2 files changed, 69 insertions, 29 deletions

diff --git a/include/linux/shrinker.h b/include/linux/shrinker.h
index ac6b8ee07825..884e76222e1b 100644
--- a/include/linux/shrinker.h
+++ b/include/linux/shrinker.h
@@ -4,6 +4,12 @@
 /*
  * This struct is used to pass information from page reclaim to the shrinkers.
  * We consolidate the values for easier extention later.
+ *
+ * The 'gfpmask' refers to the allocation we are currently trying to
+ * fulfil.
+ *
+ * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
+ * querying the cache size, so a fastpath for that case is appropriate.
  */
 struct shrink_control {
         gfp_t gfp_mask;
@@ -12,23 +18,37 @@ struct shrink_control {
         unsigned long nr_to_scan;
 };
 
+#define SHRINK_STOP (~0UL)
 /*
  * A callback you can register to apply pressure to ageable caches.
  *
- * 'sc' is passed shrink_control which includes a count 'nr_to_scan'
- * and a 'gfpmask'.  It should look through the least-recently-used
- * 'nr_to_scan' entries and attempt to free them up.  It should return
- * the number of objects which remain in the cache.  If it returns -1, it means
- * it cannot do any scanning at this time (eg. there is a risk of deadlock).
+ * @shrink() should look through the least-recently-used 'nr_to_scan' entries
+ * and attempt to free them up.  It should return the number of objects which
+ * remain in the cache.  If it returns -1, it means it cannot do any scanning at
+ * this time (eg. there is a risk of deadlock).
  *
- * The 'gfpmask' refers to the allocation we are currently trying to
- * fulfil.
+ * @count_objects should return the number of freeable items in the cache. If
+ * there are no objects to free or the number of freeable items cannot be
+ * determined, it should return 0. No deadlock checks should be done during the
+ * count callback - the shrinker relies on aggregating scan counts that couldn't
+ * be executed due to potential deadlocks to be run at a later call when the
+ * deadlock condition is no longer pending.
  *
- * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
- * querying the cache size, so a fastpath for that case is appropriate.
+ * @scan_objects will only be called if @count_objects returned a non-zero
+ * value for the number of freeable objects. The callout should scan the cache
+ * and attempt to free items from the cache. It should then return the number
+ * of objects freed during the scan, or SHRINK_STOP if progress cannot be made
+ * due to potential deadlocks. If SHRINK_STOP is returned, then no further
+ * attempts to call the @scan_objects will be made from the current reclaim
+ * context.
  */
 struct shrinker {
         int (*shrink)(struct shrinker *, struct shrink_control *sc);
+        unsigned long (*count_objects)(struct shrinker *,
+                                       struct shrink_control *sc);
+        unsigned long (*scan_objects)(struct shrinker *,
+                                      struct shrink_control *sc);
+
         int seeks;      /* seeks to recreate an obj */
         long batch;     /* reclaim batch size, 0 = default */
 
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 2cff0d491c6d..4d4e859b4b9c 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -205,19 +205,24 @@ static inline int do_shrinker_shrink(struct shrinker *shrinker,
  *
  * Returns the number of slab objects which we shrunk.
  */
-unsigned long shrink_slab(struct shrink_control *shrink,
+unsigned long shrink_slab(struct shrink_control *shrinkctl,
                           unsigned long nr_pages_scanned,
                           unsigned long lru_pages)
 {
         struct shrinker *shrinker;
-        unsigned long ret = 0;
+        unsigned long freed = 0;
 
         if (nr_pages_scanned == 0)
                 nr_pages_scanned = SWAP_CLUSTER_MAX;
 
         if (!down_read_trylock(&shrinker_rwsem)) {
-                /* Assume we'll be able to shrink next time */
-                ret = 1;
+                /*
+                 * If we would return 0, our callers would understand that we
+                 * have nothing else to shrink and give up trying. By returning
+                 * 1 we keep it going and assume we'll be able to shrink next
+                 * time.
+                 */
+                freed = 1;
                 goto out;
         }
 
@@ -225,14 +230,16 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                 unsigned long long delta;
                 long total_scan;
                 long max_pass;
-                int shrink_ret = 0;
                 long nr;
                 long new_nr;
                 long batch_size = shrinker->batch ? shrinker->batch
                                                   : SHRINK_BATCH;
 
-                max_pass = do_shrinker_shrink(shrinker, shrink, 0);
-                if (max_pass <= 0)
+                if (shrinker->count_objects)
+                        max_pass = shrinker->count_objects(shrinker, shrinkctl);
+                else
+                        max_pass = do_shrinker_shrink(shrinker, shrinkctl, 0);
+                if (max_pass == 0)
                         continue;
 
                 /*
@@ -248,8 +255,8 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                 do_div(delta, lru_pages + 1);
                 total_scan += delta;
                 if (total_scan < 0) {
-                        printk(KERN_ERR "shrink_slab: %pF negative objects to "
-                               "delete nr=%ld\n",
+                        printk(KERN_ERR
+                        "shrink_slab: %pF negative objects to delete nr=%ld\n",
                                shrinker->shrink, total_scan);
                         total_scan = max_pass;
                 }
@@ -277,20 +284,33 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                 if (total_scan > max_pass * 2)
                         total_scan = max_pass * 2;
 
-                trace_mm_shrink_slab_start(shrinker, shrink, nr,
+                trace_mm_shrink_slab_start(shrinker, shrinkctl, nr,
                                         nr_pages_scanned, lru_pages,
                                         max_pass, delta, total_scan);
 
                 while (total_scan >= batch_size) {
-                        int nr_before;
 
-                        nr_before = do_shrinker_shrink(shrinker, shrink, 0);
-                        shrink_ret = do_shrinker_shrink(shrinker, shrink,
-                                                        batch_size);
-                        if (shrink_ret == -1)
-                                break;
-                        if (shrink_ret < nr_before)
-                                ret += nr_before - shrink_ret;
+                        if (shrinker->scan_objects) {
+                                unsigned long ret;
+                                shrinkctl->nr_to_scan = batch_size;
+                                ret = shrinker->scan_objects(shrinker, shrinkctl);
+
+                                if (ret == SHRINK_STOP)
+                                        break;
+                                freed += ret;
+                        } else {
+                                int nr_before;
+                                long ret;
+
+                                nr_before = do_shrinker_shrink(shrinker, shrinkctl, 0);
+                                ret = do_shrinker_shrink(shrinker, shrinkctl,
+                                                                batch_size);
+                                if (ret == -1)
+                                        break;
+                                if (ret < nr_before)
+                                        freed += nr_before - ret;
+                        }
+
                         count_vm_events(SLABS_SCANNED, batch_size);
                         total_scan -= batch_size;
 
@@ -308,12 +328,12 @@ unsigned long shrink_slab(struct shrink_control *shrink,
                 else
                         new_nr = atomic_long_read(&shrinker->nr_in_batch);
 
-                trace_mm_shrink_slab_end(shrinker, shrink_ret, nr, new_nr);
+                trace_mm_shrink_slab_end(shrinker, freed, nr, new_nr);
         }
         up_read(&shrinker_rwsem);
 out:
         cond_resched();
-        return ret;
+        return freed;
 }
 
 static inline int is_page_cache_freeable(struct page *page)