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

[Simon Horman]

On Wed, Sep 02, 2015 at 10:48:39AM +0200, Michael Holzheu wrote:
> 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.

Thanks, I am convinced.

Could you repost the patch as a non-RFC?