[RFC 01/26] mm, slub: allocate private object map for sysfs listings

[Vlastimil Babka]

Slub has a static spinlock protected bitmap for marking which objects are on
freelist when it wants to list them, for situations where dynamically
allocating such map can lead to recursion or locking issues, and on-stack
bitmap would be too large.

The handlers of sysfs files alloc_calls and free_calls also currently use this
shared bitmap, but their syscall context makes it straightforward to allocate a
private map before entering locked sections, so switch these processing paths
to use a private bitmap.

Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
---
 mm/slub.c | 43 +++++++++++++++++++++++++++++--------------
 1 file changed, 29 insertions(+), 14 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index 3f96e099817a..4c876749f322 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -448,6 +448,18 @@ static inline bool cmpxchg_double_slab(struct kmem_cache *s, struct page *page,
 static unsigned long object_map[BITS_TO_LONGS(MAX_OBJS_PER_PAGE)];
 static DEFINE_SPINLOCK(object_map_lock);
 
+static void __fill_map(unsigned long *obj_map, struct kmem_cache *s,
+		       struct page *page)
+{
+	void *addr = page_address(page);
+	void *p;
+
+	bitmap_zero(obj_map, page->objects);
+
+	for (p = page->freelist; p; p = get_freepointer(s, p))
+		set_bit(__obj_to_index(s, addr, p), obj_map);
+}
+
 /*
  * Determine a map of object in use on a page.
  *
@@ -457,17 +469,11 @@ static DEFINE_SPINLOCK(object_map_lock);
 static unsigned long *get_map(struct kmem_cache *s, struct page *page)
 	__acquires(&object_map_lock)
 {
-	void *p;
-	void *addr = page_address(page);
-
 	VM_BUG_ON(!irqs_disabled());
 
 	spin_lock(&object_map_lock);
 
-	bitmap_zero(object_map, page->objects);
-
-	for (p = page->freelist; p; p = get_freepointer(s, p))
-		set_bit(__obj_to_index(s, addr, p), object_map);
+	__fill_map(object_map, s, page);
 
 	return object_map;
 }
@@ -4813,17 +4819,17 @@ static int add_location(struct loc_track *t, struct kmem_cache *s,
 }
 
 static void process_slab(struct loc_track *t, struct kmem_cache *s,
-		struct page *page, enum track_item alloc)
+		struct page *page, enum track_item alloc,
+		unsigned long *obj_map)
 {
 	void *addr = page_address(page);
 	void *p;
-	unsigned long *map;
 
-	map = get_map(s, page);
+	__fill_map(obj_map, s, page);
+
 	for_each_object(p, s, addr, page->objects)
-		if (!test_bit(__obj_to_index(s, addr, p), map))
+		if (!test_bit(__obj_to_index(s, addr, p), obj_map))
 			add_location(t, s, get_track(s, p, alloc));
-	put_map(map);
 }
 
 static int list_locations(struct kmem_cache *s, char *buf,
@@ -4834,11 +4840,18 @@ static int list_locations(struct kmem_cache *s, char *buf,
 	struct loc_track t = { 0, 0, NULL };
 	int node;
 	struct kmem_cache_node *n;
+	unsigned long *obj_map;
+
+	obj_map = bitmap_alloc(oo_objects(s->oo), GFP_KERNEL);
+	if (!obj_map)
+		return sysfs_emit(buf, "Out of memory\n");
 
 	if (!alloc_loc_track(&t, PAGE_SIZE / sizeof(struct location),
 			     GFP_KERNEL)) {
+		bitmap_free(obj_map);
 		return sysfs_emit(buf, "Out of memory\n");
 	}
+
 	/* Push back cpu slabs */
 	flush_all(s);
 
@@ -4851,12 +4864,14 @@ static int list_locations(struct kmem_cache *s, char *buf,
 
 		spin_lock_irqsave(&n->list_lock, flags);
 		list_for_each_entry(page, &n->partial, slab_list)
-			process_slab(&t, s, page, alloc);
+			process_slab(&t, s, page, alloc, obj_map);
 		list_for_each_entry(page, &n->full, slab_list)
-			process_slab(&t, s, page, alloc);
+			process_slab(&t, s, page, alloc, obj_map);
 		spin_unlock_irqrestore(&n->list_lock, flags);
 	}
 
+	bitmap_free(obj_map);
+
 	for (i = 0; i < t.count; i++) {
 		struct location *l = &t.loc[i];
 
-- 
2.31.1