[RFC PATCH] kexec: use mmap instead of read for slurp_file()

[Michael Holzheu]

On Wed, 2 Sep 2015 10:07:21 +0900
Simon Horman <horms at verge.net.au> wrote:
[snip]
> > 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
> > 
> > Therefore use mmap() for non-character devices to reduce the memory
> > consumption of the kexec tool.
> 
> I'm not opposed to this change but I also don't see a strong motivation for
> it.  I would imagine that the memory saving is not that large. And that the
> memory consumption disappears when the presumably short-lived kexec process
> exits.

Correct it will disappear.

The reason for the the patch is that we have the following scanario:

1) Boot a 4 GB Linux system
2) Read kernel and 1,5 GB ramdisk from external source into local tmpfs (ram)
3) kexec the kernel and ramdisk

So without the mmap patch for the kexec runtime we need:

1,5 GB (tmpfs) + 1,5 GB (kexec malloc) + 1,5 GB (kernel memory) = 4,5 GB

If we use mmap we only need 3 GB memory.

Regards,
Michael