 May 2021 00:39:35 -0700
Message-ID: <CAHk-=wiDXxQyzcJ6Xy1t8-4R0uqcanfSYetLoMNeKUTPdCNL0w@mail.gmail.com>
Subject: Re: [PATCH 3/3] mm: gup: pack has_pinned in MMF_HAS_PINNED
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Peter Xu <peterx@redhat.com>, Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Kirill Shutemov <kirill@shutemov.name>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Michal Hocko <mhocko@suse.com>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrea Arcangeli <aarcange@redhat.com>
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On Thu, May 6, 2021 at 11:43 PM John Hubbard <jhubbard@nvidia.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