Linux 3.12 compat: shrinker semantics

The new shrinker API as of Linux 3.12 modifies "struct shrinker" by
replacing the @shrink callback with the pair of @count_objects and
@scan_objects.  It also requires the return value of @count_objects to
return the number of objects actually freed whereas the previous @shrink
callback returned the number of remaining freeable objects.

This patch adds support for the new @scan_objects return value semantics
and updates the splat shrinker test case appropriately.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes openzfs#403

Conflicts:
	module/spl/spl-kmem.c
	module/splat/splat-linux.c

include/linux/mm_compat.h

@@ -311,4 +311,11 @@ fn ## _scan_objects(struct shrinker *shrink, struct shrink_control *sc)	\
 #error "Unknown shrinker callback"
 #endif
 
+#if defined(HAVE_SPLIT_SHRINKER_CALLBACK)
+typedef unsigned long	spl_shrinker_t;
+#else
+typedef int		spl_shrinker_t;
+#define	SHRINK_STOP	(-1)
+#endif
+
 #endif /* SPL_MM_COMPAT_H */

module/spl/spl-kmem.c

@@ -63,7 +63,7 @@ MODULE_PARM_DESC(spl_kmem_cache_expire, "By age (0x1) or low memory (0x2)");
  * workload this may increase the likelihood of out of memory events.
  * For those cases it is advised that this option be set to zero.
  */
-unsigned int spl_kmem_cache_reclaim = KMC_RECLAIM_ONCE;
+unsigned int spl_kmem_cache_reclaim = 0 /* KMC_RECLAIM_ONCE */;
 module_param(spl_kmem_cache_reclaim, uint, 0644);
 MODULE_PARM_DESC(spl_kmem_cache_reclaim, "Single reclaim pass (0x1)");
 
@@ -2236,14 +2236,24 @@ EXPORT_SYMBOL(spl_kmem_cache_free);
  * report that they contain unused objects.  Because of this we only
  * register one shrinker function in the shim layer for all slab caches.
  * We always attempt to shrink all caches when this generic shrinker
- * is called.  The shrinker should return the number of free objects
- * in the cache when called with nr_to_scan == 0 but not attempt to
- * free any objects.  When nr_to_scan > 0 it is a request that nr_to_scan
- * objects should be freed, which differs from Solaris semantics.
- * Solaris semantics are to free all available objects which may (and
- * probably will) be more objects than the requested nr_to_scan.
+ * is called.
+ *
+ * If sc->nr_to_scan is zero, the caller is requesting a query of the
+ * number of objects which can potentially be freed.  If it is nonzero,
+ * the request is to free that many objects.
+ *
+ * Linux kernels >= 3.12 have the count_objects and scan_objects callbacks
+ * in struct shrinker and also require the shrinker to return the number
+ * of objects freed.
+ *
+ * Older kernels require the shrinker to return the number of freeable
+ * objects following the freeing of nr_to_free.
+ *
+ * Linux semantics differ from those under Solaris, which are to
+ * free all available objects which may (and probably will) be more
+ * objects than the requested nr_to_scan.
  */
-static int
+static spl_shrinker_t
 __spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
     struct shrink_control *sc)
 {
@@ -2252,17 +2262,22 @@ __spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
 
 	down_read(&spl_kmem_cache_sem);
 	list_for_each_entry(skc, &spl_kmem_cache_list, skc_list) {
-		if (sc->nr_to_scan)
+		if (sc->nr_to_scan) {
+#ifdef HAVE_SPLIT_SHRINKER_CALLBACK
+			uint64_t oldalloc = skc->skc_obj_alloc;
 			spl_kmem_cache_reap_now(skc,
 			   MAX(sc->nr_to_scan >> fls64(skc->skc_slab_objs), 1));
-
-		/*
-		 * Presume everything alloc'ed is reclaimable, this ensures
-		 * we are called again with nr_to_scan > 0 so can try and
-		 * reclaim.  The exact number is not important either so
-		 * we forgo taking this already highly contented lock.
-		 */
-		alloc += skc->skc_obj_alloc;
+			if (oldalloc > skc->skc_obj_alloc)
+				alloc += oldalloc - skc->skc_obj_alloc;
+#else
+			spl_kmem_cache_reap_now(skc,
+			   MAX(sc->nr_to_scan >> fls64(skc->skc_slab_objs), 1));
+			alloc += skc->skc_obj_alloc;
+#endif /* HAVE_SPLIT_SHRINKER_CALLBACK */
+		} else {
+			/* Request to query number of freeable objects */
+			alloc += skc->skc_obj_alloc;
+		}
 	}
 	up_read(&spl_kmem_cache_sem);
 
@@ -2273,7 +2288,7 @@ __spl_kmem_cache_generic_shrinker(struct shrinker *shrink,
 	 * system to thrash.
 	 */
 	if ((spl_kmem_cache_reclaim & KMC_RECLAIM_ONCE) && sc->nr_to_scan)
-		return (-1);
+		return (SHRINK_STOP);
 
 	return MAX((alloc * sysctl_vfs_cache_pressure) / 100, 0);
 }
@@ -2384,7 +2399,7 @@ spl_kmem_reap(void)
 	sc.nr_to_scan = KMC_REAP_CHUNK;
 	sc.gfp_mask = GFP_KERNEL;
 
-	__spl_kmem_cache_generic_shrinker(NULL, &sc);
+	(void) __spl_kmem_cache_generic_shrinker(NULL, &sc);
 }
 EXPORT_SYMBOL(spl_kmem_reap);
 

module/splat/splat-linux.c

@@ -97,11 +97,13 @@ SPL_SHRINKER_DECLARE(splat_linux_shrinker, splat_linux_shrinker_fn, 1);
 static unsigned long splat_linux_shrinker_size = 0;
 static struct file *splat_linux_shrinker_file = NULL;
 
-static int
+static spl_shrinker_t
 __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 {
 	static int failsafe = 0;
 	static unsigned long last_splat_linux_shrinker_size = 0;
+	unsigned long size;
+	spl_shrinker_t count;
 
 	/*
 	 * shrinker_size can only decrease or stay the same between callbacks
@@ -114,13 +116,21 @@ __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 	last_splat_linux_shrinker_size = splat_linux_shrinker_size;
 
 	if (sc->nr_to_scan) {
-		splat_linux_shrinker_size = splat_linux_shrinker_size -
-		    MIN(sc->nr_to_scan, splat_linux_shrinker_size);
+		size = MIN(sc->nr_to_scan, splat_linux_shrinker_size);
+		splat_linux_shrinker_size -= size;
 
 		splat_vprint(splat_linux_shrinker_file, SPLAT_LINUX_TEST3_NAME,
 		    "Reclaimed %lu objects, size now %lu\n",
-		    sc->nr_to_scan, splat_linux_shrinker_size);
+		    size, splat_linux_shrinker_size);
+
+#ifdef HAVE_SPLIT_SHRINKER_CALLBACK
+		count = size;
+#else
+		count = splat_linux_shrinker_size;
+#endif /* HAVE_SPLIT_SHRINKER_CALLBACK */
+
 	} else {
+		count = splat_linux_shrinker_size;
 		splat_vprint(splat_linux_shrinker_file, SPLAT_LINUX_TEST3_NAME,
 		    "Cache size is %lu\n", splat_linux_shrinker_size);
 	}
@@ -130,7 +140,7 @@ __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 		splat_vprint(splat_linux_shrinker_file, SPLAT_LINUX_TEST3_NAME,
 		    "Far more calls than expected (%d), size now %lu\n",
 		   failsafe, splat_linux_shrinker_size);
-		return -1;
+		return (SHRINK_STOP);
 	} else {
 		/*
 		 * We only increment failsafe if it doesn't trigger.  This
@@ -142,7 +152,7 @@ __splat_linux_shrinker_fn(struct shrinker *shrink, struct shrink_control *sc)
 	/* Shrinker has run, so signal back to test. */
 	wake_up(&shrinker_wait);
 
-	return (int)splat_linux_shrinker_size;
+	return (count);
 }
 
 SPL_SHRINKER_CALLBACK_WRAPPER(splat_linux_shrinker_fn);