On Tue, Feb 9, 2021 at 5:34 PM Roman Gushchin [off-list ref] wrote:

On Tue, Feb 09, 2021 at 05:12:51PM -0800, Yang Shi wrote:

quoted

On Tue, Feb 9, 2021 at 4:39 PM Roman Gushchin [off-list ref] wrote:

quoted

On Tue, Feb 09, 2021 at 09:46:41AM -0800, Yang Shi wrote:

quoted

Currently registered shrinker is indicated by non-NULL shrinker->nr_deferred.
This approach is fine with nr_deferred at the shrinker level, but the following
patches will move MEMCG_AWARE shrinkers' nr_deferred to memcg level, so their
shrinker->nr_deferred would always be NULL.  This would prevent the shrinkers
from unregistering correctly.

Remove SHRINKER_REGISTERING since we could check if shrinker is registered
successfully by the new flag.

Acked-by: Kirill Tkhai <redacted>
Signed-off-by: Yang Shi <redacted>
---
 include/linux/shrinker.h |  7 ++++---
 mm/vmscan.c              | 31 +++++++++----------------------
 2 files changed, 13 insertions(+), 25 deletions(-)

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index 0f80123650e2..1eac79ce57d4 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h

@@ -79,13 +79,14 @@ struct shrinker {
 #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */

 /* Flags */
-#define SHRINKER_NUMA_AWARE  (1 << 0)
-#define SHRINKER_MEMCG_AWARE (1 << 1)
+#define SHRINKER_REGISTERED  (1 << 0)
+#define SHRINKER_NUMA_AWARE  (1 << 1)
+#define SHRINKER_MEMCG_AWARE (1 << 2)
 /*
  * It just makes sense when the shrinker is also MEMCG_AWARE for now,
  * non-MEMCG_AWARE shrinker should not have this flag set.
  */
-#define SHRINKER_NONSLAB     (1 << 2)
+#define SHRINKER_NONSLAB     (1 << 3)

 extern int prealloc_shrinker(struct shrinker *shrinker);
 extern void register_shrinker_prepared(struct shrinker *shrinker);

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 273efbf4d53c..a047980536cf 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c

@@ -315,19 +315,6 @@ void set_shrinker_bit(struct mem_cgroup *memcg, int nid, int shrinker_id)
      }
 }

-/*
- * We allow subsystems to populate their shrinker-related
- * LRU lists before register_shrinker_prepared() is called
- * for the shrinker, since we don't want to impose
- * restrictions on their internal registration order.
- * In this case shrink_slab_memcg() may find corresponding
- * bit is set in the shrinkers map.
- *
- * This value is used by the function to detect registering
- * shrinkers and to skip do_shrink_slab() calls for them.
- */
-#define SHRINKER_REGISTERING ((struct shrinker *)~0UL)
-
 static DEFINE_IDR(shrinker_idr);

 static int prealloc_memcg_shrinker(struct shrinker *shrinker)

@@ -336,7 +323,7 @@ static int prealloc_memcg_shrinker(struct shrinker *shrinker)

      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);
+     id = idr_alloc(&shrinker_idr, shrinker, 0, 0, GFP_KERNEL);
      if (id < 0)
              goto unlock;

@@ -499,10 +486,7 @@ void register_shrinker_prepared(struct shrinker *shrinker)
 {
      down_write(&shrinker_rwsem);
      list_add_tail(&shrinker->list, &shrinker_list);
-#ifdef CONFIG_MEMCG
-     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
-             idr_replace(&shrinker_idr, shrinker, shrinker->id);
-#endif
+     shrinker->flags |= SHRINKER_REGISTERED;
      up_write(&shrinker_rwsem);
 }

@@ -522,13 +506,16 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
-     if (!shrinker->nr_deferred)
+     if (!(shrinker->flags & SHRINKER_REGISTERED))
              return;
-     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
-             unregister_memcg_shrinker(shrinker);
+
      down_write(&shrinker_rwsem);
      list_del(&shrinker->list);
+     shrinker->flags &= ~SHRINKER_REGISTERED;
      up_write(&shrinker_rwsem);
+
+     if (shrinker->flags & SHRINKER_MEMCG_AWARE)
+             unregister_memcg_shrinker(shrinker);

Because unregister_memcg_shrinker() will take and release shrinker_rwsem once again,
I wonder if it's better to move it into the locked section and change the calling
convention to require the caller to take the semaphore?

I don't think we could do that since unregister_memcg_shrinker() is
called by free_prealloced_shrinker() which is called without holding
the shrinker_rwsem by fs and workingset code.

We could add a bool parameter to indicate if the rwsem was acquired or
not, but IMHO it seems not worth it.

Can free_preallocated_shrinker() just do

if (shrinker->flags & SHRINKER_MEMCG_AWARE) {
        down_write(&shrinker_rwsem);
        unregister_memcg_shrinker(shrinker);
        up_write(&shrinker_rwsem);
}

?

Aha, yes. I didn't think of that way.