linux-kernel - Re: [PATCH mm] fix swapoff breakage; however...

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Date:	Mon, 17 Sep 2007 20:51:05 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH mm] fix swapoff breakage; however...

On Tue, 18 Sep 2007, Balbir Singh wrote:
> Hugh Dickins wrote:
> > More fundamentally, it looks like any container brought over its limit in
> > unuse_pte will abort swapoff: that doesn't doesn't seem "contained" to me.
> > Maybe unuse_pte should just let containers go over their limits without
> > error?  Or swap should be counted along with RSS?  Needs reconsideration.
> 
> Thanks, for the catching this. There are three possible solutions
> 
> 1. Account each RSS page with a probable swap cache page, double
>    the RSS accounting to ensure that swapoff will not fail.
> 2. Account for the RSS page just once, do not account swap cache
>    pages

Neither of those makes sense to me, but I may be misunderstanding.

What would make sense is (what I meant when I said swap counted
along with RSS) not to count pages out and back in as they are
go out to swap and back in, just keep count of instantiated pages

I say "make sense" meaning that the numbers could be properly
accounted; but it may well be unpalatable to treat fast RAM as
equal to slow swap.

> 3. Follow your suggestion and let containers go over their limits
>    without error
> 
> With the current approach, a container over it's limit will not
> be able to call swapoff successfully, is that bad?

That's not so bad.  What's bad is that anyone else with the
CAP_SYS_ADMIN to swapoff is liable to be prevented by containers
going over their limits.

Hugh
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