linux-kernel - Re: [PATCH v4 01/11] mm: memcontrol: prepare objcg API for non-kmem usage

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Date:   Wed, 25 May 2022 16:46:17 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     mhocko@...nel.org, roman.gushchin@...ux.dev, shakeelb@...gle.com,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, duanxiongchun@...edance.com,
        longman@...hat.com
Subject: Re: [PATCH v4 01/11] mm: memcontrol: prepare objcg API for non-kmem
 usage

On Tue, May 24, 2022 at 03:01:25PM -0400, Johannes Weiner wrote:
> On Tue, May 24, 2022 at 02:05:41PM +0800, Muchun Song wrote:
> > Pagecache pages are charged at the allocation time and holding a
> > reference to the original memory cgroup until being reclaimed.
> > Depending on the memory pressure, specific patterns of the page
> > sharing between different cgroups and the cgroup creation and
> > destruction rates, a large number of dying memory cgroups can be
> > pinned by pagecache pages. It makes the page reclaim less efficient
> > and wastes memory.
> > 
> > We can convert LRU pages and most other raw memcg pins to the objcg
> > direction to fix this problem, and then the page->memcg will always
> > point to an object cgroup pointer.
> > 
> > Therefore, the infrastructure of objcg no longer only serves
> > CONFIG_MEMCG_KMEM. In this patch, we move the infrastructure of the
> > objcg out of the scope of the CONFIG_MEMCG_KMEM so that the LRU pages
> > can reuse it to charge pages.
> > 
> > We know that the LRU pages are not accounted at the root level. But
> > the page->memcg_data points to the root_mem_cgroup. So the
> > page->memcg_data of the LRU pages always points to a valid pointer.
> > But the root_mem_cgroup dose not have an object cgroup. If we use
> > obj_cgroup APIs to charge the LRU pages, we should set the
> > page->memcg_data to a root object cgroup. So we also allocate an
> > object cgroup for the root_mem_cgroup.
> > 
> > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> 
> Looks good to me. Also gets rid of some use_hierarchy cruft.
>

Thanks for taking a look.