linux-kernel - Re: [PATCH 7/9] swap_info: swap count continuations

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Date:	Fri, 16 Oct 2009 03:24:57 +0100 (BST)
From:	Hugh Dickins <hugh.dickins@...cali.co.uk>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>, hongshin@...il.com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 7/9] swap_info: swap count continuations

On Fri, 16 Oct 2009, KAMEZAWA Hiroyuki wrote:
> > 
> My concern is that small numbers of swap_map[] which has too much refcnt
> can consume too much pages.
> 
> If an entry is shared by 65535, 65535/128 = 512 page will be used.
> (I'm sorry if I don't undestand implementation correctly.)

Ah, you're thinking it's additive: perhaps because I use the name
"continuation", which may give that impression - maybe there's a
better name I can give it.

No, it's multiplicative - just like 999 is almost a thousand, not 27.

If an entry is shared by 65535, then it needs its original swap_map
page (0 to 0x3e) and a continuation page (0 to 0x7f) and another
continuation page (0 to 0x7f): if I've got my arithmetic right,
those three pages can hold a shared count up to 1032191, for
every one of that group of PAGE_SIZE neighbouring pages.

Hugh
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