Re: [PATCH] memcg: fix stale swapcache at swapout

[KAMEZAWA Hiroyuki]

On Fri, 15 May 2009 13:09:59 +0900
Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx> wrote:

> This patch is split out from Kamezawa-san's fix stale swap cache account leak
> patch set(v7).
> 
>   http://lkml.org/lkml/2009/5/11/688
> 
> I moved the definition of memcg_free_unused_swapcache() back to memcontrol.c
> (as I did in my original version) to avoid confilict with
> Kosaki-san's remove CONFIG_UNEVICTABLE_LRU config option patch.
> (it's not on mmotm yet though).
> And IMHO, this function might not be appropriate for "vmscan".
> 
> This patch is based on mmotm-2009-05-13-16-34.
> ===
> From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
> 
> Reclaiming anonymous memory in vmscan.c does following 2 steps.
>   1. add to swap and unmap.
>   2. pageout
> But above 2 steps doesn't occur at once. There are many chances
> to avoid pageout, and _really_ unused pages are swapped out by
> visit-and-check-again logic of LRU rotation.
> 
> But this behavior has troubles with memcg because memcg doesn't put
> !PageCgroupUsed swapcache back to its LRU.
> These swapcache cannot be freed in memcg's LRU scanning, so swp_entry cannot
> be freed properly as a result.
> 
> These swapcaches can be created by a race like below:
> 
>     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 swapcache.
> 
>     If this page goes thorough without being freed for some reason, this page
>     doesn't goes back to memcg's LRU because of !PageCgroupUsed.
> 
> This patch adds a hook after add_to_swap() to check the page is mapped by a
> process or not, and frees it if it has been unmapped already.
> 
> If a page has been on swap cache already when the owner process calls
> page_remove_rmap() -> mem_cgroup_uncharge_page(), the page is not uncharged.
> It goes back to memcg's LRU even if it goes through shrink_page_list()
> without being freed, so this patch ignores these case.
> 
> Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

Thank you!
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>


> ---
>  include/linux/swap.h |   12 ++++++++++++
>  mm/memcontrol.c      |   14 +++++++++++++-
>  mm/vmscan.c          |    8 ++++++++
>  3 files changed, 33 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 6ea541d..00d8066 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -336,11 +336,17 @@ static inline void disable_swap_token(void)
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>  extern void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent);
> +extern int memcg_free_unused_swapcache(struct page *page);
>  #else
>  static inline void
>  mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
>  {
>  }
> +static inline int
> +memcg_free_unused_swapcache(struct page *page)
> +{
> +	return 0;
> +}
>  #endif
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
>  extern void mem_cgroup_uncharge_swap(swp_entry_t ent);
> @@ -436,6 +442,12 @@ mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
>  {
>  }
>  
> +static inline int
> +memcg_free_unused_swapcache(struct page *page)
> +{
> +	return 0;
> +}
> +
>  #endif /* CONFIG_SWAP */
>  #endif /* __KERNEL__*/
>  #endif /* _LINUX_SWAP_H */
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0c90345..63844cd 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1578,7 +1578,19 @@ void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
>  	if (memcg)
>  		css_put(&memcg->css);
>  }
> -#endif
> +
> +int memcg_free_unused_swapcache(struct page *page)
> +{
> +	VM_BUG_ON(!PageLocked(page));
> +	VM_BUG_ON(!PageSwapCache(page));
> +
> +	if (mem_cgroup_disabled())
> +		return 0;
> +	if (!PageAnon(page) || page_mapped(page))
> +		return 0;
> +	return try_to_free_swap(page);	/* checks page_swapcount */
> +}
> +#endif /* CONFIG_SWAP */
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
>  /*
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 2f9d555..1fd0e43 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -665,6 +665,14 @@ static unsigned long shrink_page_list(struct list_head *page_list,
>  				goto keep_locked;
>  			if (!add_to_swap(page))
>  				goto activate_locked;
> +			/*
> +			 * The owner process might have uncharged the page
> +			 * (by page_remove_rmap()) before it has been added
> +			 * to swap cache.
> +			 * Check it here to avoid making it stale.
> +			 */
> +			if (memcg_free_unused_swapcache(page))
> +				goto keep_locked;
>  			may_enter_fs = 1;
>  		}
>  
> 

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