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From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Shakeel Butt <shakeel.butt@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Muchun Song <muchun.song@linux.dev>, linux-mm@kvack.org,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org,
	Meta kernel team <kernel-team@meta.com>
Subject: Re: [RFC PATCH 06/10] memcg: do obj_cgroup_put inside drain_obj_stock
Message-ID: <20250314113533.jNrVXeyr@linutronix.de>
References: <20250314061511.1308152-1-shakeel.butt@linux.dev>
 <20250314061511.1308152-7-shakeel.butt@linux.dev>
 <0b3ab5e5-e684-44ce-b6ed-276ad37784e6@suse.cz>
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In-Reply-To: <0b3ab5e5-e684-44ce-b6ed-276ad37784e6@suse.cz>

On 2025-03-14 11:17:28 [+0100], Vlastimil Babka wrote:
> 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@linux.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?

This was like this until commit
	5675114623872 ("mm/memcg: protect memcg_stock with a local_lock_t")

at which point the put had to happen outside. This "percpu_ref_exit() ->
kfree()" was also prior this commit.

Sebastian