linux-kernel - Re: [patch 1/4] memcg: use native word to represent dirtyable pages

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Message-ID: <20101108223838.GM23393@cmpxchg.org>
Date:	Mon, 8 Nov 2010 23:38:38 +0100
From:	Johannes Weiner <hannes@...xchg.org>
To:	Greg Thelen <gthelen@...gle.com>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Young <hidave.darkstar@...il.com>,
	Andrea Righi <arighi@...eler.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Wu Fengguang <fengguang.wu@...el.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] memcg: use native word to represent dirtyable pages

On Mon, Nov 08, 2010 at 02:25:15PM -0800, Greg Thelen wrote:
> Minchan Kim <minchan.kim@...il.com> writes:
> 
> > On Mon, Nov 8, 2010 at 7:14 AM, Johannes Weiner <hannes@...xchg.org> wrote:
> >> The memory cgroup dirty info calculation currently uses a signed
> >> 64-bit type to represent the amount of dirtyable memory in pages.
> >>
> >> This can instead be changed to an unsigned word, which will allow the
> >> formula to function correctly with up to 160G of LRU pages on a 32-bit
> Is is really 160G of LRU pages?  On 32-bit machine we use a 32 bit
> unsigned page number.  With a 4KiB page size, I think that maps 16TiB
> (1<<(32+12)) bytes.  Or is there some other limit?

Yes, the dirty limit we calculate from it :)

We have to be able to multiply this number by up to 100 (maximum dirty
ratio value) without overflowing.
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