Re: [PATCH V3 4/8] memcg: add per cgroup dirty pages accounting

[Michal Hocko]

On Wed 26-12-12 01:26:07, Sha Zhengju wrote:
> From: Sha Zhengju <handai.szj@xxxxxxxxxx>
> 
> This patch adds memcg routines to count dirty pages, which allows memory controller
> to maintain an accurate view of the amount of its dirty memory and can provide some
> info for users while cgroup's direct reclaim is working.

I guess you meant targeted resp. (hard/soft) limit reclaim here,
right? It is true that this is direct reclaim but it is not clear to me
why the usefulnes should be limitted to the reclaim for users. I would
understand this if the users was in fact in-kernel users.

[...]
> To prevent AB/BA deadlock mentioned by Greg Thelen in previous version
> (https://lkml.org/lkml/2012/7/30/227), we adjust the lock order:
> ->private_lock --> mapping->tree_lock --> memcg->move_lock.
> So we need to make mapping->tree_lock ahead of TestSetPageDirty in __set_page_dirty()
> and __set_page_dirty_nobuffers(). But in order to avoiding useless spinlock contention,
> a prepare PageDirty() checking is added.

But there is another AA deadlock here I believe.
page_remove_rmap
  mem_cgroup_begin_update_page_stat		<<< 1
  set_page_dirty
    __set_page_dirty_buffers
      __set_page_dirty
        mem_cgroup_begin_update_page_stat	<<< 2
	  move_lock_mem_cgroup
	    spin_lock_irqsave(&memcg->move_lock, *flags);

mem_cgroup_begin_update_page_stat is not recursive wrt. locking AFAICS
because we might race with the moving charges:
	CPU0						CPU1					
page_remove_rmap
						mem_cgroup_can_attach
  mem_cgroup_begin_update_page_stat (1)
    rcu_read_lock
						  mem_cgroup_start_move
						    atomic_inc(&memcg_moving)
						    atomic_inc(&memcg->moving_account)
						    synchronize_rcu
    __mem_cgroup_begin_update_page_stat
      mem_cgroup_stolen	<<< TRUE
      move_lock_mem_cgroup
  [...]
        mem_cgroup_begin_update_page_stat (2)
	  __mem_cgroup_begin_update_page_stat
	    mem_cgroup_stolen	  <<< still TRUE
	    move_lock_mem_cgroup  <<< DEADLOCK
  [...]
  mem_cgroup_end_update_page_stat
    rcu_unlock
    						  # wake up from synchronize_rcu
						[...]
						mem_cgroup_move_task
						  mem_cgroup_move_charge
						    walk_page_range
						      mem_cgroup_move_account
						        move_lock_mem_cgroup


Maybe I have missed some other locking which would prevent this from
happening but the locking relations are really complicated in this area
so if mem_cgroup_{begin,end}_update_page_stat might be called
recursively then we need a fat comment which justifies that.

[...]
-- 
Michal Hocko
SUSE Labs
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