Re: [PATCH] mm/memcontrol: make the slab calculation consistent

[Roman Gushchin <guro@fb.com>]

On Wed, Dec 02, 2020 at 08:14:34PM +0800, Muchun Song wrote:
> Although the ratio of the slab is one, we also should read the ratio
> from the related memory_stats instead of hard-coding. And the local
> variable of size is already the value of slab_unreclaimable. So we
> do not need to read again. Simplify the code here.
> 
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> ---
>  mm/memcontrol.c | 22 +++++++++++++++++-----
>  1 file changed, 17 insertions(+), 5 deletions(-)

Hi Muchun!

> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 9922f1510956..03a9c64560f6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1545,12 +1545,22 @@ static int __init memory_stats_init(void)
>  	int i;
>  
>  	for (i = 0; i < ARRAY_SIZE(memory_stats); i++) {
> +		switch (memory_stats[i].idx) {
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> -		if (memory_stats[i].idx == NR_ANON_THPS ||
> -		    memory_stats[i].idx == NR_FILE_THPS ||
> -		    memory_stats[i].idx == NR_SHMEM_THPS)
> +		case NR_ANON_THPS:
> +		case NR_FILE_THPS:
> +		case NR_SHMEM_THPS:
>  			memory_stats[i].ratio = HPAGE_PMD_SIZE;
> +			break;
>  #endif
> +		case NR_SLAB_UNRECLAIMABLE_B:
> +			VM_BUG_ON(i < 1);
> +			VM_BUG_ON(memory_stats[i - 1].idx != NR_SLAB_RECLAIMABLE_B);

Please, convert these to BUILD_BUG_ON(), they don't have to be runtime checks.


> +			break;
> +		default:
> +			break;
> +		}
> +
>  		VM_BUG_ON(!memory_stats[i].ratio);
>  		VM_BUG_ON(memory_stats[i].idx >= MEMCG_NR_STAT);
>  	}
> @@ -1587,8 +1597,10 @@ static char *memory_stat_format(struct mem_cgroup *memcg)
>  		seq_buf_printf(&s, "%s %llu\n", memory_stats[i].name, size);
>

Can you, please, add a small comment here stating that we're printing
unreclaimable, reclaimable and the sum of both? It will simplify the reading of the code.

>  		if (unlikely(memory_stats[i].idx == NR_SLAB_UNRECLAIMABLE_B)) {
> -			size = memcg_page_state(memcg, NR_SLAB_RECLAIMABLE_B) +
> -			       memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE_B);
> +			int idx = i - 1;
> +
> +			size += memcg_page_state(memcg, memory_stats[idx].idx) *
> +				memory_stats[idx].ratio;
>  			seq_buf_printf(&s, "slab %llu\n", size);
>  		}
>  	}

Otherwise the patch looks good to me! Please, feel free to add
Acked-by: Roman Gushchin <guro@fb.com>
after addressing my comments.

Thanks!
> -- 
> 2.11.0
>