Re: [PATCH 0/3] fix stale swap cache account leak in memcg v7

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-05-12 10:44:01]:

> I hope this version gets acks..
> ==
> As Nishimura reported, there is a race at handling swap cache.
> 
> Typical cases are following (from Nishimura's mail)
> 
> 
> == Type-1 ==
>   If some pages of processA has been swapped out, it calls free_swap_and_cache().
>   And if at the same time, processB is calling read_swap_cache_async() about
>   a swap entry *that is used by processA*, a race like below can happen.
> 
>             processA                   |           processB
>   -------------------------------------+-------------------------------------
>     (free_swap_and_cache())            |  (read_swap_cache_async())
>                                        |    swap_duplicate()
>                                        |    __set_page_locked()
>                                        |    add_to_swap_cache()
>       swap_entry_free() == 0           |
>       find_get_page() -> found         |
>       try_lock_page() -> fail & return |
>                                        |    lru_cache_add_anon()
>                                        |      doesn't link this page to memcg's
>                                        |      LRU, because of !PageCgroupUsed.
> 
>   This type of leak can be avoided by setting /proc/sys/vm/page-cluster to 0.
> 
> 
> == Type-2 ==
>     Assume processA is exiting and pte points to a page(!PageSwapCache).
>     And processB is trying reclaim the page.
> 
>               processA                   |           processB
>     -------------------------------------+-------------------------------------
>       (page_remove_rmap())               |  (shrink_page_list())
>          mem_cgroup_uncharge_page()      |
>             ->uncharged because it's not |
>               PageSwapCache yet.         |
>               So, both mem/memsw.usage   |
>               are decremented.           |
>                                          |    add_to_swap() -> added to swap cache.
> 
>     If this page goes thorough without being freed for some reason, this page
>     doesn't goes back to memcg's LRU because of !PageCgroupUsed.
> 
> 
> Considering Type-1, it's better to avoid swapin-readahead when memcg is used.
> swapin-readahead just read swp_entries which are near to requested entry. So,
> pages not to be used can be on memory (on global LRU). When memcg is used,
> this is not good behavior anyway.
> 
> Considering Type-2, the page should be freed from SwapCache right after WriteBack.
> Free swapped out pages as soon as possible is a good nature to memcg, anyway.
> 
> The patch set includes followng
>  [1/3] add mem_cgroup_is_activated() function. which tell us memcg is _really_ used.
>  [2/3] fix swap cache handling race by avoidng readahead.
>  [3/3] fix swap cache handling race by check swapcount again.
> 
> Result is good under my test.

What was the result (performance data impact) of disabling swap
readahead? Otherwise, this looks the most reasonable set of patches
for this problem.

-- 
	Balbir

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