kexec/s390x: use mmap instead of read for slurp_file()

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@linux.vnet.ibm.com>
Reviewed-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>

1 files changed, 1 insertions, 0 deletions

diff --git a/kexec/kexec.h b/kexec/kexec.h
index 7c97b25..c02ac8f 100644
--- a/kexec/kexec.h
+++ b/kexec/kexec.h
@@ -253,6 +253,7 @@ extern void die(const char *fmt, ...)
 extern void *xmalloc(size_t size);
 extern void *xrealloc(void *ptr, size_t size);
 extern char *slurp_file(const char *filename, off_t *r_size);
+extern char *slurp_file_mmap(const char *filename, off_t *r_size);
 extern char *slurp_file_len(const char *filename, off_t size, off_t *nread);
 extern char *slurp_decompress_file(const char *filename, off_t *r_size);
 extern unsigned long virt_to_phys(unsigned long addr);