kexec: 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

Therefore use mmap() for non-character devices to reduce the memory consumption of the kexec tool.

Signed-off-by Michael Holzheu <[email protected]>

---
 kexec/kexec.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

kexec/kexec.c +2 -0

index 8ce6885..fecf061 100644
@@ -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 {

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 <[email protected]>

---
 kexec/kexec.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

kexec/kexec.c +5 -3

index 8ce6885..fecf061 100644
@@ -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);
 

Signature · 1 line

2.3.0

Quoted 6 lines

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

Quoted 2 lines

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.

On Wed, 2 Sep 2015 10:07:21 +0900
Simon Horman <[email protected]> wrote:
[snip]

Quoted 6 lines

Quoted 6 lines

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

Quoted 7 lines

Quoted 2 lines

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.

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 <[email protected]>

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 <[email protected]>

---
 kexec/kexec.c | 8 +++++---

if (err < 0)

Quoted 2 lines

	> 	> 	> die("Can not seek to the begin of file %s: %s\n",
	> 	> 	> 	> 	> filename, strerror(errno));

+> > > buf = slurp_fd(fd, filename, size, &nread);

Quoted 1 line

	> } else {

-> > > size = stats.st_size;
+> > > size = nread = stats.st_size;
+> > > buf = mmap(NULL, size,

Quoted 8 lines

if (err < 0)

Quoted 2 lines

	> 	> 	> die("Can not seek to the begin of file %s: %s\n",
	> 	> 	> 	> 	> filename, strerror(errno));

+> > > buf = slurp_fd(fd, filename, size, &nread);

Quoted 1 line

	> } else {

-> > > size = stats.st_size;
+> > > size = nread = stats.st_size;
+> > > buf = mmap(NULL, size,

With this change the caller can't tell if buf was malloc'ed or mmaped. The only safe thing it can do is to not call free() on the returned buf, this will lead to memory leakage for malloc'ed buffers.