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

[Michael Holzheu]

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 runtime
memory consumption of the kexec tool.

The following use case illustrates the usefulness of this patch a bit more:

 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

 Without this patch for the kexec runtime we need:

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

Signed-off-by: Michael Holzheu <holzheu at linux.vnet.ibm.com>
---
 kexec/kexec.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kexec/kexec.c b/kexec/kexec.c
index 8ce6885..fecf061 100644
--- a/kexec/kexec.c
+++ b/kexec/kexec.c
@@ -26,6 +26,7 @@
 #include <stdlib.h>
 #include <errno.h>
 #include <limits.h>
+#include <sys/mman.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/reboot.h>
@@ -552,11 +553,12 @@ char *slurp_file(const char *filename, off_t *r_size)
 		if (err < 0)
 			die("Can not seek to the begin of file %s: %s\n",
 					filename, strerror(errno));
+		buf = slurp_fd(fd, filename, size, &nread);
 	} else {
-		size = stats.st_size;
+		size = nread = stats.st_size;
+		buf = mmap(NULL, size,
+			   PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
 	}
-
-	buf = slurp_fd(fd, filename, size, &nread);
 	if (!buf)
 		die("Cannot read %s", filename);
 
-- 
2.3.0