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

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Date:	Sat, 05 Jul 2008 12:19:11 +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:
> On Sat, 05 Jul 2008 11:11:10 +0530
> Balbir Singh <balbir@...ux.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@...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.
> 

That can be redone -- the moment a page is used by a path, refcnt (increment)
it. Undo the same when the page is no longer in use.

I expect

rmap path to increment/decrement it on mapping
radix-tree (cache's) to do the same


Using a kref we should be able to get this logic right - no?

> 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 ;)

I think that is easier to simply, instead of adding the complex checks we have
right now. refcnt is easier to prove as working correct than the checks.

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