Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230527103126.398267-1-linmiaohe@huawei.com> <ZHGAcaqOx/e8lqwV@casper.infradead.org>
 <CAJD7tkYSrVkAONXko0eE6LWS__kK_Xeto9MVGwTxuqT5j6N8RQ@mail.gmail.com> <ZHIcnOV/mrkcerlG@casper.infradead.org>
In-Reply-To: <ZHIcnOV/mrkcerlG@casper.infradead.org>
From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Sat, 27 May 2023 11:54:50 -0700
Message-ID: <CAJD7tkZ2Q1ZCqNchpiiC6FCE08dYH6tzANA=VqujeDgT8YhRUA@mail.gmail.com>
Subject: Re: [PATCH] memcg: remove unused mem_cgroup_from_obj()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Miaohe Lin <linmiaohe@huawei.com>,
        Vasily Averin <vasily.averin@linux.dev>, hannes@cmpxchg.org,
        mhocko@kernel.org, roman.gushchin@linux.dev, shakeelb@google.com,
        akpm@linux-foundation.org, muchun.song@linux.dev,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Sat, May 27, 2023 at 8:07=E2=80=AFAM Matthew Wilcox <willy@infradead.org=
> wrote:
>
> On Fri, May 26, 2023 at 09:13:05PM -0700, Yosry Ahmed wrote:
> > On Fri, May 26, 2023 at 9:01=E2=80=AFPM Matthew Wilcox <willy@infradead=
.org> wrote:
> > >
> > > On Sat, May 27, 2023 at 06:31:26PM +0800, Miaohe Lin wrote:
> > > > The function mem_cgroup_from_obj() is not used anymore. Remove it a=
nd
> > > > clean up relevant comments.
> > >
> > > You should have looked at the git history to see why it was created
> > > and who used it.
> > >
> > > Shakeel, Vasily, are you going to retry adding commit 1d0403d20f6c?
> >
> > That commit did not introduce the function though, no? It was
> > introduced before it and replaced by other variants over time (like
> > mem_cgroup_from_slab_obj()). It looks like that commit was reverted ~9
> > months ago. We can always bring it back if/when needed.
>
> The commit immediately preceding it is fc4db90fe71e.
>
> Of course we can bring it back.  It's just code.  But avoiding
> unnecessary churn is also good.  Let's wait to hear from Vasily.
>
> > It also looks to me that 1d0403d20f6c was using mem_cgroup_from_obj()
> > on a struct net object, which is allocated in net_alloc() from a slab
> > cache, so mem_cgroup_from_slab_obj() should be sufficient, no?
>
> Clearly not.

I dived deeper into the history on LKML, and you are right:
https://lore.kernel.org/all/Yp4F6n2Ie32re7Ed@qian/

I still do not understand why mem_cgroup_from_slab_obj() would not be
sufficient, so I am hoping Vasily or Shakeel can help me understand
here. Seems to be something arch-specific.

Thanks for digging this up, Matthew.