Re: [BUGFIX][PATCH] memcg: ensure list is empty at rmdir

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 13 Jan 2010 14:27:07 +0900
Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> wrote:
> For example:
> - Pages can be uncharged by its owner process while they are on LRU.
> - race between mem_cgroup_add_lru_list() and __mem_cgroup_uncharge_common().
> 
> So there can be a case in which the usage is zero but some of the LRUs are not empty.
> 
> OTOH, mem_cgroup_del_lru_list(), which can be called asynchronously with rmdir,
> accesses the mem_cgroup, so this access can cause a problem if it races with
> rmdir because the mem_cgroup might have been freed by rmdir.
> 
 
> The problem here is pages on LRU may contain pointer to stale memcg.
> To make res->usage to be 0, all pages on memcg must be uncharged or moved to
> another(parent) memcg. Moved page_cgroup have already removed from original LRU,
> but uncharged page_cgroup contains pointer to memcg withou PCG_USED bit. (This
> asynchronous LRU work is for improving performance.) If PCG_USED bit is not set,
> page_cgroup will never be added to memcg's LRU. So, about pages not on LRU, they
> never access stale pointer. Then, what we have to take care of is page_cgroup
> _on_ LRU list. This patch fixes this problem by making mem_cgroup_force_empty()
> visit all LRUs before exiting its loop and guarantee there are no pages on its LRU.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
> Cc: stable@kernel.org

Thank you. very nice.

Regards,
-Kame

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