From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 4C03C2E0407 for ; Wed, 19 Nov 2025 07:43:06 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763538189; cv=none; b=t5OVTjsUZxuikgYInhl3IJBKbmnLLZFCrlc7AEOJHvh5Fp1Fv+32JjYlJIiD3LLf/RnindrWB1iKFS9NPQ8NRqbk3eZFKMtmINiNpzx/N1u3M3YKBF2t4u4bwZbrsK7xZSUJ+n+IaA8QX8W6cFhKOp0qisgGTralVtO5nset1q4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1763538189; c=relaxed/simple; bh=vBlhwB9uH6UZ/hN/r8t5S1orQpa2gP51uqcQEPSBRZs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AZqGowsWRjd1neUtU4fc9fT2gjDJmoPq1rN5Y/CBM24rzel3+m4PcG5aZdFX2GeaZHaHIkVLv80vmmuPwn4R9lnuwLcW0zFZNkrAkhhlsShlm67Bh55GWzcT7C4HZAUkUcBwr6PY7nx2fj8mAu76dUVo2QGxMsYg6Lc61do9oZA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=l3SMmJV7; arc=none smtp.client-ip=91.218.175.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="l3SMmJV7" Message-ID: <938180ff-7e24-4d73-87d5-fca4bbaa4ced@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1763538184; 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=1I7cA2fVYEnU5tKmvwQ5dB4/kePKxlMyfJS/i/M6zD8=; b=l3SMmJV7Df/YlzQosv4zjnQtzbyfuENKua4z6s08PbQodhAK/In9ZUM9Y4NRbSBMPVkJWa 1sExjcueECfivNB8hdidZd1kutWfWzzwrDPl+CgFixJsdFwbKXBqpujVjgPrjI/JsEU0qa gDayA0fLuY27pEbzURnI6Awwg70OeaE= Date: Wed, 19 Nov 2025 15:42:51 +0800 Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v1 06/26] mm: memcontrol: return root object cgroup for root memory cgroup To: Harry Yoo Cc: hannes@cmpxchg.org, hughd@google.com, mhocko@suse.com, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, david@redhat.com, lorenzo.stoakes@oracle.com, ziy@nvidia.com, imran.f.khan@oracle.com, kamalesh.babulal@oracle.com, axelrasmussen@google.com, yuanchu@google.com, weixugc@google.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Muchun Song , Qi Zheng References: <5e9743f291e7ca7b8f052775e993090ed66cfa80.1761658310.git.zhengqi.arch@bytedance.com> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Qi Zheng In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On 11/19/25 3:24 PM, Harry Yoo wrote: > On Tue, Nov 18, 2025 at 08:11:04PM +0800, Qi Zheng wrote: >> >> >> On 11/18/25 7:28 PM, Qi Zheng wrote: >>> Hi Harry, >>> >>> On 11/17/25 5:17 PM, Harry Yoo wrote: >>>> On Tue, Oct 28, 2025 at 09:58:19PM +0800, Qi Zheng wrote: >>>>> From: Muchun Song >>>>> >>>>> Memory cgroup functions such as get_mem_cgroup_from_folio() and >>>>> get_mem_cgroup_from_mm() return a valid memory cgroup pointer, >>>>> even for the root memory cgroup. In contrast, the situation for >>>>> object cgroups has been different. >>>>> >>>>> Previously, the root object cgroup couldn't be returned because >>>>> it didn't exist. Now that a valid root object cgroup exists, for >>>>> the sake of consistency, it's necessary to align the behavior of >>>>> object-cgroup-related operations with that of memory cgroup APIs. >>>>> >>>>> Signed-off-by: Muchun Song >>>>> Signed-off-by: Qi Zheng >>>>> --- >>>>>   include/linux/memcontrol.h | 29 +++++++++++++++++------- >>>>>   mm/memcontrol.c            | 45 ++++++++++++++++++++------------------ >>>>>   mm/percpu.c                |  2 +- >>>>>   3 files changed, 46 insertions(+), 30 deletions(-) >>>>> >>>>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h >>>>> index 6185d8399a54e..9fdbd4970021d 100644 >>>>> --- a/include/linux/memcontrol.h >>>>> +++ b/include/linux/memcontrol.h >>>>> @@ -332,6 +332,7 @@ struct mem_cgroup { >>>>>   #define MEMCG_CHARGE_BATCH 64U >>>>>   extern struct mem_cgroup *root_mem_cgroup; >>>>> +extern struct obj_cgroup *root_obj_cgroup; >>>>>   enum page_memcg_data_flags { >>>>>       /* page->memcg_data is a pointer to an slabobj_ext vector */ >>>>> @@ -549,6 +550,11 @@ static inline bool >>>>> mem_cgroup_is_root(struct mem_cgroup *memcg) >>>>>       return (memcg == root_mem_cgroup); >>>>>   } >>>>> +static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg) >>>>> +{ >>>>> +    return objcg == root_obj_cgroup; >>>>> +} >>>> >>>> After reparenting, an objcg may satisfy objcg->memcg == root_mem_cgroup >>>> while objcg != root_obj_cgroup. Should they be considered as >>>> root objcgs? >>> >>> Indeed, it's pointless to charge to root_mem_cgroup (objcg->memcg). >>> >>> So it should be: >>> >>> static inline bool obj_cgroup_is_root(const struct obj_cgroup *objcg) >>> { >>>     return (objcg == root_obj_cgroup) || (objcg->memcg == >>> root_mem_cgroup); >>> } >> >> Oh, we can't do that because we still need to consider this objcg when >> uncharging. Some pages may be charged before reparenting. > > Ouch, right. We don't know if it's charged before reparenting and so > it can break statistics in a few places if we skip uncharging it after > repareting. Right. > > And I think we don't charge new pages to objcgs that satisfy > (objcg->memcg == root_mem_cgroup) && (objcg != root_obj_cgroup) > after they're reparented anyway... The charge and uncharge operations must be symmetrical, so we cannot control the charge operation independently. Otherwise: charge ====== if ((objcg->memcg == root_mem_cgroup)) skip charge this page uncharge ======== we can't decide whether to skip this page. Thanks, Qi >