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

[Michal Hocko]

On Wed 02-01-13 11:44:21, Michal Hocko wrote:
> 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);

JFYI since abf09bed (s390/mm: implement software dirty bits) this is no
longer possible. I haven't checked wheter there are other cases like
this one and it should be better if mem_cgroup_begin_update_page_stat
was recursive safe if that can be done without too many hacks.
I will have a look at this (hopefully) sometimes next week.

[...]
-- 
Michal Hocko
SUSE Labs
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