Re: [RFC][PATCH] fix swap entries is not reclaimed in proper way for memg v3.

From: Balbir Singh <balbir@linux.vnet.ibm.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"hugh@veritas.com" <hugh@veritas.com>
Subject: Re: [RFC][PATCH] fix swap entries is not reclaimed in proper way for memg v3.
Date: Mon, 27 Apr 2009 13:42:06 +0530	[thread overview]
Message-ID: <20090427081206.GI4454@balbir.in.ibm.com> (raw)
In-Reply-To: <20090424162840.2ad06d8a.kamezawa.hiroyu@jp.fujitsu.com>

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-04-24 16:28:40]:

> This is new one. (using new logic.) Maybe enough light-weight and caches all cases.

You sure mean catches above :)

> 
> Thanks,
> -Kame
> ==
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> 
> Because free_swap_and_cache() function is called under spinlocks,
> it can't sleep and use trylock_page() instead of lock_page().
> By this, swp_entry which is not used after zap_xx can exists as
> SwapCache, which will be never used.
> This kind of SwapCache is reclaimed by global LRU when it's found
> at LRU rotation.
> 
> When memory cgroup is used,  the global LRU will not be kicked and
> stale Swap Caches will not be reclaimed. This is problematic because
> memcg's swap entry accounting is leaked and memcg can't know it.
> To catch this stale SwapCache, we have to chase it and check the
> swap is alive or not again.
> 
> This patch adds a function to chase stale swap cache and reclaim it
> in modelate way. When zap_xxx fails to remove swap ent, it will be
> recoreded into buffer and memcg's "work" will reclaim it later.
> No sleep, no memory allocation under free_swap_and_cache().
> 
> This patch also adds stale-swap-cache-congestion logic and try to avoid having
> too much stale swap caches at the same time.
> 
> Implementation is naive but maybe the cost meets trade-off.
> 
> How to test:
>   1. set limit of memory to very small (1-2M?). 
>   2. run some amount of program and run page reclaim/swap-in.
>   3. kill programs by SIGKILL etc....then, Stale Swap Cache will
>      be increased. After this patch, stale swap caches are reclaimed
>      and mem+swap controller will not go to OOM.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

Quick comment on the design

1. I like the marking of swap cache entries as stale
2. Can't we reclaim stale entries during memcg LRU reclaim? Why write
a GC for it?

-- 
	Balbir