Re: [PATCH][mmotm] memcg fix LRU accounting for SwapCache.

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Mon, 22 Dec 2008 15:55:18 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> This works well in my environment. Nishimura-san, could you test this ?
> 
> -Kame
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Now, a page can be deleted from SwapCache while do_swap_page().
> memcg-fix-swap-accounting-leak-v3.patch handles that, but, LRU handling
> is still broken.
> (above behavior broke assumption of memcg-synchronized-lru patch.)
> 
> This patch is a fix for LRU handling (especially for per-zone counters).
> At charging SwapCache,
>  - Remove page_cgroup from LRU if it's not used.
>  - Add page cgroup to LRU if it's not linked to.
> 
> Reported-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/memcontrol.c |   59 +++++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 50 insertions(+), 9 deletions(-)
> 
> Index: mmotm-2.6.28-Dec19/mm/memcontrol.c
> ===================================================================
> --- mmotm-2.6.28-Dec19.orig/mm/memcontrol.c
> +++ mmotm-2.6.28-Dec19/mm/memcontrol.c
> @@ -331,8 +331,12 @@ void mem_cgroup_del_lru_list(struct page
>  		return;
>  	pc = lookup_page_cgroup(page);
>  	/* can happen while we handle swapcache. */
> -	if (list_empty(&pc->lru))
> +	if (list_empty(&pc->lru) || !pc->mem_cgroup)
>  		return;
> +	/*
> +	 * We don't check PCG_USED bit. It's cleared when the "page" is finally
> +	 * removed from global LRU.
> +	 */
>  	mz = page_cgroup_zoneinfo(pc);
>  	mem = pc->mem_cgroup;
>  	MEM_CGROUP_ZSTAT(mz, lru) -= 1;
> @@ -379,16 +383,44 @@ void mem_cgroup_add_lru_list(struct page
>  	MEM_CGROUP_ZSTAT(mz, lru) += 1;
>  	list_add(&pc->lru, &mz->lists[lru]);
>  }
> +
>  /*
> - * To add swapcache into LRU. Be careful to all this function.
> - * zone->lru_lock shouldn't be held and irq must not be disabled.
> + * At handling SwapCache, pc->mem_cgroup may be changed while it's linked to
> + * lru because the page may.be reused after it's fully uncharged (because of
> + * SwapCache behavior).To handle that, unlink page_cgroup from LRU at chargin
> + * it again.This function is only used for charging SwapCache. It's done under
> + * lock_page and expected that zone->lru_lock is never held.
>   */
> -static void mem_cgroup_lru_fixup(struct page *page)
> +static bool mem_cgroup_lru_del_before_commit_swapcache(struct page *page)
          ^^^
should be void...I'll post v2, sorry.

-Kame

> +{
> +	unsigned long flags;
> +	struct zone *zone = page_zone(page);
> +	struct page_cgroup *pc = lookup_page_cgroup(page);
> +
> +	spin_lock_irqsave(&zone->lru_lock, flags);
> +	/*
> +	 * Forget old LRU when this page_cgroup is *not* used. This Used bit
> +	 * is guarded by lock_page() because the page is SwapCache.
> +	 */
> +	if (!PageCgroupUsed(pc))
> +		mem_cgroup_del_lru_list(page, page_lru(page));
> +	spin_unlock_irqrestore(&zone->lru_lock, flags);
> +}
> +
> +static void mem_cgroup_lru_add_after_commit_swapcache(struct page *page)
>  {
> -	if (!isolate_lru_page(page))
> -		putback_lru_page(page);
> +	unsigned long flags;
> +	struct zone *zone = page_zone(page);
> +	struct page_cgroup *pc = lookup_page_cgroup(page);
> +
> +	spin_lock_irqsave(&zone->lru_lock, flags);
> +	/* link when the page is linked to LRU but page_cgroup isn't */
> +	if (PageLRU(page) && list_empty(&pc->lru))
> +		mem_cgroup_add_lru_list(page, page_lru(page));
> +	spin_unlock_irqrestore(&zone->lru_lock, flags);
>  }
>  
> +
>  void mem_cgroup_move_lists(struct page *page,
>  			   enum lru_list from, enum lru_list to)
>  {
> @@ -1161,8 +1193,11 @@ int mem_cgroup_cache_charge_swapin(struc
>  					mem = NULL; /* charge to current */
>  			}
>  		}
> +		/* SwapCache may be still linked to LRU now. */
> +		mem_cgroup_lru_del_before_commit_swapcache(page);
>  		ret = mem_cgroup_charge_common(page, mm, mask,
>  				MEM_CGROUP_CHARGE_TYPE_SHMEM, mem);
> +		mem_cgroup_lru_add_after_commit_swapcache(page);
>  		/* drop extra refcnt from tryget */
>  		if (mem)
>  			css_put(&mem->css);
> @@ -1178,8 +1213,6 @@ int mem_cgroup_cache_charge_swapin(struc
>  	}
>  	if (!locked)
>  		unlock_page(page);
> -	/* add this page(page_cgroup) to the LRU we want. */
> -	mem_cgroup_lru_fixup(page);
>  
>  	return ret;
>  }
> @@ -1194,7 +1227,9 @@ void mem_cgroup_commit_charge_swapin(str
>  	if (!ptr)
>  		return;
>  	pc = lookup_page_cgroup(page);
> +	mem_cgroup_lru_del_before_commit_swapcache(page);
>  	__mem_cgroup_commit_charge(ptr, pc, MEM_CGROUP_CHARGE_TYPE_MAPPED);
> +	mem_cgroup_lru_add_after_commit_swapcache(page);
>  	/*
>  	 * Now swap is on-memory. This means this page may be
>  	 * counted both as mem and swap....double count.
> @@ -1213,7 +1248,7 @@ void mem_cgroup_commit_charge_swapin(str
>  
>  	}
>  	/* add this page(page_cgroup) to the LRU we want. */
> -	mem_cgroup_lru_fixup(page);
> +
>  }
>  
>  void mem_cgroup_cancel_charge_swapin(struct mem_cgroup *mem)
> @@ -1281,6 +1316,12 @@ __mem_cgroup_uncharge_common(struct page
>  
>  	mem_cgroup_charge_statistics(mem, pc, false);
>  	ClearPageCgroupUsed(pc);
> +	/*
> +	 * pc->mem_cgroup is not cleared here. It will be accessed when it's
> +	 * freed from LRU. This is safe because uncharged page is expected not
> +	 * to be reused (freed soon). Exception is SwapCache, it's handled by
> +	 * special functions.
> +	 */
>  
>  	mz = page_cgroup_zoneinfo(pc);
>  	unlock_page_cgroup(pc);
> 
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