Messages in this thread | | | Date | Tue, 12 May 2009 13:32:38 +0900 | From | Daisuke Nishimura <> | Subject | Re: [PATCH 2/3] fix swap cache account leak at swapin-readahead |
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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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