Re: [PATCH] memcg: fix stale swap cache leak v5

[Balbir Singh]

* Daisuke Nishimura <d-nishimura@xxxxxxxxxxxxxxxxx> [2009-05-01 13:33:17]:

>               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.

For some reason could use some clarification.

> 
> These swap cache cannot be freed in memcg's LRU scanning, and swp_entry cannot
> be freed properly as a result.
> 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>
> ---
>  include/linux/swap.h |   12 ++++++++++++
>  mm/memcontrol.c      |   14 ++++++++++++++
>  mm/vmscan.c          |    8 ++++++++
>  3 files changed, 34 insertions(+), 0 deletions(-)
> 
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index caf0767..8e75d7a 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);
> @@ -431,6 +437,12 @@ static inline swp_entry_t get_swap_page(void)
>  #define has_swap_token(x) 0
>  #define disable_swap_token() do { } while(0)
> 
> +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 01c2d8f..4f7e5b6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1488,6 +1488,7 @@ void mem_cgroup_uncharge_cache_page(struct page *page)
>  	__mem_cgroup_uncharge_common(page, MEM_CGROUP_CHARGE_TYPE_CACHE);
>  }
> 
> +#ifdef CONFIG_SWAP
>  /*
>   * called from __delete_from_swap_cache() and drop "page" account.
>   * memcg information is recorded to swap_cgroup of "ent"
> @@ -1507,6 +1508,19 @@ void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
>  		css_put(&memcg->css);
>  }
> 
> +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;

Do we need these checks? Isn't PageSwapCache() check and
page_swapcount() check enough in try_to_free_swap()?

> +	return try_to_free_swap(page);	/* checks page_swapcount */

try_to_free_swap() marks the page as dirty, do you know why?

> +}
> +#endif /* CONFIG_SWAP */
> +
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR_SWAP
>  /*
>   * called from swap_entry_free(). remove record in swap_cgroup and
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index eac9577..c1a7a6f 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -656,6 +656,14 @@ 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;

Seems reasonable, but I think it is better to check for
scan_global_lru().. no?

>  			may_enter_fs = 1;
>  		}
> 
> 

-- 
	Balbir
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