Re: [BUGFIX][PATCH] fix race in file_mapped accounting in memcg

[Balbir Singh]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-03-24 15:43:24]:

> A fix for race in file_mapped statistics. I noticed this race while discussing
> Andrea's dirty accounting patch series. 
> At the end of discusstion, I said "please don't touch file mapped". So, this bugfix
> should be posted as an independent patch.
> Tested on the latest mmotm.
> 
> Thanks,
> -Kame
> 
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> Now, memcg's FILE_MAPPED accounting has following race with
> move_account (happens at rmdir()).
> 
>     increment page->mapcount (rmap.c)
>     mem_cgroup_update_file_mapped()           move_account()
> 					      lock_page_cgroup()
> 					      check page_mapped() if
> 					      page_mapped(page)>1 {
> 						FILE_MAPPED -1 from old memcg
> 						FILE_MAPPED +1 to old memcg
> 					      }
> 					      .....
> 					      overwrite pc->mem_cgroup
> 					      unlock_page_cgroup()
>     lock_page_cgroup()
>     FILE_MAPPED + 1 to pc->mem_cgroup
>     unlock_page_cgroup()
> 
> Then,
> 	old memcg (-1 file mapped)
> 	new memcg (+2 file mapped)
>

Good catch!


Reviewed-by: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx>
 
 
-- 
	Three Cheers,
	Balbir
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