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Subject[245/289] memcg: avoid deadlock between move charge and try_charge()
2.6.36-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>

commit b1dd693e5b9348bd68a80e679e03cf9c0973b01b upstream.

__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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
mm/memcontrol.c | 43 ++++++++++++++++++++++++++-----------------
1 file changed, 26 insertions(+), 17 deletions(-)

--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -269,13 +269,14 @@ 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),
@@ -4445,7 +4446,7 @@ static unsigned long mem_cgroup_count_pr
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,
@@ -4457,7 +4458,6 @@ static unsigned long mem_cgroup_count_pr
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;
@@ -4508,11 +4508,16 @@ 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;
memcg_oom_recover(from);
memcg_oom_recover(to);
wake_up_all(&mc.waitq);
@@ -4537,26 +4542,37 @@ static int mem_cgroup_can_attach(struct
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);
+
spin_lock(&mc.lock);
mc.from = from;
mc.to = mem;
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;
}
@@ -4644,7 +4660,7 @@ static void mem_cgroup_move_charge(struc
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 = {
@@ -4663,7 +4679,6 @@ static void mem_cgroup_move_charge(struc
*/
break;
}
- up_read(&mm->mmap_sem);
}

static void mem_cgroup_move_task(struct cgroup_subsys *ss,
@@ -4672,17 +4687,11 @@ static void mem_cgroup_move_task(struct
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 */



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