Re: [v3 PATCH 04/11] mm: vmscan: remove memcg_shrinker_map_size

[Kirill Tkhai]

On 06.01.2021 01:58, Yang Shi wrote:
> Both memcg_shrinker_map_size and shrinker_nr_max is maintained, but actually 
> the
> map size can be calculated via shrinker_nr_max, so it seems unnecessary to 
> keep both.
> Remove memcg_shrinker_map_size since shrinker_nr_max is also used by 
> iterating the
> bit map.
> 
> Signed-off-by: Yang Shi <shy828...@gmail.com>
> ---
>  mm/vmscan.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index ddb9f972f856..8da765a85569 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -185,8 +185,7 @@ static LIST_HEAD(shrinker_list);
>  static DECLARE_RWSEM(shrinker_rwsem);
>  
>  #ifdef CONFIG_MEMCG
> -
> -static int memcg_shrinker_map_size;
> +static int shrinker_nr_max;
>  
>  static void memcg_free_shrinker_map_rcu(struct rcu_head *head)
>  {
> @@ -248,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
>               return 0;
>  
>       down_read(&shrinker_rwsem);
> -     size = memcg_shrinker_map_size;
> +     size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned 
> long);
>       for_each_node(nid) {
>               map = kvzalloc(sizeof(*map) + size, GFP_KERNEL);
>               if (!map) {
> @@ -269,7 +268,7 @@ static int memcg_expand_shrinker_maps(int new_id)
>       struct mem_cgroup *memcg;
>  
>       size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
> -     old_size = memcg_shrinker_map_size;
> +     old_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * 
> sizeof(unsigned long);
>       if (size <= old_size)
>               return 0;

These bunch of DIV_ROUND_UP() looks too complex. Since now all the shrinker 
maps allocation
logic in the only file, can't we simplify this to look better? I mean something 
like below
to merge in your patch:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b951c289ef3a..27b6371a1656 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -247,7 +247,7 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
                return 0;
 
        down_read(&shrinker_rwsem);
-       size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * sizeof(unsigned 
long);
+       size = shrinker_nr_max / BITS_PER_BYTE;
        for_each_node(nid) {
                map = kvzalloc(sizeof(*map) + size, GFP_KERNEL);
                if (!map) {
@@ -264,13 +264,11 @@ int memcg_alloc_shrinker_maps(struct mem_cgroup *memcg)
 
 static int memcg_expand_shrinker_maps(int new_id)
 {
-       int size, old_size, ret = 0;
+       int size, old_size, new_nr_max, ret = 0;
        struct mem_cgroup *memcg;
 
        size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
-       old_size = DIV_ROUND_UP(shrinker_nr_max, BITS_PER_LONG) * 
sizeof(unsigned long);
-       if (size <= old_size)
-               return 0;
+       new_nr_max = size * BITS_PER_BYTE;
 
        if (!root_mem_cgroup)
                goto out;
@@ -287,6 +285,9 @@ static int memcg_expand_shrinker_maps(int new_id)
        } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
 
 out:
+       if (ret == 0)
+               shrinker_nr_max = new_nr_max;
+
        return ret;
 }
 
@@ -334,8 +335,6 @@ static int prealloc_memcg_shrinker(struct shrinker 
*shrinker)
                        idr_remove(&shrinker_idr, id);
                        goto unlock;
                }
-
-               shrinker_nr_max = id + 1;
        }
        shrinker->id = id;
        ret = 0;