memcg: fix race at move_parent around compound_order()

A fix up mem_cgroup_move_parent() which use compound_order() in
asynchronous manner.  This compound_order() may return unknown value
because we don't take lock.  Use PageTransHuge() and HPAGE_SIZE instead
of it.

Also clean up for mem_cgroup_move_parent().
 - remove unnecessary initialization of local variable.
 - rename charge_size -> page_size
 - remove unnecessary (wrong) comment.
 - added a comment about THP.

Note:
 Current design take compound_page_lock() in caller of move_account().
 This should be revisited when we implement direct move_task of hugepage
 without splitting.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 9 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8ab1d42664fb..3878cfe399dc 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2236,7 +2236,12 @@ static int mem_cgroup_move_account(struct page_cgroup *pc,
 {
         int ret = -EINVAL;
         unsigned long flags;
-
+        /*
+         * The page is isolated from LRU. So, collapse function
+         * will not handle this page. But page splitting can happen.
+         * Do this check under compound_page_lock(). The caller should
+         * hold it.
+         */
         if ((charge_size > PAGE_SIZE) && !PageTransHuge(pc->page))
                 return -EBUSY;
 
@@ -2268,7 +2273,7 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
         struct cgroup *cg = child->css.cgroup;
         struct cgroup *pcg = cg->parent;
         struct mem_cgroup *parent;
-        int charge = PAGE_SIZE;
+        int page_size = PAGE_SIZE;
         unsigned long flags;
         int ret;
 
@@ -2281,22 +2286,24 @@ static int mem_cgroup_move_parent(struct page_cgroup *pc,
                 goto out;
         if (isolate_lru_page(page))
                 goto put;
-        /* The page is isolated from LRU and we have no race with splitting */
-        charge = PAGE_SIZE << compound_order(page);
+
+        if (PageTransHuge(page))
+                page_size = HPAGE_SIZE;
 
         parent = mem_cgroup_from_cont(pcg);
-        ret = __mem_cgroup_try_charge(NULL, gfp_mask, &parent, false, charge);
+        ret = __mem_cgroup_try_charge(NULL, gfp_mask,
+                                &parent, false, page_size);
         if (ret || !parent)
                 goto put_back;
 
-        if (charge > PAGE_SIZE)
+        if (page_size > PAGE_SIZE)
                 flags = compound_lock_irqsave(page);
 
-        ret = mem_cgroup_move_account(pc, child, parent, true, charge);
+        ret = mem_cgroup_move_account(pc, child, parent, true, page_size);
         if (ret)
-                mem_cgroup_cancel_charge(parent, charge);
+                mem_cgroup_cancel_charge(parent, page_size);
 
-        if (charge > PAGE_SIZE)
+        if (page_size > PAGE_SIZE)
                 compound_unlock_irqrestore(page, flags);
 put_back:
         putback_lru_page(page);