linux-kernel - Re: [RFC PATCH 06/10] memcg: do obj_cgroup_put inside drain_obj

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Message-ID: <0b3ab5e5-e684-44ce-b6ed-276ad37784e6@suse.cz>
Date: Fri, 14 Mar 2025 11:17:28 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Shakeel Butt <shakeel.butt@...ux.dev>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Hocko <mhocko@...nel.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>, linux-mm@...ck.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Meta kernel team <kernel-team@...a.com>
Subject: Re: [RFC PATCH 06/10] memcg: do obj_cgroup_put inside drain_obj_stock

On 3/14/25 07:15, Shakeel Butt wrote:
> Previously we could not call obj_cgroup_put() inside the local lock
> because on the put on the last reference, the release function
> obj_cgroup_release() may try to re-acquire the local lock. However that
> chain has been broken. Now simply do obj_cgroup_put() inside
> drain_obj_stock() instead of returning the old objcg.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@...ux.dev>

Hm is this really safe? I can see obj_cgroup_release() doing
percpu_ref_exit() -> kfree(), do we have guaranteed that allocation won't be
also in a kmemcg and recurse?