linux-kernel - Re: [patch v2 02/10] kdump: Make kimage_load_crash

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Message-ID: <20110801203143.GH3805@redhat.com>
Date:	Mon, 1 Aug 2011 16:31:43 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Michael Holzheu <holzheu@...ux.vnet.ibm.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 v2 02/10] kdump: Make kimage_load_crash_segment() weak

On Wed, Jul 27, 2011 at 02:55:06PM +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.
> 
> Signed-off-by: Michael Holzheu <holzheu@...ux.vnet.ibm.com>
> ---
>  kernel/kexec.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> --- a/kernel/kexec.c
> +++ b/kernel/kexec.c
> @@ -842,8 +842,8 @@ out:
>  	return result;
>  }
>  
> -static int kimage_load_crash_segment(struct kimage *image,
> -					struct kexec_segment *segment)
> +int __weak kimage_load_crash_segment(struct kimage *image,
> +				     struct kexec_segment *segment)

A comment here why we are making it weak should help in increasing
the code readability.

Thanks
Vivek

>  {
>  	/* For crash dumps kernels we simply copy the data from
>  	 * user space to it's destination.
--
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