linux-kernel - Re: [PATCH v2 06/28] mm: memcontrol: allocate object cgroup for non-kmem case

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Message-ID: <0088cfa4-7a60-4a44-afbf-7ee0b6ebb2c4@linux.dev>
Date: Thu, 18 Dec 2025 14:25:26 +0800
From: Qi Zheng <qi.zheng@...ux.dev>
To: Johannes Weiner <hannes@...xchg.org>
Cc: hughd@...gle.com, mhocko@...e.com, roman.gushchin@...ux.dev,
 shakeel.butt@...ux.dev, muchun.song@...ux.dev, david@...nel.org,
 lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com,
 imran.f.khan@...cle.com, kamalesh.babulal@...cle.com,
 axelrasmussen@...gle.com, yuanchu@...gle.com, weixugc@...gle.com,
 chenridong@...weicloud.com, mkoutny@...e.com, akpm@...ux-foundation.org,
 hamzamahfooz@...ux.microsoft.com, apais@...ux.microsoft.com,
 lance.yang@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 cgroups@...r.kernel.org, Muchun Song <songmuchun@...edance.com>,
 Qi Zheng <zhengqi.arch@...edance.com>
Subject: Re: [PATCH v2 06/28] mm: memcontrol: allocate object cgroup for
 non-kmem case



On 12/18/25 5:22 AM, Johannes Weiner wrote:
> On Wed, Dec 17, 2025 at 03:27:30PM +0800, Qi Zheng wrote:
>> From: Muchun Song <songmuchun@...edance.com>
>>
>> Pagecache pages are charged at allocation time and hold a reference
>> to the original memory cgroup until reclaimed. Depending on memory
>> pressure, page sharing patterns between different cgroups and cgroup
>> creation/destruction rates, many dying memory cgroups can be pinned
>> by pagecache pages, reducing page reclaim efficiency and wasting
>> memory. Converting LRU folios and most other raw memory cgroup pins
>> to the object cgroup direction can fix this long-living problem.
> 
> Not a big deal, but since the coverletter will be preserved in git, I
> don't think you need to repeat the full thesis.

Got it.

> 
>> As a result, the objcg infrastructure is no longer solely applicable
>> to the kmem case. In this patch, we extend the scope of the objcg
>> infrastructure beyond the kmem case, enabling LRU folios to reuse
>> it for folio charging purposes.
> 
> "To allow LRU page reparenting, the objcg infrastructure [...]"

OK, will do.

> 
>> It should be noted that LRU folios are not accounted for at the root
>> level, yet the folio->memcg_data points to the root_mem_cgroup. Hence,
>> the folio->memcg_data of LRU folios always points to a valid pointer.
>> However, the root_mem_cgroup does not possess an object cgroup.
>> Therefore, we also allocate an object cgroup for the root_mem_cgroup.
>>
>> Signed-off-by: Muchun Song <songmuchun@...edance.com>
>> Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
>> Reviewed-by: Harry Yoo <harry.yoo@...cle.com>
> 
> Looks good to me.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>

Thanks!