memcg: avoid deadlock between move charge and try_charge()

__mem_cgroup_try_charge() can be called under down_write(&mmap_sem)(e.g.
mlock does it). This means it can cause deadlock if it races with move charge:

Ex.1)
                move charge             |        try charge
  --------------------------------------+------------------------------
    mem_cgroup_can_attach()             |  down_write(&mmap_sem)
      mc.moving_task = current          |    ..
      mem_cgroup_precharge_mc()         |  __mem_cgroup_try_charge()
        mem_cgroup_count_precharge()    |    prepare_to_wait()
          down_read(&mmap_sem)          |    if (mc.moving_task)
          -> cannot aquire the lock     |    -> true
                                        |      schedule()

Ex.2)
                move charge             |        try charge
  --------------------------------------+------------------------------
    mem_cgroup_can_attach()             |
      mc.moving_task = current          |
      mem_cgroup_precharge_mc()         |
        mem_cgroup_count_precharge()    |
          down_read(&mmap_sem)          |
          ..                            |
          up_read(&mmap_sem)            |
                                        |  down_write(&mmap_sem)
    mem_cgroup_move_task()              |    ..
      mem_cgroup_move_charge()          |  __mem_cgroup_try_charge()
        down_read(&mmap_sem)            |    prepare_to_wait()
        -> cannot aquire the lock       |    if (mc.moving_task)
                                        |    -> true
                                        |      schedule()

To avoid this deadlock, we do all the move charge works (both can_attach() and
attach()) under one mmap_sem section.
And after this patch, we set/clear mc.moving_task outside mc.lock, because we
use the lock only to check mc.from/to.

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

1 files changed, 26 insertions, 17 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 62d1880f6992..26218df8d19d 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -278,13 +278,14 @@ enum move_type {
 
 /* "mc" and its members are protected by cgroup_mutex */
 static struct move_charge_struct {
-        spinlock_t        lock; /* for from, to, moving_task */
+        spinlock_t        lock; /* for from, to */
         struct mem_cgroup *from;
         struct mem_cgroup *to;
         unsigned long precharge;
         unsigned long moved_charge;
         unsigned long moved_swap;
         struct task_struct *moving_task;        /* a task moving charges */
+        struct mm_struct *mm;
         wait_queue_head_t waitq;                /* a waitq for other context */
 } mc = {
         .lock = __SPIN_LOCK_UNLOCKED(mc.lock),
@@ -4631,7 +4632,7 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
         unsigned long precharge;
         struct vm_area_struct *vma;
 
-        down_read(&mm->mmap_sem);
+        /* We've already held the mmap_sem */
         for (vma = mm->mmap; vma; vma = vma->vm_next) {
                 struct mm_walk mem_cgroup_count_precharge_walk = {
                         .pmd_entry = mem_cgroup_count_precharge_pte_range,
@@ -4643,7 +4644,6 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm)
                 walk_page_range(vma->vm_start, vma->vm_end,
                                         &mem_cgroup_count_precharge_walk);
         }
-        up_read(&mm->mmap_sem);
 
         precharge = mc.precharge;
         mc.precharge = 0;
@@ -4694,11 +4694,16 @@ static void mem_cgroup_clear_mc(void)
 
                 mc.moved_swap = 0;
         }
+        if (mc.mm) {
+                up_read(&mc.mm->mmap_sem);
+                mmput(mc.mm);
+        }
         spin_lock(&mc.lock);
         mc.from = NULL;
         mc.to = NULL;
-        mc.moving_task = NULL;
         spin_unlock(&mc.lock);
+        mc.moving_task = NULL;
+        mc.mm = NULL;
         mem_cgroup_end_move(from);
         memcg_oom_recover(from);
         memcg_oom_recover(to);
@@ -4724,12 +4729,21 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
                         return 0;
                 /* We move charges only when we move a owner of the mm */
                 if (mm->owner == p) {
+                        /*
+                         * We do all the move charge works under one mmap_sem to
+                         * avoid deadlock with down_write(&mmap_sem)
+                         * -> try_charge() -> if (mc.moving_task) -> sleep.
+                         */
+                        down_read(&mm->mmap_sem);
+
                         VM_BUG_ON(mc.from);
                         VM_BUG_ON(mc.to);
                         VM_BUG_ON(mc.precharge);
                         VM_BUG_ON(mc.moved_charge);
                         VM_BUG_ON(mc.moved_swap);
                         VM_BUG_ON(mc.moving_task);
+                        VM_BUG_ON(mc.mm);
+
                         mem_cgroup_start_move(from);
                         spin_lock(&mc.lock);
                         mc.from = from;
@@ -4737,14 +4751,16 @@ static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
                         mc.precharge = 0;
                         mc.moved_charge = 0;
                         mc.moved_swap = 0;
-                        mc.moving_task = current;
                         spin_unlock(&mc.lock);
+                        mc.moving_task = current;
+                        mc.mm = mm;
 
                         ret = mem_cgroup_precharge_mc(mm);
                         if (ret)
                                 mem_cgroup_clear_mc();
-                }
-                mmput(mm);
+                        /* We call up_read() and mmput() in clear_mc(). */
+                } else
+                        mmput(mm);
         }
         return ret;
 }
@@ -4832,7 +4848,7 @@ static void mem_cgroup_move_charge(struct mm_struct *mm)
         struct vm_area_struct *vma;
 
         lru_add_drain_all();
-        down_read(&mm->mmap_sem);
+        /* We've already held the mmap_sem */
         for (vma = mm->mmap; vma; vma = vma->vm_next) {
                 int ret;
                 struct mm_walk mem_cgroup_move_charge_walk = {
@@ -4851,7 +4867,6 @@ static void mem_cgroup_move_charge(struct mm_struct *mm)
                          */
                         break;
         }
-        up_read(&mm->mmap_sem);
 }
 
 static void mem_cgroup_move_task(struct cgroup_subsys *ss,
@@ -4860,17 +4875,11 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss,
                                 struct task_struct *p,
                                 bool threadgroup)
 {
-        struct mm_struct *mm;
-
-        if (!mc.to)
+        if (!mc.mm)
                 /* no need to move charge */
                 return;
 
-        mm = get_task_mm(p);
-        if (mm) {
-                mem_cgroup_move_charge(mm);
-                mmput(mm);
-        }
+        mem_cgroup_move_charge(mc.mm);
         mem_cgroup_clear_mc();
 }
 #else   /* !CONFIG_MMU */