mm: page_cgroup: check page_cgroup arrays in lookup_page_cgroup() only when necessary

lookup_page_cgroup() is usually used only against pages that are used in
userspace.

The exception is the CONFIG_DEBUG_VM-only memcg check from the page
allocator: it can run on pages without page_cgroup descriptors allocated
when the pages are fed into the page allocator for the first time during
boot or memory hotplug.

Include the array check only when CONFIG_DEBUG_VM is set and save the
unnecessary check in production kernels.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index f0559e049e002e48eda0160c41a71abc18a95460..e910524e5a086c1969f25f70fa00777ff6c706e7 100644 (file)
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -28,9 +28,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
        struct page_cgroup *base;
 
        base = NODE_DATA(page_to_nid(page))->node_page_cgroup;
+#ifdef CONFIG_DEBUG_VM
+       /*
+        * The sanity checks the page allocator does upon freeing a
+        * page can reach here before the page_cgroup arrays are
+        * allocated when feeding a range of pages to the allocator
+        * for the first time during bootup or memory hotplug.
+        */
        if (unlikely(!base))
                return NULL;
-
+#endif
        offset = pfn - NODE_DATA(page_to_nid(page))->node_start_pfn;
        return base + offset;
 }
@@ -85,9 +92,16 @@ struct page_cgroup *lookup_page_cgroup(struct page *page)
 {
        unsigned long pfn = page_to_pfn(page);
        struct mem_section *section = __pfn_to_section(pfn);
-
+#ifdef CONFIG_DEBUG_VM
+       /*
+        * The sanity checks the page allocator does upon freeing a
+        * page can reach here before the page_cgroup arrays are
+        * allocated when feeding a range of pages to the allocator
+        * for the first time during bootup or memory hotplug.
+        */
        if (!section->page_cgroup)
                return NULL;
+#endif
        return section->page_cgroup + pfn;
 }