linux-kernel - Re: [PATCH] remove memory.limit v.s. memsw.limit comparison.

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Message-Id: <20090604173918.3b2c68f5.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 4 Jun 2009 17:39:18 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>
Subject: Re: [PATCH] remove memory.limit v.s. memsw.limit comparison.

On Thu, 4 Jun 2009 16:31:10 +0800
Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> [2009-06-04 14:10:43]:
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > 
> > Removes memory.limit < memsw.limit at setting limit check completely.
> > 
> > The limitation "memory.limit <= memsw.limit" was added just because
> > it seems sane ...if memory.limit > memsw.limit, only memsw.limit works.
> > 
> > But To implement this limitation, we needed to use private mutex and make
> > the code a bit complated.
> > As Nishimura pointed out, in real world, there are people who only want
> > to use memsw.limit.
> > 
> > Then, this patch removes the check. user-land library or middleware can check
> > this in userland easily if this really concerns.
> > 
> > And this is a good change to charge-and-reclaim.
> > 
> > Now, memory.limit is always checked before memsw.limit
> > and it may do swap-out. But, if memory.limit == memsw.limit, swap-out is
> > finally no help and hits memsw.limit again. So, let's allow the condition
> > memory.limit > memsw.limit. Then we can skip unnecesary swap-out.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> We can't change behaviour this way without breaking userspace scripts,
> API and code. What does it mean for people already using these
> features? Does it break their workflow?
> 

Hopefully no breaks to current users's workflow.
Because this just has influences to "error path" like below

 echo 200M > memory.memsw.limit
 echo 300M > memory.limit
 => ERROR

If the user program made in sane way, above case will never happens because
they set memsw.limit to be greater than memory.limit and above is treated as error.


Thanks,
-Kame


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