linux-kernel - Re: [PATCH v3] mm: kmem: add lockdep assertion to obj_cgroup

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Message-ID: <ZqKN4jF7eQ-o-mNH@google.com>
Date: Thu, 25 Jul 2024 17:39:46 +0000
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Muchun Song <songmuchun@...edance.com>
Cc: hannes@...xchg.org, mhocko@...nel.org, shakeel.butt@...ux.dev,
	muchun.song@...ux.dev, akpm@...ux-foundation.org, vbabka@...nel.org,
	cgroups@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: kmem: add lockdep assertion to obj_cgroup_memcg

On Thu, Jul 25, 2024 at 05:43:30PM +0800, Muchun Song wrote:
> The obj_cgroup_memcg() is supposed to safe to prevent the returned
> memory cgroup from being freed only when the caller is holding the
> rcu read lock or objcg_lock or cgroup_mutex. It is very easy to
> ignore thoes conditions when users call some upper APIs which call
> obj_cgroup_memcg() internally like mem_cgroup_from_slab_obj() (See
> the link below). So it is better to add lockdep assertion to
> obj_cgroup_memcg() to find those issues ASAP.
> 
> Because there is no user of obj_cgroup_memcg() holding objcg_lock
> to make the returned memory cgroup safe, do not add objcg_lock
> assertion (We should export objcg_lock if we really want to do).
> Additionally, this is some internal implementation detail of memcg
> and should not be accessible outside memcg code.
> 
> Some users like __mem_cgroup_uncharge() do not care the lifetime
> of the returned memory cgroup, which just want to know if the
> folio is charged to a memory cgroup, therefore, they do not need
> to hold the needed locks. In which case, introduce a new helper
> folio_memcg_charged() to do this. Compare it to folio_memcg(), it
> could eliminate a memory access of objcg->memcg for kmem, actually,
> a really small gain.
> 
> Link: https://lore.kernel.org/all/20240718083607.42068-1-songmuchun@bytedance.com/
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Acked-by: Roman Gushchin <roman.gushchin@...ux.dev>

Thanks!