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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <v4ihmnzafzwfgdbk6gcnl76bjqko2t4wdz3vy32vjuzffzzt6i@uxecpmupkqh3>
Date: Fri, 26 Sep 2025 09:36:57 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Qi Zheng <zhengqi.arch@...edance.com>
Cc: Zi Yan <ziy@...dia.com>, David Hildenbrand <david@...hat.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 Fri, Sep 26, 2025 at 02:57:39PM +0800, Qi Zheng wrote:
> 
> 
> On 9/26/25 6:35 AM, Shakeel Butt wrote:
[...]
> > > 
> > > 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).
> 
> My mistake, now I think using css_is_dying() is safe.
> 
> > 
> > BTW if this period is not acceptable and we don't want to add is_dying
> > to struct deferred_split, we can use something similar to what list_lru
> > does in the similar situation i.e. set a special value (LONG_MIN) in its
> > nr_items variable. That is make split_queue_len a long and set it to
> > LONG_MIN during memcg offlining/reparenting.
> 
> I've considered this option, but I am concerned about the risk of
> overflow.

If you modify it only within a lock then why would it over or underflow?

> 
> So I will try to use css_is_dying() in the next version.

No objection from me but add a comment on the potential window where the
deferred thps will be hidden from the shrinkers.