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From:   Mina Almasry <almasrymina@google.com>
Date:   Wed, 17 Nov 2021 20:15:17 -0800
Message-ID: <CAHS8izMsQyTs=OKa-rD=MTZOHNbRXRP80jUUSt4+QOuvMSsHyQ@mail.gmail.com>
Subject: Re: [PATCH v7] hugetlb: Add hugetlb.*.numa_stat file
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Mike Kravetz <mike.kravetz@oracle.com>, 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 Wed, Nov 17, 2021 at 7:55 PM Muchun Song <songmuchun@bytedance.com> wrote:
>
> 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.
>

Hmm to be honest I would have not thought it would be preferable to
duplicate a long string like
h_cg->nodeinfo[page_to_nid(page)]->usage[idx], and then for future
code changes to keep them in sync. I think Marco had the same thinking
and that was his initial suggestion, but I don't mind much either way.
I'll submit another iteration with the change :-)

> >
> > 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