Re: [PATCH v6 03/17] mm: Assign id to every memcg-aware shrinker

[Vladimir Davydov]

Hello Kirill,

Generally, all patches in the series look OK to me, but I'm going to do
some nitpicking before I ack them. See below.

On Fri, May 18, 2018 at 11:42:08AM +0300, Kirill Tkhai wrote:
> The patch introduces shrinker::id number, which is used to enumerate
> memcg-aware shrinkers. The number start from 0, and the code tries
> to maintain it as small as possible.
> 
> This will be used as to represent a memcg-aware shrinkers in memcg
> shrinkers map.
> 
> Since all memcg-aware shrinkers are based on list_lru, which is per-memcg
> in case of !CONFIG_MEMCG_KMEM only, the new functionality will be under
> this config option.
> 
> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
> ---
>  include/linux/shrinker.h |    4 +++
>  mm/vmscan.c              |   60 ++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+)
> 
> diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
> index 6794490f25b2..7ca9c18cf130 100644
> --- a/include/linux/shrinker.h
> +++ b/include/linux/shrinker.h
> @@ -66,6 +66,10 @@ struct shrinker {
>  
>  	/* These are for internal use */
>  	struct list_head list;
> +#ifdef CONFIG_MEMCG_KMEM
> +	/* ID in shrinker_idr */
> +	int id;
> +#endif
>  	/* objs pending delete, per node */
>  	atomic_long_t *nr_deferred;
>  };
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 50055d72f294..3de12a9bdf85 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -169,6 +169,48 @@ unsigned long vm_total_pages;
>  static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +static DEFINE_IDR(shrinker_idr);
> +
> +static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> +{
> +	int id, ret;
> +
> +	shrinker->id = -1;
> +	down_write(&shrinker_rwsem);
> +	ret = id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
> +	if (ret < 0)
> +		goto unlock;
> +	shrinker->id = id;
> +	ret = 0;
> +unlock:
> +	up_write(&shrinker_rwsem);
> +	return ret;
> +}
> +
> +static void unregister_memcg_shrinker(struct shrinker *shrinker)
> +{
> +	int id = shrinker->id;
> +
> +	if (id < 0)
> +		return;

Nit: Please replace this with BUG_ON(id >= 0) - this function can only
be called for a memcg-aware shrinker that has been fully initialized
(prealloc_shrinker() sets nr_deferred after id; unregister_shrinker()
returns immediately if nr_deferred is NULL).

> +
> +	down_write(&shrinker_rwsem);
> +	idr_remove(&shrinker_idr, id);
> +	up_write(&shrinker_rwsem);

> +	shrinker->id = -1;

Nit: I'd move this under shrinker_rwsem as you set it with the rwsem
taken.

> +}
> +#else /* CONFIG_MEMCG_KMEM */
> +static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> +{
> +	return 0;
> +}
> +
> +static void unregister_memcg_shrinker(struct shrinker *shrinker)
> +{
> +}
> +#endif /* CONFIG_MEMCG_KMEM */
> +
>  #ifdef CONFIG_MEMCG
>  static bool global_reclaim(struct scan_control *sc)
>  {
> @@ -306,6 +348,7 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone
>  int prealloc_shrinker(struct shrinker *shrinker)
>  {
>  	size_t size = sizeof(*shrinker->nr_deferred);
> +	int ret;
>  
>  	if (shrinker->flags & SHRINKER_NUMA_AWARE)
>  		size *= nr_node_ids;
> @@ -313,11 +356,26 @@ int prealloc_shrinker(struct shrinker *shrinker)
>  	shrinker->nr_deferred = kzalloc(size, GFP_KERNEL);
>  	if (!shrinker->nr_deferred)
>  		return -ENOMEM;
> +
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
> +		ret = prealloc_memcg_shrinker(shrinker);
> +		if (ret)
> +			goto free_deferred;

Nit: 'ret' is not really needed here.

> +	}
> +
>  	return 0;
> +
> +free_deferred:
> +	kfree(shrinker->nr_deferred);
> +	shrinker->nr_deferred = NULL;
> +	return -ENOMEM;
>  }
>  
>  void free_prealloced_shrinker(struct shrinker *shrinker)
>  {
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> +		unregister_memcg_shrinker(shrinker);
> +
>  	kfree(shrinker->nr_deferred);
>  	shrinker->nr_deferred = NULL;
>  }
> @@ -347,6 +405,8 @@ void unregister_shrinker(struct shrinker *shrinker)
>  {
>  	if (!shrinker->nr_deferred)
>  		return;
> +	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
> +		unregister_memcg_shrinker(shrinker);
>  	down_write(&shrinker_rwsem);
>  	list_del(&shrinker->list);
>  	up_write(&shrinker_rwsem);
>