Date:	Thu, 30 Apr 2009 16:35:39 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hugh@...itas.com" <hugh@...itas.com>
Subject: Re: [PATCH] memcg: fix stale swap cache leak v5

On Thu, 30 Apr 2009 16:16:27 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> This is v5 but all codes are rewritten.
> 
> After this patch, when memcg is used,
>  1. page's swapcount is checked after I/O (without locks). If the page is
>     stale swap cache, freeing routine will be scheduled.
>  2. vmscan.c calls try_to_free_swap() when __remove_mapping() fails.
> 
> Works well for me. no extra resources and no races.
> 
> Because my office will be closed until May/7, I'll not be able to make a
> response. Posting this for showing what I think of now.
> 
I found a hole immediately after posted this...sorry. plz ignore this patch/
see you again in the next month.

-Kame

> This should be fixed before posting softlimit etc...
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> In general, Linux's swp_entry handling is done by combination of lazy techniques
> and global LRU. It works well but when we use mem+swap controller, some more
> strict cotroll is appropriate. Otherwise, swp_entry used by a cgroup will be
> never freed until global LRU works. In a system where memcg is well-configured,
> global LRU doesn't work frequently.
> 
>   Example A) Assume a swap cache which is not mapped.
>               CPU0                            CPU1
> 	   zap_pte()....                  shrink_page_list()
> 	    free_swap_and_cache()           lock_page()
> 		page seems busy.
> 
>   Example B) Assume swapin-readahed.
> 	      CPU0			      CPU1
> 	   zap_pte()			  read_swap_cache_async()
> 					  swap_duplicate().
>            swap_entry_free() = 1
> 	   find_get_page()=> NULL.
> 					  add_to_swap_cache().
> 					  issue swap I/O. 
> 
> There are many patterns of this kind of race (but no problems).
> 
> free_swap_and_cache() is called for freeing swp_entry. But it is a best-effort
> function. If the swp_entry/page seems busy, swp_entry is not freed.
> This is not a problem because global-LRU will find SwapCache at page reclaim.
> But...
> 
> If memcg is used, on the other hand, global LRU may not work. Then, above
> unused SwapCache will not be freed.
> (unmapped SwapCache occupy swp_entry but never be freed if not on memcg's LRU)
> 
> So, even if there are no tasks in a cgroup, swp_entry usage still remains.
> In bad case, OOM by mem+swap controller is triggerred by this "leak" of
> swp_entry as Nishimura repoted.
> 
> This patch tries to fix racy case of free_swap_and_cache() and I/O by checking
> swap's refnct again after I/O. And add a hook to vmscan.c.
> After this patch applied, follwoing test works well.
> 
>   # echo 1-2M > ../memory.limit_in_bytes
>   # run tasks under memcg.
>   # kill all tasks and make memory.tasks empty
>   # check memory.memsw.usage_in_bytes == memory.usage_in_bytes and
>     there is no _used_ swp_entry.
> 
> Changelog: v4->v5
>  - completely new design.
>  - added nolock page_swapcount.
>  - checks all swap I/O.
> 
> Reported-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  include/linux/swap.h |    1 
>  mm/page_io.c         |  120 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/swapfile.c        |   37 +++++++++++++++
>  mm/vmscan.c          |   31 ++++++++++++-
>  4 files changed, 187 insertions(+), 2 deletions(-)
> 
> Index: mmotm-2.6.30-Apr24/mm/page_io.c
> ===================================================================
> --- mmotm-2.6.30-Apr24.orig/mm/page_io.c
> +++ mmotm-2.6.30-Apr24/mm/page_io.c
> @@ -19,6 +19,123 @@
>  #include <linux/writeback.h>
>  #include <asm/pgtable.h>
>  
> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR
> +/*
> + * When memory cgroup is used, race between read/write-swap and zap-swap can
> + * be a leak of swp_entry accounted. So we have to check the status at
> + * the end of swap-io. If memory cgroup is not used, Global LRU will find
> + * unused swap-cache finally. (But this is too lazy for memcg.)
> + */
> +
> +struct swapio_check {
> +	spinlock_t	lock;
> +	void		*swap_bio_list;
> +	struct delayed_work work;
> +} stale_swap_check;
> +
> +
> +static void mem_cgroup_check_stale_swap(struct work_struct *work)
> +{
> +	struct bio *bio;
> +	struct page *page;
> +	struct swapio_check *sc;
> +	int nr = SWAP_CLUSTER_MAX;
> +	swp_entry_t entry;
> +
> +	sc = &stale_swap_check;
> +
> +	while (nr--) {
> +		cond_resched();
> +		spin_lock_irq(&sc->lock);
> +		bio = sc->swap_bio_list;
> +		if (bio)
> +			sc->swap_bio_list = bio->bi_next;
> +		spin_unlock_irq(&sc->lock);
> +		if (!bio)
> +			break;
> +		entry.val = (unsigned long)bio->bi_private;
> +		bio_put(bio);
> +
> +		page = find_get_page(&swapper_space, entry.val);
> +		if (!page || page_mapped(page))
> +			continue;
> +		lock_page(page);
> +		/*
> +		 * When it's mapped, this page passed checks in do_swap_page()
> +		 * and we don't have to do any more. All other necessary checks
> +		 * will be done in try_to_free_swap().
> +		 */
> +		if (!page_mapped(page))
> +			try_to_free_swap(page);
> +		unlock_page(page);
> +		put_page(page);
> +	}
> +	if (sc->swap_bio_list)
> +		schedule_delayed_work(&sc->work, HZ/10);
> +}
> +
> +/*
> + * We can't call try_to_free_swap directly here because of caller's context.
> + */
> +static void mem_cgroup_swapio_check_again(struct bio *bio, struct page *page)
> +{
> +	unsigned long flags;
> +	struct swapio_check *sc;
> +	swp_entry_t entry;
> +	int ret;
> +
> +	/* check swap count here. If swp_entry is stable, nothing to do.*/
> +	if (likely(mem_cgroup_staleswap_hint(page)))
> +		return;
> +	/* reuse bio if this bio is ready to be freed. */
> +	ret = atomic_inc_return(&bio->bi_cnt);
> +	/* Any other reference other than us ? */
> +	if (unlikely(ret > 2)) {
> +		bio_put(bio);
> +		return;
> +	}
> +	/*
> +	 * We don't want to grab this page....record swp_entry instead of page.
> +	 */
> +	entry.val = page_private(page);
> +	bio->bi_private = (void *)entry.val;
> +
> +	sc = &stale_swap_check;
> +	spin_lock_irqsave(&sc->lock, flags);
> +	/* link bio */
> +	bio->bi_next = sc->swap_bio_list;
> +	sc->swap_bio_list = bio;
> +	spin_unlock_irqrestore(&sc->lock, flags);
> +	/*
> +	 * Swap I/O is tend to be countinous. Do check in batched manner.
> +	 */
> +	if (!delayed_work_pending(&sc->work))
> +		schedule_delayed_work(&sc->work, HZ/10);
> +}
> +
> +static int __init setup_stale_swap_check(void)
> +{
> +	struct swapio_check *sc;
> +
> +	sc = &stale_swap_check;
> +	spin_lock_init(&sc->lock);
> +	sc->swap_bio_list = NULL;
> +	INIT_DELAYED_WORK(&sc->work, mem_cgroup_check_stale_swap);
> +	return 0;
> +}
> +late_initcall(setup_stale_swap_check);
> +
> +
> +#else /* CONFIG_CGROUP_MEM_RES_CTRL */
> +
> +static inline
> +void mem_cgroup_swapio_check_again(struct bio *bio, struct page *page)
> +{
> +}
> +#endif
> +
> +
> +
>  static struct bio *get_swap_bio(gfp_t gfp_flags, pgoff_t index,
>  				struct page *page, bio_end_io_t end_io)
>  {
> @@ -66,6 +183,8 @@ static void end_swap_bio_write(struct bi
>  				(unsigned long long)bio->bi_sector);
>  		ClearPageReclaim(page);
>  	}
> +	/* While PG_writeback, this page is stable ...then, call this here */
> +	mem_cgroup_swapio_check_again(bio, page);
>  	end_page_writeback(page);
>  	bio_put(bio);
>  }
> @@ -85,6 +204,7 @@ void end_swap_bio_read(struct bio *bio, 
>  	} else {
>  		SetPageUptodate(page);
>  	}
> +	mem_cgroup_swapio_check_again(bio, page);
>  	unlock_page(page);
>  	bio_put(bio);
>  }
> Index: mmotm-2.6.30-Apr24/mm/vmscan.c
> ===================================================================
> --- mmotm-2.6.30-Apr24.orig/mm/vmscan.c
> +++ mmotm-2.6.30-Apr24/mm/vmscan.c
> @@ -586,6 +586,30 @@ void putback_lru_page(struct page *page)
>  }
>  #endif /* CONFIG_UNEVICTABLE_LRU */
>  
> +#ifdef CONFIG_CGROUP_MEM_RES_CTRL
> +/*
> + * Even if we don't call this, global LRU will finally find this SwapCache and
> + * free swap entry in the next loop. But, when memcg is used, we may have
> + * smaller chance to call global LRU's memory reclaim code.
> + * Freeing unused swap entry in aggresive way is good for avoid "leak" of swap
> + * entry accounting.
> + */
> +static inline void unuse_swapcache_check_again(struct page *page)
> +{
> +	/*
> +	 * The page is locked, but have extra reference from somewhere.
> +	 * In typical case, rotate_reclaimable_page()'s extra refcnt makes
> +	 * __remove_mapping fail. (see mm/swap.c)
> +	 */
> +	if (PageSwapCache(page))
> +		try_to_free_swap(page);
> +}
> +#else
> +static inline void unuse_swapcache_check_again(struct page *page)
> +{
> +}
> +#endif
> +
>  
>  /*
>   * shrink_page_list() returns the number of reclaimed pages
> @@ -758,9 +782,12 @@ static unsigned long shrink_page_list(st
>  			}
>  		}
>  
> -		if (!mapping || !__remove_mapping(mapping, page))
> +		if (!mapping)
>  			goto keep_locked;
> -
> +		if (!__remove_mapping(mapping, page)) {
> +			unuse_swapcache_check_again(page);
> +			goto keep_locked;
> +		}
>  		/*
>  		 * At this point, we have no other references and there is
>  		 * no way to pick any more up (removed from LRU, removed
> Index: mmotm-2.6.30-Apr24/mm/swapfile.c
> ===================================================================
> --- mmotm-2.6.30-Apr24.orig/mm/swapfile.c
> +++ mmotm-2.6.30-Apr24/mm/swapfile.c
> @@ -528,6 +528,43 @@ static inline int page_swapcount(struct 
>  	return count;
>  }
>  
> +#ifdef CONFIG_CGROUP_MEM_RES_CTLR
> +static inline int page_swapused_nolock(struct page *page)
> +{
> +	swp_entry_t entry;
> +	unsigned long type, offset;
> +	struct swap_info_struct *p;
> +
> +	entry.val = page_private(page);
> +	type = swp_type(entry);
> +	VM_BUG_ON(type >= nr_swapfiles);
> +
> +	offset = swp_offset(entry);
> +	p = &swap_info[type];
> +	VM_BUG_ON(!(p->flags & SWP_USED));
> +	VM_BUG_ON(!(p->swap_map[offset]));
> +
> +	smp_rmb();
> +	return p->swap_map[offset] != 1;
> +}
> +/*
> + * Use a lapping function not to allow reuse this function other than memcg.
> + */
> +int mem_cgroup_staleswap_hint(struct page *page)
> +{
> +	/*
> +	 * The page may not under lock_page() but Writeback is set in that case.
> +	 * Then, swap_map is stable when this is called.
> +	 * Very terrible troube will not occur even if page_swapused_nolock()
> +	 * returns wrong value.
> +	 * Because this can be called via interrupt context, we use nolock
> +	 * version of swap's refcnt check.
> +	 */
> +	if (!PageSwapCache(page) || page_mapped(page))
> +		return 1;
> +	return page_swapused_nolock(page);
> +}
> +#endif
>  /*
>   * We can write to an anon page without COW if there are no other references
>   * to it.  And as a side-effect, free up its swap: because the old content
> Index: mmotm-2.6.30-Apr24/include/linux/swap.h
> ===================================================================
> --- mmotm-2.6.30-Apr24.orig/include/linux/swap.h
> +++ mmotm-2.6.30-Apr24/include/linux/swap.h
> @@ -336,6 +336,7 @@ static inline void disable_swap_token(vo
>  
>  #ifdef CONFIG_CGROUP_MEM_RES_CTLR
>  extern void mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent);
> +extern int mem_cgroup_staleswap_hint(struct page *page);
>  #else
>  static inline void
>  mem_cgroup_uncharge_swapcache(struct page *page, swp_entry_t ent)
> 
> --
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