linux-kernel - Re: [PATCH v4 30/31] mm: memcontrol: eliminate the problem of dying memory cgroup for LRU folios

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Message-ID: <aYoezZdqL_AofUgP@linux.dev>
Date: Mon, 9 Feb 2026 09:53:47 -0800
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <qi.zheng@...ux.dev>
Cc: hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com, 
	roman.gushchin@...ux.dev, muchun.song@...ux.dev, david@...nel.org, 
	lorenzo.stoakes@...cle.com, ziy@...dia.com, harry.yoo@...cle.com, yosry.ahmed@...ux.dev, 
	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, bhe@...hat.com, 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 v4 30/31] mm: memcontrol: eliminate the problem of dying
 memory cgroup for LRU folios

On Mon, Feb 09, 2026 at 11:49:43AM +0800, Qi Zheng wrote:
> 
> 
> On 2/8/26 6:25 AM, Shakeel Butt wrote:
> > On Thu, Feb 05, 2026 at 05:01:49PM +0800, Qi Zheng wrote:
> > > From: Muchun Song <songmuchun@...edance.com>
> > > 
> > > Now that everything is set up, switch folio->memcg_data pointers to
> > > objcgs, update the accessors, and execute reparenting on cgroup death.
> > > 
> > > Finally, folio->memcg_data of LRU folios and kmem folios will always
> > > point to an object cgroup pointer. The folio->memcg_data of slab
> > > folios will point to an vector of object cgroups.
> > > 
> > > Signed-off-by: Muchun Song <songmuchun@...edance.com>
> > > Signed-off-by: Qi Zheng <zhengqi.arch@...edance.com>
> > >   /*
> > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > index e7d4e4ff411b6..0e0efaa511d3d 100644
> > > --- a/mm/memcontrol.c
> > > +++ b/mm/memcontrol.c
> > > @@ -247,11 +247,25 @@ static inline void reparent_state_local(struct mem_cgroup *memcg, struct mem_cgr
> > >   static inline void reparent_locks(struct mem_cgroup *memcg, struct mem_cgroup *parent)
> > >   {
> > > +	int nid, nest = 0;
> > > +
> > >   	spin_lock_irq(&objcg_lock);
> > > +	for_each_node(nid) {
> > > +		spin_lock_nested(&mem_cgroup_lruvec(memcg,
> > > +				 NODE_DATA(nid))->lru_lock, nest++);
> > > +		spin_lock_nested(&mem_cgroup_lruvec(parent,
> > > +				 NODE_DATA(nid))->lru_lock, nest++);
> > 
> > Is there a reason to acquire locks for all the node together? Why not do
> > the for_each_node(nid) in memcg_reparent_objcgs() and then reparent the
> > LRUs for each node one by one and taking and releasing lock
> > individually. Though the lock for the offlining memcg might not be
> 
> To do this, we first need to convert objcg from per-memcg to per-memcg
> per-node. In this way, we can hold the lru lock and objcg lock for
> each node to reparent the folio and the corresponding objcg together.

Oh we want reparenting of both objcg and folio atomic. Let's add a
comment here with the explanation.