linux-kernel - Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1

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Message-ID: <486F0976.7010104@linux.vnet.ibm.com>
Date:	Sat, 05 Jul 2008 11:11:10 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	"hugh@...itas.com" <hugh@...itas.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	"yamamoto@...inux.co.jp" <yamamoto@...inux.co.jp>
Subject: Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1

KAMEZAWA Hiroyuki wrote:
> My swapcache accounting under memcg patch failed to catch tmpfs(shmem)'s one.
> Can I test this under -mm tree ?
> (If -mm is busy, I'm not in hurry.)
> This patch works well in my box.
> =
> SwapCache handling fix.
> 
> shmem's swapcache behavior is a little different from anonymous's one and
> memcg failed to handle it. This patch tries to fix it.
> 
> After this:
> 
> Any page marked as SwapCache is not uncharged. (delelte_from_swap_cache()
> delete the SwapCache flag.)
> 
> To check a shmem-page-cache is alive or not we use
>  page->mapping && !PageAnon(page) instead of
>  pc->flags & PAGE_CGROUP_FLAG_CACHE.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

Though I am not opposed to this, I do sit up and think if keeping the reference
count around could avoid this complexity and from my point, the maintenance
overhead of this logic/code (I fear there might be more special cases :( )

The trade-off is complexity versus the overhead of reference counting.

-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL
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