cgroups: use css id in swap cgroup for saving memory v5

Try to use CSS ID for records in swap_cgroup.  By this, on 64bit machine,
size of swap_cgroup goes down to 2 bytes from 8bytes.

This means, when 2GB of swap is equipped, (assume the page size is 4096bytes)

	From size of swap_cgroup = 2G/4k * 8 = 4Mbytes.
	To   size of swap_cgroup = 2G/4k * 2 = 1Mbytes.

Reduction is large.  Of course, there are trade-offs.  This CSS ID will
add overhead to swap-in/swap-out/swap-free.

But in general,
  - swap is a resource which the user tend to avoid use.
  - If swap is never used, swap_cgroup area is not used.
  - Reading traditional manuals, size of swap should be proportional to
    size of memory. Memory size of machine is increasing now.

I think reducing size of swap_cgroup makes sense.

Note:
  - ID->CSS lookup routine has no locks, it's under RCU-Read-Side.
  - memcg can be obsolete at rmdir() but not freed while refcnt from
    swap_cgroup is available.

Changelog v4->v5:
 - reworked on to memcg-charge-swapcache-to-proper-memcg.patch
Changlog ->v4:
 - fixed not configured case.
 - deleted unnecessary comments.
 - fixed NULL pointer bug.
 - fixed message in dmesg.

[nishimura@mxp.nes.nec.co.jp: css_tryget can be called twice in !PageCgroupUsed case]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 18 deletions

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index ceecfbb143f..ebf81074bed 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -285,12 +285,8 @@ struct swap_cgroup_ctrl {
 
 struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
 
-/*
- * This 8bytes seems big..maybe we can reduce this when we can use "id" for
- * cgroup rather than pointer.
- */
 struct swap_cgroup {
-        struct mem_cgroup       *val;
+        unsigned short          id;
 };
 #define SC_PER_PAGE     (PAGE_SIZE/sizeof(struct swap_cgroup))
 #define SC_POS_MASK     (SC_PER_PAGE - 1)
@@ -342,10 +338,10 @@ not_enough_page:
  * @ent: swap entry to be recorded into
  * @mem: mem_cgroup to be recorded
  *
- * Returns old value at success, NULL at failure.
- * (Of course, old value can be NULL.)
+ * Returns old value at success, 0 at failure.
+ * (Of course, old value can be 0.)
  */
-struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
+unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
 {
         int type = swp_type(ent);
         unsigned long offset = swp_offset(ent);
@@ -354,18 +350,18 @@ struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
         struct swap_cgroup_ctrl *ctrl;
         struct page *mappage;
         struct swap_cgroup *sc;
-        struct mem_cgroup *old;
+        unsigned short old;
 
         if (!do_swap_account)
-                return NULL;
+                return 0;
 
         ctrl = &swap_cgroup_ctrl[type];
 
         mappage = ctrl->map[idx];
         sc = page_address(mappage);
         sc += pos;
-        old = sc->val;
-        sc->val = mem;
+        old = sc->id;
+        sc->id = id;
 
         return old;
 }
@@ -374,9 +370,9 @@ struct mem_cgroup *swap_cgroup_record(swp_entry_t ent, struct mem_cgroup *mem)
  * lookup_swap_cgroup - lookup mem_cgroup tied to swap entry
  * @ent: swap entry to be looked up.
  *
- * Returns pointer to mem_cgroup at success. NULL at failure.
+ * Returns CSS ID of mem_cgroup at success. 0 at failure. (0 is invalid ID)
  */
-struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent)
+unsigned short lookup_swap_cgroup(swp_entry_t ent)
 {
         int type = swp_type(ent);
         unsigned long offset = swp_offset(ent);
@@ -385,16 +381,16 @@ struct mem_cgroup *lookup_swap_cgroup(swp_entry_t ent)
         struct swap_cgroup_ctrl *ctrl;
         struct page *mappage;
         struct swap_cgroup *sc;
-        struct mem_cgroup *ret;
+        unsigned short ret;
 
         if (!do_swap_account)
-                return NULL;
+                return 0;
 
         ctrl = &swap_cgroup_ctrl[type];
         mappage = ctrl->map[idx];
         sc = page_address(mappage);
         sc += pos;
-        ret = sc->val;
+        ret = sc->id;
         return ret;
 }
 
@@ -432,7 +428,7 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
 
         printk(KERN_INFO
                 "swap_cgroup: uses %ld bytes of vmalloc for pointer array space"
-                " and %ld bytes to hold mem_cgroup pointers on swap\n",
+                " and %ld bytes to hold mem_cgroup information per swap ents\n",
                 array_size, length * PAGE_SIZE);
         printk(KERN_INFO
         "swap_cgroup can be disabled by noswapaccount boot option.\n");