mm: slab: make page_cgroup_ino() to recognize non-compound slab pages properly

page_cgroup_ino() doesn't return a valid memcg pointer for non-compound
slab pages, because it depends on PgHead AND PgSlab flags to be set to
determine the memory cgroup from the kmem_cache.  It's correct for
compound pages, but not for generic small pages.  Those don't have PgHead
set, so it ends up returning zero.

Fix this by replacing the condition to PageSlab() && !PageTail().

Before this patch:
  [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/[email protected]/ | grep slab
  0x0000000000000080	        38        0  _______S___________________________________	slab

After this patch:
  [root@localhost ~]# ./page-types -c /sys/fs/cgroup/user.slice/user-0.slice/[email protected]/ | grep slab
  0x0000000000000080	       147        0  _______S___________________________________	slab

Also, hwpoison_filter_task() uses output of page_cgroup_ino() in order
to filter error injection events based on memcg.  So if
page_cgroup_ino() fails to return memcg pointer, we just fail to inject
memory error.  Considering that hwpoison filter is for testing, affected
users are limited and the impact should be marginal.

[[email protected]: changelog additions]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: 4d96ba3 ("mm: memcg/slab: stop setting page->mem_cgroup pointer for slab pages")
Signed-off-by: Roman Gushchin <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: David Rientjes <[email protected]>
Cc: Vladimir Davydov <[email protected]>
Cc: Daniel Jordan <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: rgushchin

mm/memcontrol.c

@@ -484,7 +484,7 @@ ino_t page_cgroup_ino(struct page *page)
 	unsigned long ino = 0;
 
 	rcu_read_lock();
-	if (PageHead(page) && PageSlab(page))
+	if (PageSlab(page) && !PageTail(page))
 		memcg = memcg_from_slab_page(page);
 	else
 		memcg = READ_ONCE(page->mem_cgroup);

mm/slab.h

@@ -323,8 +323,8 @@ static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
  * Expects a pointer to a slab page. Please note, that PageSlab() check
  * isn't sufficient, as it returns true also for tail compound slab pages,
  * which do not have slab_cache pointer set.
- * So this function assumes that the page can pass PageHead() and PageSlab()
- * checks.
+ * So this function assumes that the page can pass PageSlab() && !PageTail()
+ * check.
  *
  * The kmem_cache can be reparented asynchronously. The caller must ensure
  * the memcg lifetime, e.g. by taking rcu_read_lock() or cgroup_mutex.