memcg: avoid use cmpxchg in swap cgroup maintainance

swap_cgroup uses 2bytes data and uses cmpxchg in a new operation.  2byte
cmpxchg/xchg is not available on some archs.  This patch replaces
cmpxchg/xchg with operations under lock.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reported-by: Sachin Sant <sachinp@in.ibm.com> wrote:
Acked-by: Balbir Singh <balbir@in.ibm.com>
Acked-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 16 insertions, 4 deletions

diff --git a/mm/page_cgroup.c b/mm/page_cgroup.c
index 3dd88539a0e6..6c0081441a32 100644
--- a/mm/page_cgroup.c
+++ b/mm/page_cgroup.c
@@ -284,6 +284,7 @@ static DEFINE_MUTEX(swap_cgroup_mutex);
 struct swap_cgroup_ctrl {
         struct page **map;
         unsigned long length;
+        spinlock_t      lock;
 };
 
 struct swap_cgroup_ctrl swap_cgroup_ctrl[MAX_SWAPFILES];
@@ -353,16 +354,22 @@ unsigned short swap_cgroup_cmpxchg(swp_entry_t ent,
         struct swap_cgroup_ctrl *ctrl;
         struct page *mappage;
         struct swap_cgroup *sc;
+        unsigned long flags;
+        unsigned short retval;
 
         ctrl = &swap_cgroup_ctrl[type];
 
         mappage = ctrl->map[idx];
         sc = page_address(mappage);
         sc += pos;
-        if (cmpxchg(&sc->id, old, new) == old)
-                return old;
+        spin_lock_irqsave(&ctrl->lock, flags);
+        retval = sc->id;
+        if (retval == old)
+                sc->id = new;
         else
-                return 0;
+                retval = 0;
+        spin_unlock_irqrestore(&ctrl->lock, flags);
+        return retval;
 }
 
 /**
@@ -383,13 +390,17 @@ unsigned short swap_cgroup_record(swp_entry_t ent, unsigned short id)
         struct page *mappage;
         struct swap_cgroup *sc;
         unsigned short old;
+        unsigned long flags;
 
         ctrl = &swap_cgroup_ctrl[type];
 
         mappage = ctrl->map[idx];
         sc = page_address(mappage);
         sc += pos;
-        old = xchg(&sc->id, id);
+        spin_lock_irqsave(&ctrl->lock, flags);
+        old = sc->id;
+        sc->id = id;
+        spin_unlock_irqrestore(&ctrl->lock, flags);
 
         return old;
 }
@@ -441,6 +452,7 @@ int swap_cgroup_swapon(int type, unsigned long max_pages)
         mutex_lock(&swap_cgroup_mutex);
         ctrl->length = length;
         ctrl->map = array;
+        spin_lock_init(&ctrl->lock);
         if (swap_cgroup_prepare(type)) {
                 /* memory shortage */
                 ctrl->map = NULL;