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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Thu, 18 Nov 2021 11:54:37 +0800
Message-ID: <CAMZfGtXgYPVOsGUE8OOzkx8K14BjHoMS1hLvxXX77+5cSycrPw@mail.gmail.com>
Subject: Re: [PATCH v7] hugetlb: Add hugetlb.*.numa_stat file
To:     Mike Kravetz <mike.kravetz@oracle.com>,
        Mina Almasry <almasrymina@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shuah Khan <shuah@kernel.org>,
        Miaohe Lin <linmiaohe@huawei.com>,
        Oscar Salvador <osalvador@suse.de>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Shakeel Butt <shakeelb@google.com>, Jue Wang <juew@google.com>,
        Yang Yao <ygyao@google.com>, Joanna Li <joannali@google.com>,
        Cannon Matthews <cannonmatthews@google.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>, linux-doc@vger.kernel.org
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On Thu, Nov 18, 2021 at 8:13 AM Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> On 11/17/21 12:18, Mina Almasry wrote:
> ...
> > diff --git a/mm/hugetlb_cgroup.c b/mm/hugetlb_cgroup.c
> ...
> > @@ -288,11 +317,21 @@ static void __hugetlb_cgroup_commit_charge(int idx, unsigned long nr_pages,
> >                                          struct hugetlb_cgroup *h_cg,
> >                                          struct page *page, bool rsvd)
> >  {
> > +     unsigned long *usage;
> > +
>
> I assume the use of a pointer is just to make the following WRITE_ONCE
> look better?  I prefer the suggestion by Muchun:
>
> unsigned long usage = h_cg->nodeinfo[page_to_nid(page)]->usage[idx];
>
> usage += nr_pages;
> WRITE_ONCE(h_cg->nodeinfo[page_to_nid(page)]->usage[idx], usage);
>
> I had to think for just a second 'why are we using/passing a pointer?'.
> Not insisting we use Muchun's suggestion, it just caused me to think
> a little more than necessary.

At least I have the same question here. For me I think it's
unnecessary to use a pointer.

>
> In any case, I would move the variable usage inside the
> 'if (!rsvd)' block.
>
> >       if (hugetlb_cgroup_disabled() || !h_cg)
> >               return;
> >
> >       __set_hugetlb_cgroup(page, h_cg, rsvd);
> > -     return;
> > +     if (!rsvd) {
> > +             usage = &h_cg->nodeinfo[page_to_nid(page)]->usage[idx];
> > +             /*
> > +              * This write is not atomic due to fetching *usage and writing
> > +              * to it, but that's fine because we call this with
> > +              * hugetlb_lock held anyway.
> > +              */
> > +             WRITE_ONCE(*usage, *usage + nr_pages);
> > +     }
> >  }
> >
> >  void hugetlb_cgroup_commit_charge(int idx, unsigned long nr_pages,
> > @@ -316,6 +355,7 @@ static void __hugetlb_cgroup_uncharge_page(int idx, unsigned long nr_pages,
> >                                          struct page *page, bool rsvd)
> >  {
> >       struct hugetlb_cgroup *h_cg;
> > +     unsigned long *usage;
>
> Same here.
>
> Otherwise, looks good to me.
> --
> Mike Kravetz