[v6 PATCH 09/11] mm: vmscan: don't need allocate shrinker->nr_deferred for memcg aware shrinkers

[Yang Shi]

Now nr_deferred is available on per memcg level for memcg aware shrinkers, so 
don't need
allocate shrinker->nr_deferred for such shrinkers anymore.

The prealloc_memcg_shrinker() would return -ENOSYS if !CONFIG_MEMCG or memcg is 
disabled
by kernel command line, then shrinker's SHRINKER_MEMCG_AWARE flag would be 
cleared.
This makes the implementation of this patch simpler.

Acked-by: Vlastimil Babka <vba...@suse.cz>
Signed-off-by: Yang Shi <shy828...@gmail.com>
---
 mm/vmscan.c | 31 ++++++++++++++++---------------
 1 file changed, 16 insertions(+), 15 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 545422d2aeec..20a35d26ae12 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -334,6 +334,9 @@ static int prealloc_memcg_shrinker(struct shrinker 
*shrinker)
 {
        int id, ret = -ENOMEM;
 
+       if (mem_cgroup_disabled())
+               return -ENOSYS;
+
        down_write(&shrinker_rwsem);
        /* This may call shrinker, so it must use down_read_trylock() */
        id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
@@ -414,7 +417,7 @@ static bool writeback_throttling_sane(struct scan_control 
*sc)
 #else
 static int prealloc_memcg_shrinker(struct shrinker *shrinker)
 {
-       return 0;
+       return -ENOSYS;
 }
 
 static void unregister_memcg_shrinker(struct shrinker *shrinker)
@@ -525,8 +528,18 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum 
lru_list lru, int zone
  */
 int prealloc_shrinker(struct shrinker *shrinker)
 {
-       unsigned int size = sizeof(*shrinker->nr_deferred);
+       unsigned int size;
+       int err;
+
+       if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
+               err = prealloc_memcg_shrinker(shrinker);
+               if (err != -ENOSYS)
+                       return err;
 
+               shrinker->flags &= ~SHRINKER_MEMCG_AWARE;
+       }
+
+       size = sizeof(*shrinker->nr_deferred);
        if (shrinker->flags & SHRINKER_NUMA_AWARE)
                size *= nr_node_ids;
 
@@ -534,26 +547,14 @@ int prealloc_shrinker(struct shrinker *shrinker)
        if (!shrinker->nr_deferred)
                return -ENOMEM;
 
-       if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
-               if (prealloc_memcg_shrinker(shrinker))
-                       goto free_deferred;
-       }
 
        return 0;
-
-free_deferred:
-       kfree(shrinker->nr_deferred);
-       shrinker->nr_deferred = NULL;
-       return -ENOMEM;
 }
 
 void free_prealloced_shrinker(struct shrinker *shrinker)
 {
-       if (!shrinker->nr_deferred)
-               return;
-
        if (shrinker->flags & SHRINKER_MEMCG_AWARE)
-               unregister_memcg_shrinker(shrinker);
+               return unregister_memcg_shrinker(shrinker);
 
        kfree(shrinker->nr_deferred);
        shrinker->nr_deferred = NULL;
-- 
2.26.2