On Mon, Jan 25, 2021 at 12:36 AM Kirill Tkhai [off-list ref] wrote:

On 22.01.2021 02:06, Yang Shi wrote:

quoted

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 <redacted>
---
 mm/vmscan.c | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d3f3701dfcd2..40e7751ef961 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 free_shrinker_map_rcu(struct rcu_head *head)
 {

@@ -248,7 +247,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
              return 0;

      down_write(&shrinker_rwsem);
-     size = memcg_shrinker_map_size;
+     size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
      for_each_node(nid) {
              map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid);
              if (!map) {

@@ -266,10 +265,11 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg)
 static int expand_shrinker_maps(int new_id)
 {
      int size, old_size, ret = 0;
+     int new_nr_max = new_id + 1;
      struct mem_cgroup *memcg;

-     size = DIV_ROUND_UP(new_id + 1, BITS_PER_LONG) * sizeof(unsigned long);
-     old_size = memcg_shrinker_map_size;
+     size = (new_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);
+     old_size = (shrinker_nr_max / BITS_PER_LONG + 1) * sizeof(unsigned long);

      if (size <= old_size)
              return 0;

This looks a BUG:

expand_shrinker_maps(id == 1)
{
        old_size = 64;
        size = 64;

        ===>return 0 and shrinker_nr_max remains 0.
}

Then shrink_slab_memcg() misses this shrinker since shrinker_nr_max == 0.

Yes, thanks for catching this. It should be fixed by the below patch:

diff --git a/mm/vmscan.c b/mm/vmscan.c
index bb254d39339f..47010a69b400 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -287,7 +287,7 @@ static int expand_shrinker_info(int new_id)
        old_d_size = shrinker_nr_max * sizeof(atomic_long_t);
        old_size = old_m_size + old_d_size;
        if (size <= old_size)
-               return 0;
+               goto out;

        if (!root_mem_cgroup)
                goto out;

quoted

@@ -286,9 +286,10 @@ static int expand_shrinker_maps(int new_id)
                      goto out;
              }
      } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
+
 out:
      if (!ret)
-             memcg_shrinker_map_size = size;
+             shrinker_nr_max = new_nr_max;

      return ret;
 }

@@ -321,7 +322,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
 #define SHRINKER_REGISTERING ((struct shrinker *)~0UL)

 static DEFINE_IDR(shrinker_idr);
-static int shrinker_nr_max;

 static int prealloc_memcg_shrinker(struct shrinker *shrinker)
 {

@@ -338,8 +338,6 @@ 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;