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Date: Fri, 15 May 2009 13:30:26 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Balbir Singh <balbir@...ibm.com>,
Hugh Dickins <hugh@...itas.com>, linux-mm <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] memcg: fix stale swapcache at swapout
On Fri, 15 May 2009 13:09:59 +0900
Daisuke Nishimura <nishimura@....nes.nec.co.jp> 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@....nes.nec.co.jp>
>
> 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@....nes.nec.co.jp>
Thank you!
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
> 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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