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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 1 Feb 2021 09:09:45 -0800
Message-ID: <CAHbLzkpCLxzy-whY8jiSw59V-tmq=VgDuxyUcdRkWrgkZnxC2w@mail.gmail.com>
Subject: Re: [v5 PATCH 07/11] mm: vmscan: add per memcg shrinker nr_deferred
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Roman Gushchin <guro@fb.com>, Kirill Tkhai <ktkhai@virtuozzo.com>,
        Shakeel Butt <shakeelb@google.com>,
        Dave Chinner <david@fromorbit.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Mon, Feb 1, 2021 at 7:17 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 1/29/21 7:04 PM, Yang Shi wrote:
>
> >> > > @@ -209,9 +214,15 @@ static int expand_one_shrinker_info(struct mem_cgroup *memcg,
> >> > >               if (!new)
> >> > >                       return -ENOMEM;
> >> > >
> >> > > -             /* Set all old bits, clear all new bits */
> >> > > -             memset(new->map, (int)0xff, old_size);
> >> > > -             memset((void *)new->map + old_size, 0, size - old_size);
> >> > > +             new->map = (unsigned long *)(new + 1);
> >> > > +             new->nr_deferred = (void *)new->map + m_size;
> >> >
> >> > This better be aligned to sizeof(atomic_long_t). Can we be sure about that?
> >>
> >> Good point. No, if unsigned long is 32 bit on some 64 bit machines.
> >
> > I think we could just change map to "u64" and guarantee struct
> > shrinker_info is aligned to 64 bit.
>
> What about changing to order, nr_deferred before map? Then the atomics are at
> the beginning of allocated area, thus aligned.

Yes, it works too. The rcu_head is guaranteed to have aligned at sizeof(void *).

Will fix in v6.