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

From: Balbir Singh
Date: Mon May 04 2009 - 22:21:37 EST


* 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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