On Wed, Jan 6, 2021 at 3:16 AM Kirill Tkhai [off-list ref] wrote:

On 06.01.2021 01:58, Yang Shi wrote:

quoted

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.

Signed-off-by: Yang Shi <redacted>
---
 mm/vmscan.c | 33 ++++++++++++++++++---------------
 1 file changed, 18 insertions(+), 15 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index f20ed8e928c2..d9795fb0f1c5 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -340,6 +340,9 @@ 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_REGISTERING, 0, 0, GFP_KERNEL);

@@ -424,7 +427,7 @@ 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)

@@ -535,8 +538,20 @@ 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)
+                     return 0;
+             if (err != -ENOSYS)
+                     return err;
+
+             shrinker->flags &= ~SHRINKER_MEMCG_AWARE;

This looks very confusing.

In case of you want to disable preallocation branch for !MEMCG case,
you should firstly consider something like the below:

Not only !CONFIG_MEMCG, but also "cgroup_disable=memory" case.

#ifdef CONFIG_MEMCG
#define SHRINKER_MEMCG_AWARE    (1 << 2)
#else
#define SHRINKER_MEMCG_AWARE    0
#endif

This could handle !CONFIG_MEMCG case, but can't deal with
"cgroup_disable=memory" case. We could consider check
mem_cgroup_disabled() when initializing shrinker, but this may result
in touching fs codes like below:

--- a/fs/super.c
+++ b/fs/super.c

@@ -266,7 +266,9 @@ static struct super_block *alloc_super(struct

file_system_type *type, int flags,
        s->s_shrink.scan_objects = super_cache_scan;
        s->s_shrink.count_objects = super_cache_count;
        s->s_shrink.batch = 1024;
-       s->s_shrink.flags = SHRINKER_NUMA_AWARE | SHRINKER_MEMCG_AWARE;
+       s->s_shrink.flags = SHRINKER_NUMA_AWARE;
+       if (!mem_cgroup_disabled())
+               s->s_shrink.flags |= SHRINKER_MEMCG_AWARE;
        if (prealloc_shrinker(&s->s_shrink))
                goto fail;
        if (list_lru_init_memcg(&s->s_dentry_lru, &s->s_shrink))

quoted

+     }

+     size = sizeof(*shrinker->nr_deferred);
      if (shrinker->flags & SHRINKER_NUMA_AWARE)
              size *= nr_node_ids;

@@ -544,26 +559,14 @@ 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;