Re: [PATCH v5 01/13] mm: Assign id to every memcg-aware shrinker

[Vladimir Davydov]

On Thu, May 10, 2018 at 12:52:18PM +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 !SLOB only, the new functionality will be under MEMCG && !SLOB
> ifdef (symlinked to CONFIG_MEMCG_SHRINKER).

Using MEMCG && !SLOB instead of introducing a new config option was done
deliberately, see:

  http://lkml.kernel.org/r/20151210202244.GA4809@xxxxxxxxxxx

I guess, this doesn't work well any more, as there are more and more
parts depending on kmem accounting, like shrinkers. If you really want
to introduce a new option, I think you should call it CONFIG_MEMCG_KMEM
and use it consistently throughout the code instead of MEMCG && !SLOB.
And this should be done in a separate patch.

> diff --git a/fs/super.c b/fs/super.c
> index 122c402049a2..16c153d2f4f1 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -248,6 +248,9 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
>  	s->s_time_gran = 1000000000;
>  	s->cleancache_poolid = CLEANCACHE_NO_POOL;
>  
> +#ifdef CONFIG_MEMCG_SHRINKER
> +	s->s_shrink.id = -1;
> +#endif

No point doing that - you are going to overwrite the id anyway in
prealloc_shrinker().

>  	s->s_shrink.seeks = DEFAULT_SEEKS;
>  	s->s_shrink.scan_objects = super_cache_scan;
>  	s->s_shrink.count_objects = super_cache_count;

> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 10c8a38c5eef..d691beac1048 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -169,6 +169,47 @@ unsigned long vm_total_pages;
>  static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
> +#ifdef CONFIG_MEMCG_SHRINKER
> +static DEFINE_IDR(shrinker_idr);
> +
> +static int prealloc_memcg_shrinker(struct shrinker *shrinker)
> +{
> +	int id, ret;
> +
> +	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 del_memcg_shrinker(struct shrinker *shrinker)

Nit: IMO unregister_memcg_shrinker() would be a better name as it
matches unregister_shrinker(), just like prealloc_memcg_shrinker()
matches prealloc_shrinker().

> +{
> +	int id = shrinker->id;
> +

> +	if (id < 0)
> +		return;

Nit: I think this should be BUG_ON(id >= 0) as this function is only
called for memcg-aware shrinkers AFAICS.

> +
> +	down_write(&shrinker_rwsem);
> +	idr_remove(&shrinker_idr, id);
> +	up_write(&shrinker_rwsem);
> +	shrinker->id = -1;
> +}