Re: [PATCH] fix leak of swap accounting as stale swap cache under memcg

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

On Tue, 28 Apr 2009 00:47:31 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> Thanks for the detailed explanation of the possible race conditions. I
> am beginning to wonder why we don't have any hooks in add_to_swap.*.
> for charging a page. If the page is already charged and if it is a
> context issue (charging it to the right cgroup) that is already
> handled from what I see. Won't that help us solve the !PageCgroupUsed
> issue?
> 

For adding hook to add_to_swap_cache, we need to know which cgroup the swap cache
should be charged. Then, we have to remove CONFIG_CGROUP_MEM_RES_CTRL_SWAP_EXT
and enable memsw control always.

When using swap_cgroup, we'll know which cgroup the new swap cache should be charged.
Then, the new page readed in will be charged to recorded cgroup in swap_cgroup.
One bad thing of this method is a cgroup which swap_cgroup point to is different from
a cgroup which the task calls do_swap_fault(). This means that a page-fault by a
task can cause memory-reclaim under another cgroup and moreover, OOM.
I don't think it's sane behavior. So, current design of swap accounting waits until the
page is mapped.

Thanks,
-Kame

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