page_cgroup: reduce allocation overhead for page_cgroup array for CONFIG_SPARSEMEM

Currently we are allocating a single page_cgroup array per memory section
(stored in mem_section->base) when CONFIG_SPARSEMEM is selected.  This is
correct but memory inefficient solution because the allocated memory
(unless we fall back to vmalloc) is not kmalloc friendly:

        - 32b - 16384 entries (20B per entry) fit into 327680B so the
          524288B slab cache is used
        - 32b with PAE - 131072 entries with 2621440B fit into 4194304B
        - 64b - 32768 entries (40B per entry) fit into 2097152 cache

This is ~37% wasted space per memory section and it sumps up for the whole
memory.  On a x86_64 machine it is something like 6MB per 1GB of RAM.

We can reduce the internal fragmentation by using alloc_pages_exact which
allocates PAGE_SIZE aligned blocks so we will get down to <4kB wasted
memory per section which is much better.

We still need a fallback to vmalloc because we have no guarantees that we
will have a continuous memory of that size (order-10) later on during the
hotplug events.

[hannes@cmpxchg.org: do not define unused free_page_cgroup() without memory hotplug]
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Dave Hansen <dave@linux.vnet.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 36 insertions, 22 deletions

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 6c3f7a6a481a..8e7577cdf5e4 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -130,7 +130,38 @@ struct page *lookup_cgroup_page(struct page_cgroup *pc)
         return page;
 }
 
-/* __alloc_bootmem...() is protected by !slab_available() */
+static void *__init_refok alloc_page_cgroup(size_t size, int nid)
+{
+        void *addr = NULL;
+
+        addr = alloc_pages_exact(size, GFP_KERNEL | __GFP_NOWARN);
+        if (addr)
+                return addr;
+
+        if (node_state(nid, N_HIGH_MEMORY))
+                addr = vmalloc_node(size, nid);
+        else
+                addr = vmalloc(size);
+
+        return addr;
+}
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+static void free_page_cgroup(void *addr)
+{
+        if (is_vmalloc_addr(addr)) {
+                vfree(addr);
+        } else {
+                struct page *page = virt_to_page(addr);
+                if (!PageReserved(page)) { /* Is bootmem ? */
+                        size_t table_size =
+                                sizeof(struct page_cgroup) * PAGES_PER_SECTION;
+                        free_pages_exact(addr, table_size);
+                }
+        }
+}
+#endif
+
 static int __init_refok init_section_page_cgroup(unsigned long pfn)
 {
         struct page_cgroup *base, *pc;
@@ -147,17 +178,8 @@ static int __init_refok init_section_page_cgroup(unsigned long pfn)
 
         nid = page_to_nid(pfn_to_page(pfn));
         table_size = sizeof(struct page_cgroup) * PAGES_PER_SECTION;
-        VM_BUG_ON(!slab_is_available());
-        if (node_state(nid, N_HIGH_MEMORY)) {
-                base = kmalloc_node(table_size,
-                                    GFP_KERNEL | __GFP_NOWARN, nid);
-                if (!base)
-                        base = vmalloc_node(table_size, nid);
-        } else {
-                base = kmalloc(table_size, GFP_KERNEL | __GFP_NOWARN);
-                if (!base)
-                        base = vmalloc(table_size);
-        }
+        base = alloc_page_cgroup(table_size, nid);
+
         /*
          * The value stored in section->page_cgroup is (base - pfn)
          * and it does not point to the memory block allocated above,
@@ -189,16 +211,8 @@ void __free_page_cgroup(unsigned long pfn)
         if (!ms || !ms->page_cgroup)
                 return;
         base = ms->page_cgroup + pfn;
-        if (is_vmalloc_addr(base)) {
-                vfree(base);
-                ms->page_cgroup = NULL;
-        } else {
-                struct page *page = virt_to_page(base);
-                if (!PageReserved(page)) { /* Is bootmem ? */
-                        kfree(base);
-                        ms->page_cgroup = NULL;
-                }
-        }
+        free_page_cgroup(base);
+        ms->page_cgroup = NULL;
 }
 
 int __meminit online_page_cgroup(unsigned long start_pfn,