linux-kernel - Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg offline

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Message-ID: <tyl5nag4exta7mmxejhzd5xduulfy5pjzde4mpklscqoygkaso@zdyadmle3wjj>
Date: Thu, 25 Sep 2025 15:15:26 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Zi Yan <ziy@...dia.com>
Cc: David Hildenbrand <david@...hat.com>, 
	Qi Zheng <zhengqi.arch@...edance.com>, hannes@...xchg.org, hughd@...gle.com, mhocko@...e.com, 
	roman.gushchin@...ux.dev, muchun.song@...ux.dev, lorenzo.stoakes@...cle.com, 
	harry.yoo@...cle.com, baolin.wang@...ux.alibaba.com, Liam.Howlett@...cle.com, 
	npache@...hat.com, ryan.roberts@....com, dev.jain@....com, baohua@...nel.org, 
	lance.yang@...ux.dev, akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 4/4] mm: thp: reparent the split queue during memcg
 offline

On Thu, Sep 25, 2025 at 03:49:52PM -0400, Zi Yan wrote:
> On 25 Sep 2025, at 15:35, David Hildenbrand wrote:
> 
> > On 25.09.25 08:11, Qi Zheng wrote:
> >> Hi David,
> >
> > Hi :)
> >
> > [...]
> >
> >>>> +++ b/include/linux/mmzone.h
> >>>> @@ -1346,6 +1346,7 @@ struct deferred_split {
> >>>>        spinlock_t split_queue_lock;
> >>>>        struct list_head split_queue;
> >>>>        unsigned long split_queue_len;
> >>>> +    bool is_dying;
> >>>
> >>> It's a bit weird to query whether the "struct deferred_split" is dying.
> >>> Shouldn't this be a memcg property? (and in particular, not exist for
> >>
> >> There is indeed a CSS_DYING flag. But we must modify 'is_dying' under
> >> the protection of the split_queue_lock, otherwise the folio may be added
> >> back to the deferred_split of child memcg.
> >
> > Is there no way to reuse the existing mechanisms, and find a way to have the shrinker / queue locking sync against that?
> >
> > There is also the offline_css() function where we clear CSS_ONLINE. But it happens after calling ss->css_offline(css);
> 
> I see CSS_DYING will be set by kill_css() before offline_css() is called.
> Probably the code can check CSS_DYING instead.
> 
> >
> > Being able to query "is the memcg going offline" and having a way to sync against that would be probably cleanest.
> 
> So basically, something like:
> 1. at folio_split_queue_lock*() time, get folio’s memcg or
>    its parent memcg until there is no CSS_DYING set or CSS_ONLINE is set.
> 2. return the associated deferred_split_queue.
> 

Yes, css_is_dying() can be used but please note that there is a rcu
grace period between setting CSS_DYING and clearing CSS_ONLINE (i.e.
reparenting deferred split queue) and during that period the deferred
split THPs of the dying memcg will be hidden from shrinkers (which
might be fine).