To: Hirokazu Takahashi <taka@valinux.co.jp>
Cc: vgoyal@in.ibm.com, akpm@osdl.org, fastboot@lists.osdl.org,
       linux-kernel@vger.kernel.org, maneesh@in.ibm.com, hari@in.ibm.com,
       suparna@in.ibm.com
Subject: Re: [Fastboot] [PATCH] Reserving backup region for kexec based crashdumps.
References: <1106833527.15652.146.camel@2fwv946.in.ibm.com>
	<20050203.160252.104031714.taka@valinux.co.jp>
	<1107421303.11609.183.camel@2fwv946.in.ibm.com>
	<20050203.183747.85413533.taka@valinux.co.jp>
From: ebiederm@xmission.com (Eric W. Biederman)
Date: 03 Feb 2005 03:07:58 -0700
In-Reply-To: <20050203.183747.85413533.taka@valinux.co.jp>
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Hirokazu Takahashi <taka@valinux.co.jp> writes:

> Hi Vivek, 
> 
> > > Hi Vivek and Eric,
> > > 
> > > IMHO, why don't we swap not only the contents of the top 640K
> > > but also kernel working memory for kdump kernel?
> > 
> > 
> > Initial patches of kdump had adopted the same approach but given the
> > fact devices are not stopped during transition to new kernel after a
> > panic, it carried inherent risk of some DMA going on and corrupting the
> > new kernel/data structures. Hence the idea of running the kernel from a
> > reserved location came up. This should be DMA safe as long as DMA is not
> > misdirected.
> 
> I see, that makes sense.
> But I'm not sure yet that it's safe to access the top of 640MB.
640K?

> I wonder how kmalloc(GFP_DMA) works in a kdump kernel.

All that happens there is a one line change to vmlinux.lds.S  that
causes the kernel to live at a different physical and virtual
address.  So everything works as normal.

I do agree that it is risky to use the first 640K for normal work.
But on the list of things to fix it is a minor war, and even if we
back up that region of memory we don't need to use it.

There are still remain a lot of code reviews to ensure the code is
generally safe.

Eric
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