memcg: cleanup mem_cgroup_move_parent()

mem_cgroup_move_parent() calls try_charge first and cancel_charge on
failure.  IMHO, charge/uncharge(especially charge) is high cost operation,
so we should avoid it as far as possible.

This patch tries to delay try_charge in mem_cgroup_move_parent() by
re-ordering checks it does.

And this patch renames mem_cgroup_move_account() to
__mem_cgroup_move_account(), changes the return value of
__mem_cgroup_move_account() from int to void, and adds a new
wrapper(mem_cgroup_move_account()), which checks whether a @pc is valid
for moving account and calls __mem_cgroup_move_account().

This patch removes the last caller of trylock_page_cgroup(), so removes
its definition too.

Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 35 insertions, 49 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 2d6b4a912a6d..6273984f2e34 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1613,27 +1613,22 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *mem,
 }
 
 /**
- * mem_cgroup_move_account - move account of the page
+ * __mem_cgroup_move_account - move account of the page
  * @pc: page_cgroup of the page.
  * @from: mem_cgroup which the page is moved from.
  * @to: mem_cgroup which the page is moved to. @from != @to.
  *
  * The caller must confirm following.
  * - page is not on LRU (isolate_page() is useful.)
- *
- * returns 0 at success,
- * returns -EBUSY when lock is busy or "pc" is unstable.
+ * - the pc is locked, used, and ->mem_cgroup points to @from.
  *
  * This function does "uncharge" from old cgroup but doesn't do "charge" to
  * new cgroup. It should be done by a caller.
  */
 
-static int mem_cgroup_move_account(struct page_cgroup *pc,
+static void __mem_cgroup_move_account(struct page_cgroup *pc,
         struct mem_cgroup *from, struct mem_cgroup *to)
 {
-        struct mem_cgroup_per_zone *from_mz, *to_mz;
-        int nid, zid;
-        int ret = -EBUSY;
         struct page *page;
         int cpu;
         struct mem_cgroup_stat *stat;
@@ -1641,20 +1636,9 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
 
         VM_BUG_ON(from == to);
         VM_BUG_ON(PageLRU(pc->page));
-
-        nid = page_cgroup_nid(pc);
-        zid = page_cgroup_zid(pc);
-        from_mz =  mem_cgroup_zoneinfo(from, nid, zid);
-        to_mz =  mem_cgroup_zoneinfo(to, nid, zid);
-
-        if (!trylock_page_cgroup(pc))
-                return ret;
-
-        if (!PageCgroupUsed(pc))
-                goto out;
-
-        if (pc->mem_cgroup != from)
-                goto out;
+        VM_BUG_ON(!PageCgroupLocked(pc));
+        VM_BUG_ON(!PageCgroupUsed(pc));
+        VM_BUG_ON(pc->mem_cgroup != from);
 
         if (!mem_cgroup_is_root(from))
                 res_counter_uncharge(&from->res, PAGE_SIZE);
@@ -1683,15 +1667,28 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
         css_get(&to->css);
         pc->mem_cgroup = to;
         mem_cgroup_charge_statistics(to, pc, true);
-        ret = 0;
-out:
-        unlock_page_cgroup(pc);
         /*
          * We charges against "to" which may not have any tasks. Then, "to"
          * can be under rmdir(). But in current implementation, caller of
          * this function is just force_empty() and it's garanteed that
          * "to" is never removed. So, we don't check rmdir status here.
          */
+}
+
+/*
+ * check whether the @pc is valid for moving account and call
+ * __mem_cgroup_move_account()
+ */
+static int mem_cgroup_move_account(struct page_cgroup *pc,
+                                struct mem_cgroup *from, struct mem_cgroup *to)
+{
+        int ret = -EINVAL;
+        lock_page_cgroup(pc);
+        if (PageCgroupUsed(pc) && pc->mem_cgroup == from) {
+                __mem_cgroup_move_account(pc, from, to);
+                ret = 0;
+        }
+        unlock_page_cgroup(pc);
         return ret;
 }
 
@@ -1713,38 +1710,27 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
         if (!pcg)
                 return -EINVAL;
 
+        ret = -EBUSY;
+        if (!get_page_unless_zero(page))
+                goto out;
+        if (isolate_lru_page(page))
+                goto put;
 
         parent = mem_cgroup_from_cont(pcg);
-
-
         ret = __mem_cgroup_try_charge(NULL, gfp_mask, &parent, false, page);
         if (ret || !parent)
-                return ret;
-
-        if (!get_page_unless_zero(page)) {
-                ret = -EBUSY;
-                goto uncharge;
-        }
-
-        ret = isolate_lru_page(page);
-
-        if (ret)
-                goto cancel;
+                goto put_back;
 
         ret = mem_cgroup_move_account(pc, child, parent);
-
+        if (!ret)
+                css_put(&parent->css);  /* drop extra refcnt by try_charge() */
+        else
+                mem_cgroup_cancel_charge(parent);       /* does css_put */
+put_back:
         putback_lru_page(page);
-        if (!ret) {
-                put_page(page);
-                /* drop extra refcnt by try_charge() */
-                css_put(&parent->css);
-                return 0;
-        }
-
-cancel:
+put:
         put_page(page);
-uncharge:
-        mem_cgroup_cancel_charge(parent);
+out:
         return ret;
 }