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SubjectRe: [PATCH 2/3] fix swap cache account leak at swapin-readahead
On Tue, 12 May 2009 10:46:03 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> In general, Linux's swp_entry handling is done by combination of lazy techniques
> and global LRU. It works well but when we use mem+swap controller, some more
> strict control is appropriate. Otherwise, swp_entry used by a cgroup will be
> never freed until global LRU works. In a system where memcg is well-configured,
> global LRU doesn't work frequently.
>
> Example) Assume swapin-readahead.
> CPU0 CPU1
> zap_pte() read_swap_cache_async()
> swap_duplicate().
> swap_entry_free() = 1
> find_get_page()=> NULL.
> add_to_swap_cache().
> issue swap I/O.
>
> There are many patterns of this kind of race (but no problems).
>
> free_swap_and_cache() is called for freeing swp_entry. But it is a best-effort
> function. If the swp_entry/page seems busy, swp_entry is not freed.
> This is not a problem because global-LRU will find SwapCache at page reclaim.
>
> If memcg is used, on the other hand, global LRU may not work. Then, above
> unused SwapCache will not be freed.
> (unmapped SwapCache occupy swp_entry but never be freed if not on memcg's LRU)
>
> So, even if there are no tasks in a cgroup, swp_entry usage still remains.
> In bad case, OOM by mem+swap controller is triggered by this "leak" of
> swp_entry as Nishimura reported.
>
> Considering this issue, swapin-readahead itself is not very good for memcg.
> It read swap cache which will not be used. (and _unused_ swapcache will
> not be accounted.) Even if we account swap cache at add_to_swap_cache(),
> we need to account page to several _unrelated_ memcg. This is bad.
>
> This patch tries to fix racy case of free_swap_and_cache() and page status.
>
> After this patch applied, following test works well.
>
> # echo 1-2M > ../memory.limit_in_bytes
> # run tasks under memcg.
> # kill all tasks and make memory.tasks empty
> # check memory.memsw.usage_in_bytes == memory.usage_in_bytes and
> there is no _used_ swp_entry.
>
> What this patch does is
> - avoid swapin-readahead when memcg is activated.
>
> Changelog: v6 -> v7
> - just handle races in readahead.
> - races in writeback is handled in the next patch.
>
> Changelog: v5 -> v6
> - works only when memcg is activated.
> - check after I/O works only after writeback.
> - avoid swapin-readahead when memcg is activated.
> - fixed page refcnt issue.
> Changelog: v4->v5
> - completely new design.
>
> Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Looks good to me.

Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>



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