[PATCH 2/4] Switch caches notification dynamically

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

The /sys/slab/<name>/cache_notify attribute controls
whether the cache <name> is to be accounted or not.

For the reasons described before kmalloc caches cannot
be turned on.

By default no caches are accountable. Simply make
  # echo -n 1 >  /sys/slab/<name>cache_notify
to turn notification of this cache on.

If we turn accounting on on some cache and this cache 
is merged with some other, this "other" will be notified
as well. We can solve this by disabling of cache merging, 
but maybe we can do it some other way.

Turning the notification off is possible only when this 
cache is empty. The reason for this is that the pages,
that are full of objects are not linked in any list, so
we wouldn't be able to walk these pages and notify others
that these objects are no longer tracked.

Signed-off-by: Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>

---

 mm/slub.c |   45 +++++++++++++++++++++++++++++++++++++++++++++
 1 files changed, 45 insertions(+)

diff --git a/mm/slub.c b/mm/slub.c
index 1802645..bfb7c21 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2338,6 +2440,14 @@ EXPORT_SYMBOL(kmem_cache_destroy);
 struct kmem_cache kmalloc_caches[PAGE_SHIFT] __cacheline_aligned;
 EXPORT_SYMBOL(kmalloc_caches);
 
+static inline int is_kmalloc_cache(struct kmem_cache *s)
+{
+	int km_idx;
+
+	km_idx = s - kmalloc_caches;
+	return km_idx >= 0 && km_idx < ARRAY_SIZE(kmalloc_caches);
+}
+
 #ifdef CONFIG_ZONE_DMA
 static struct kmem_cache *kmalloc_caches_dma[PAGE_SHIFT];
 #endif
@@ -3753,6 +3874,42 @@ static ssize_t defrag_ratio_store(struct
 SLAB_ATTR(defrag_ratio);
 #endif
 
+static ssize_t cache_notify_show(struct kmem_cache *s, char *buf)
+{
+	return sprintf(buf, "%d\n", !!(s->flags & SLAB_NOTIFY));
+}
+
+static ssize_t cache_notify_store(struct kmem_cache *s,
+		const char *buf, size_t length)
+{
+	if (buf[0] == '1') {
+		if (is_kmalloc_cache(s))
+			/*
+			 * cannot just make these caches accountable
+			 */
+			return -EINVAL;
+
+		s->flags |= SLAB_NOTIFY;
+		return length;
+	}
+
+	if (buf[0] == '0') {
+		if (any_slab_objects(s))
+			/*
+			 * we cannot turn this off because of the
+			 * full slabs cannot be found in this case
+			 */
+			return -EBUSY;
+
+		s->flags &= ~SLAB_NOTIFY;
+		return length;
+	}
+
+	return -EINVAL;
+}
+
+SLAB_ATTR(cache_notify);
+
 static struct attribute * slab_attrs[] = {
 	&slab_size_attr.attr,
 	&object_size_attr.attr,
@@ -3783,6 +3940,7 @@ static struct attribute * slab_attrs[] =
 #ifdef CONFIG_NUMA
 	&defrag_ratio_attr.attr,
 #endif
+	&cache_notify_attr.attr,
 	NULL
 };