linux-kernel - Re: [PATCH] arm64: Enable CONFIG

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Message-ID: <20141205123116.GD28339@e104818-lin.cambridge.arm.com>
Date:	Fri, 5 Dec 2014 12:31:17 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Olof Johansson <olof@...om.net>,
	"agraf@...e.de" <agraf@...e.de>, Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: Enable CONFIG_COMPAT also for 64k page size

On Fri, Dec 05, 2014 at 12:24:49PM +0000, Arnd Bergmann wrote:
> On Friday 05 December 2014 11:05:07 Catalin Marinas wrote:
> > 
> > > Should we add support for 64k-pages in the arm32 kernel as well now?
> > 
> > 32-bit LPAE doesn't support 64K pages but IIRC the classic MMU does
> > (though I have to check whether it was optional). But it's not feasible
> > to enable this in a 32-bit environment because of the memory wasted with
> > large pages (on average 32KB per mapped file, possibly even more if you
> > have lots of small files).
> 
> That sounds like a tradeoff that some users might be willing to make.
> 
> I remember that with ps3-linux, a lot of people wanted to enable 64k-pages
> because the Cell processor has such a small number of TLB entries (256 or
> 1024), but Linux only has around 200MB of memory available. I believe
> there are arm32 machines with more RAM and fewer TLBs.

I've heard that the WD MyCloud NAS (Cortex-A9 based) moved to 64K pages
and they rebuilt the filesystem (which annoyed some people that were
previously installing Debian packages directly, they now need
recompilation).

-- 
Catalin
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