[PATCH v3] kexec/s390x: use mmap instead of read for slurp_file()

[Simon Horman]

On Fri, Oct 30, 2015 at 04:02:04PM +0100, Michael Holzheu wrote:
> The slurp_fd() function allocates memory and uses the read() system call.
> This results in double memory consumption for image and initrd:
> 
>  1) Memory allocated in user space by the kexec tool
>  2) Memory allocated in kernel by the kexec() system call
> 
> The following illustrates the use case that we have on s390x:
> 
>  1) Boot a 4 GB Linux system
>  2) Copy kernel and 1,5 GB ramdisk from external source into tmpfs (ram)
>  3) Use kexec to boot kernel with ramdisk
> 
>  Therefore for kexec runtime we need:
> 
>  1,5 GB (tmpfs) + 1,5 GB (kexec malloc) + 1,5 GB (kernel memory) = 4,5 GB
> 
> This patch introduces slurp_file_mmap() which for "normal" files uses
> mmap() instead of malloc()/read(). This reduces the runtime memory
> consumption of the kexec tool as follows:
> 
>  1,5 GB (tmpfs) + 1,5 GB (kernel memory) = 3 GB
> 
> Signed-off-by: Michael Holzheu <holzheu at linux.vnet.ibm.com>
> Reviewed-by: Dave Young <dyoung at redhat.com>

Thanks, applied.