linux-kernel - Re: [PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual memory layout

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Message-ID: <20140728181456.GO15536@arm.com>
Date:	Mon, 28 Jul 2014 19:14:56 +0100
From:	Will Deacon <will.deacon@....com>
To:	Konstantin Khlebnikov <k.khlebnikov@...sung.com>
Cc:	Russell King <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Vitaly Andrianov <vitalya@...com>,
	Cyril Chemparathy <cyril@...com>
Subject: Re: [PATCH 2/2] ARM: LPAE: reduce damage caused by idmap to virtual
 memory layout

On Tue, Jul 22, 2014 at 04:36:35PM +0100, Konstantin Khlebnikov wrote:
> idmap layout combines both phisical and virtual addresses.
> Everything works fine if ram physically lays below PAGE_OFFSET.
> Otherwise idmap starts punching huge holes in virtual memory layout.
> It maps ram by 2MiB sections, but when it allocates new pmd page it
> cuts 1GiB at once.
> 
> This patch makes a copy of all affected pmds from init_mm.
> Only few (usually one) 2MiB sections will be lost.
> This is not eliminates problem but makes it 512 times less likely.

I'm struggling to understand your commit message, but making a problem `512
times less likely' does sound like a bit of a hack to me. Can't we fix this
properly instead?

Will
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