Re: [PATCH] mm: kmem: add lockdep assertion to obj_cgroup_memcg

[Shakeel Butt <shakeel.butt@linux.dev>]

On Mon, Jul 22, 2024 at 03:08:10PM GMT, 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)
> and leave a comment to indicate it is intentional.
> 

Do we expect non-memcg code to access objcg_lock? To me this is some
internal implementation detail of memcg and should not be accessible
outside memcg code. So, I would recommend to not mention objcg_lock at
all.

> 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@bytedance.com>
> ---
>  include/linux/memcontrol.h | 22 +++++++++++++++++++---
>  mm/memcontrol.c            |  6 +++---
>  2 files changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index fc94879db4dff..d616c50025098 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -360,11 +360,13 @@ static inline bool folio_memcg_kmem(struct folio *folio);
>   * After the initialization objcg->memcg is always pointing at
>   * a valid memcg, but can be atomically swapped to the parent memcg.
>   *
> - * The caller must ensure that the returned memcg won't be released:
> - * e.g. acquire the rcu_read_lock or css_set_lock.
> + * The caller must ensure that the returned memcg won't be released.
>   */
>  static inline struct mem_cgroup *obj_cgroup_memcg(struct obj_cgroup *objcg)
>  {
> +	WARN_ON_ONCE(!rcu_read_lock_held() &&
> +		  /* !lockdep_is_held(&objcg_lock) && */

> +		     !lockdep_is_held(&cgroup_mutex));
>  	return READ_ONCE(objcg->memcg);
>  }
>  
> @@ -438,6 +440,19 @@ static inline struct mem_cgroup *folio_memcg(struct folio *folio)
>  	return __folio_memcg(folio);
>  }
>  
> +/*
> + * folio_memcg_charged - If a folio is charged to a memory cgroup.
> + * @folio: Pointer to the folio.
> + *
> + * Returns true if folio is charged to a memory cgroup, otherwise returns false.
> + */
> +static inline bool folio_memcg_charged(struct folio *folio)
> +{
> +	if (folio_memcg_kmem(folio))
> +		return __folio_objcg(folio) != NULL;
> +	return __folio_memcg(folio) != NULL;
> +}
> +
>  /**
>   * folio_memcg_rcu - Locklessly get the memory cgroup associated with a folio.
>   * @folio: Pointer to the folio.
> @@ -454,7 +469,6 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
>  	unsigned long memcg_data = READ_ONCE(folio->memcg_data);
>  
>  	VM_BUG_ON_FOLIO(folio_test_slab(folio), folio);
> -	WARN_ON_ONCE(!rcu_read_lock_held());
>  
>  	if (memcg_data & MEMCG_DATA_KMEM) {
>  		struct obj_cgroup *objcg;
> @@ -463,6 +477,8 @@ static inline struct mem_cgroup *folio_memcg_rcu(struct folio *folio)
>  		return obj_cgroup_memcg(objcg);
>  	}
>  
> +	WARN_ON_ONCE(!rcu_read_lock_held());
> +
>  	return (struct mem_cgroup *)(memcg_data & ~OBJEXTS_FLAGS_MASK);
>  }
>  
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 622d4544edd24..3da0284573857 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2366,7 +2366,7 @@ void mem_cgroup_cancel_charge(struct mem_cgroup *memcg, unsigned int nr_pages)
>  
>  static void commit_charge(struct folio *folio, struct mem_cgroup *memcg)
>  {
> -	VM_BUG_ON_FOLIO(folio_memcg(folio), folio);
> +	VM_BUG_ON_FOLIO(folio_memcg_charged(folio), folio);
>  	/*
>  	 * Any of the following ensures page's memcg stability:
>  	 *
> @@ -4617,7 +4617,7 @@ void __mem_cgroup_uncharge(struct folio *folio)
>  	struct uncharge_gather ug;
>  
>  	/* Don't touch folio->lru of any random page, pre-check: */
> -	if (!folio_memcg(folio))
> +	if (!folio_memcg_charged(folio))
>  		return;
>  
>  	uncharge_gather_clear(&ug);
> @@ -4662,7 +4662,7 @@ void mem_cgroup_replace_folio(struct folio *old, struct folio *new)
>  		return;
>  
>  	/* Page cache replacement: new folio already charged? */
> -	if (folio_memcg(new))
> +	if (folio_memcg_charged(new))
>  		return;
>  
>  	memcg = folio_memcg(old);
> -- 
> 2.20.1
>