[RFC 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 memory
consumption of the kexec tool.

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