Date: Sat, 5 Jul 2008 15:11:46 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
To: balbir@linux.vnet.ibm.com
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, "hugh@veritas.com" <hugh@veritas.com>,
       "nishimura@mxp.nes.nec.co.jp" <nishimura@mxp.nes.nec.co.jp>,
       "yamamoto@valinux.co.jp" <yamamoto@valinux.co.jp>
Subject: Re: [PATCH] memcg: handle shmem's swap cache (Was 2.6.26-rc8-mm1
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On Sat, 05 Jul 2008 11:11:10 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> 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@jp.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 :( )

yes, to me. but we have to fix..

But I don't like old code's refcnt handling which does
   - increment
     - does this increment was really neccesary ?
       No? ok, decrement it again.

This was much more complex to me than current code.

And old ones will needs the check at treating swap-cache. (it couldn't but if we want)

> 
> The trade-off is complexity versus the overhead of reference counting.
> 
refcnt was also very complex ;)

Thanks,
-Kame

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

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