From mboxrd@z Thu Jan 1 00:00:00 1970 From: Muchun Song Subject: Re: [PATCH] memcg: remove unused mem_cgroup_from_obj() Date: Sun, 28 May 2023 21:01:37 +0800 Message-ID: References: <20230527103126.398267-1-linmiaohe@huawei.com> Mime-Version: 1.0 Content-Transfer-Encoding: quoted-printable Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1685278936; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=yZXa+Q1xIzbQ2IAdpFSYo7sWvHd3UcTbXBMlCfmZbUY=; b=RMKThOY2ZnoegXflMWRJU6adp4QipmRp12PejT2eSBK5bGcQuCW/uwq5m7XRDS/fpn8M+H TlgkD1l4KBJ4QFavGpsbbNmOnikoASMtE6nscS8YaMUy2OFyi2uR4ZJNY1V4REsxC7cOO2 Io2kSEYTrQJnHO5WUV1QMF8iulfouBQ= In-Reply-To: List-ID: Content-Type: text/plain; charset="utf-8" To: Yosry Ahmed Cc: Matthew Wilcox , Miaohe Lin , Vasily Averin , hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, roman.gushchin-fxUVXftIFDnyG1zEObXtfA@public.gmane.org, shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org, akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org > On May 28, 2023, at 02:54, Yosry Ahmed wrote: >=20 > On Sat, May 27, 2023 at 8:07=E2=80=AFAM Matthew Wilcox = wrote: >>=20 >> 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 = wrote: >>>>=20 >>>> 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 = and >>>>> clean up relevant comments. >>>>=20 >>>> You should have looked at the git history to see why it was created >>>> and who used it. >>>>=20 >>>> Shakeel, Vasily, are you going to retry adding commit 1d0403d20f6c? >>>=20 >>> 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. >>=20 >> The commit immediately preceding it is fc4db90fe71e. >>=20 >> 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. >>=20 >>> 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? >>=20 >> Clearly not. >=20 > I dived deeper into the history on LKML, and you are right: > https://lore.kernel.org/all/Yp4F6n2Ie32re7Ed@qian/ >=20 > 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. I think it is because *init_net* which is not allocated from slab meant its address does not belong to linear mapping addresses on arm64. = However, virt_to_page() is only applicable to linear mapping addresses. So, mem_cgroup_from_slab_obj() is not sufficient. mem_cgroup_from_obj() is = used in this case, which will use vmalloc_to_page() for the page associated with *init_net*. If Vasily does not want to bring commit 1d0403d20f6c = back, this patch LGTM. Otherwise, let's wait for Vasily. Thanks.