memcg: swapout refcnt fix

css's refcnt is dropped before end of following access.
Hold it until end of access.

Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 6ad309e..964a700 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1171,7 +1171,9 @@ __mem_cgroup_uncharge_common(struct page *page, enum charge_type ctype)
        mz = page_cgroup_zoneinfo(pc);
        unlock_page_cgroup(pc);
 
-       css_put(&mem->css);
+       /* at swapout, this memcg will be accessed to record to swap */
+       if (ctype != MEM_CGROUP_CHARGE_TYPE_SWAPOUT)
+               css_put(&mem->css);
 
        return mem;
 
@@ -1212,6 +1214,8 @@ void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
                swap_cgroup_record(ent, memcg);
                mem_cgroup_get(memcg);
        }
+       if (memcg)
+               css_put(&memcg->css);
 }
 
 #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP