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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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