linux-kernel - Re: [patch v3 2/8] kdump: Make kimage_load_crash

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Date:	Fri, 19 Aug 2011 15:27:52 +0200
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	ebiederm@...ssion.com, mahesh@...ux.vnet.ibm.com, hbabu@...ibm.com,
	oomichi@....nes.nec.co.jp, horms@...ge.net.au,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-s390@...r.kernel.org
Subject: Re: [patch v3 2/8] kdump: Make kimage_load_crash_segment() weak

Hello Vivek,

On Thu, 2011-08-18 at 13:15 -0400, Vivek Goyal wrote:
> On Fri, Aug 12, 2011 at 03:48:51PM +0200, Michael Holzheu wrote:
> > From: Michael Holzheu <holzheu@...ux.vnet.ibm.com>
> > 
> > On s390 we do not create page tables at all for the crashkernel memory.
> > This requires a s390 specific version for kimage_load_crash_segment().
> > Therefore this patch declares this function as "__weak". The s390 version is
> > very simple. It just copies the kexec segment to real memory without using
> > page tables:
> > 
> > int kimage_load_crash_segment(struct kimage *image,
> >                               struct kexec_segment *segment)
> > {
> >         return copy_from_user_real((void *) segment->mem, segment->buf,
> >                                    segment->bufsz);
> > }
> > 
> > There are two main advantages of not creating page tables for the
> > crashkernel memory:
> > 
> > a) It saves memory. We have scenarios in mind, where crashkernel
> >    memory can be very large and saving page table space is important.
> > b) We protect the crashkernel memory from being overwritten.
> 
> Michael,
> 
> Thinking more about it. Can't we provide a arch specific version of
> kmap() and kunmap() so that we create temporary mappings to copy
> the pages and then these are torn off.

Isn't kmap/kunmap() used for higmem? These functions are called from
many different functions in the Linux kernel, not only for kdump. I
would assume that creating and removing mappings with these functions is
not what a caller would expect and probably would break the Linux kernel
at many other places, no?

Perhaps we can finish this discussion after my vacation. I will change
my patch series that we even do not need this patch...

So only two common code patches are remaining. I will send the common
code patches again and will ask Andrew Morton to integrate them in the
next merge window. The s390 patches will be integrated by Martin.

Michael

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