Commit dfe076b0 authored by Daisuke Nishimura's avatar Daisuke Nishimura Committed by Linus Torvalds
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memcg: fix deadlock between cpuset and memcg 



Commit b1dd693e ("memcg: avoid deadlock between move charge and
try_charge()") can cause another deadlock about mmap_sem on task migration
if cpuset and memcg are mounted onto the same mount point.

After the commit, cgroup_attach_task() has sequence like:

cgroup_attach_task()
  ss->can_attach()
    cpuset_can_attach()
    mem_cgroup_can_attach()
      down_read(&mmap_sem)        (1)
  ss->attach()
    cpuset_attach()
      mpol_rebind_mm()
        down_write(&mmap_sem)     (2)
        up_write(&mmap_sem)
      cpuset_migrate_mm()
        do_migrate_pages()
          down_read(&mmap_sem)
          up_read(&mmap_sem)
    mem_cgroup_move_task()
      mem_cgroup_clear_mc()
        up_read(&mmap_sem)

We can cause deadlock at (2) because we've already aquire the mmap_sem at (1).

But the commit itself is necessary to fix deadlocks which have existed
before the commit like:

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()
                                        |      -> move charge should wake it up

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()
                                        |      -> move charge should wake it up

This patch fixes all of these problems by:
1. revert the commit.
2. To fix the Ex.1, we set mc.moving_task after mem_cgroup_count_precharge()
   has released the mmap_sem.
3. To fix the Ex.2, we use down_read_trylock() instead of down_read() in
   mem_cgroup_move_charge() and, if it has failed to aquire the lock, cancel
   all extra charges, wake up all waiters, and retry trylock.

Signed-off-by: default avatarDaisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Reported-by: default avatarBen Blum <bblum@andrew.cmu.edu>
Cc: Miao Xie <miaox@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Paul Menage <menage@google.com>
Cc: Hiroyuki Kamezawa <kamezawa.hiroyuki@gmail.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 043d18b1

mm/memcontrol.c

+49 −35
Original line number Diff line number Diff line
@@ -292,7 +292,6 @@ static struct move_charge_struct { 
	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),
@@ -4681,7 +4680,7 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm) 
	unsigned long precharge;

	struct vm_area_struct *vma;



	/* We've already held the mmap_sem */

	down_read(&mm->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,
@@ -4693,6 +4692,7 @@ 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;
@@ -4702,10 +4702,15 @@ static unsigned long mem_cgroup_count_precharge(struct mm_struct *mm) 


static int mem_cgroup_precharge_mc(struct mm_struct *mm)

{

	return mem_cgroup_do_precharge(mem_cgroup_count_precharge(mm));

	unsigned long precharge = mem_cgroup_count_precharge(mm);



	VM_BUG_ON(mc.moving_task);

	mc.moving_task = current;

	return mem_cgroup_do_precharge(precharge);

}



static void mem_cgroup_clear_mc(void)

/* cancels all extra charges on mc.from and mc.to, and wakes up all waiters. */

static void __mem_cgroup_clear_mc(void)

{

	struct mem_cgroup *from = mc.from;

	struct mem_cgroup *to = mc.to;
@@ -4740,23 +4745,28 @@ static void mem_cgroup_clear_mc(void) 
						PAGE_SIZE * mc.moved_swap);

		}

		/* we've already done mem_cgroup_get(mc.to) */



		mc.moved_swap = 0;

	}

	if (mc.mm) {

		up_read(&mc.mm->mmap_sem);

		mmput(mc.mm);

	memcg_oom_recover(from);

	memcg_oom_recover(to);

	wake_up_all(&mc.waitq);

}



static void mem_cgroup_clear_mc(void)

{

	struct mem_cgroup *from = mc.from;



	/*

	 * we must clear moving_task before waking up waiters at the end of

	 * task migration.

	 */

	mc.moving_task = NULL;

	__mem_cgroup_clear_mc();

	spin_lock(&mc.lock);

	mc.from = NULL;

	mc.to = 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);

	wake_up_all(&mc.waitq);

}



static int mem_cgroup_can_attach(struct cgroup_subsys *ss,
@@ -4778,37 +4788,22 @@ 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;

			mc.to = mem;

			mc.precharge = 0;

			mc.moved_charge = 0;

			mc.moved_swap = 0;

			spin_unlock(&mc.lock);

			mc.moving_task = current;

			mc.mm = mm;

			/* We set mc.moving_task later */



			ret = mem_cgroup_precharge_mc(mm);

			if (ret)

				mem_cgroup_clear_mc();

			/* We call up_read() and mmput() in clear_mc(). */

		} else

		}

		mmput(mm);

	}

	return ret;
@@ -4898,7 +4893,19 @@ static void mem_cgroup_move_charge(struct mm_struct *mm) 
	struct vm_area_struct *vma;



	lru_add_drain_all();

	/* We've already held the mmap_sem */

retry:

	if (unlikely(!down_read_trylock(&mm->mmap_sem))) {

		/*

		 * Someone who are holding the mmap_sem might be waiting in

		 * waitq. So we cancel all extra charges, wake up all waiters,

		 * and retry. Because we cancel precharges, we might not be able

		 * to move enough charges, but moving charge is a best-effort

		 * feature anyway, so it wouldn't be a big problem.

		 */

		__mem_cgroup_clear_mc();

		cond_resched();

		goto retry;

	}

	for (vma = mm->mmap; vma; vma = vma->vm_next) {

		int ret;

		struct mm_walk mem_cgroup_move_charge_walk = {
@@ -4917,6 +4924,7 @@ 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,
@@ -4925,11 +4933,17 @@ static void mem_cgroup_move_task(struct cgroup_subsys *ss, 
				struct task_struct *p,

				bool threadgroup)

{

	if (!mc.mm)

	struct mm_struct *mm;



	if (!mc.to)

		/* no need to move charge */

		return;



	mem_cgroup_move_charge(mc.mm);

	mm = get_task_mm(p);

	if (mm) {

		mem_cgroup_move_charge(mm);

		mmput(mm);

	}

	mem_cgroup_clear_mc();

}

#else	/* !CONFIG_MMU */

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