Re: [RFC] rootmpfs

[Byron Stanoszek <bstanoszek@comtime.com>]

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Rob,

FWIW I have a patch to do something like this. It even gives you a rdsize=xxx
tunable kernel parameter that lets you specify the size of the tmpfs, which
acts like the -osize= mount flag (so phrases like 100M or 20% works). So doing
things like 'cat /dev/zero > filename' will not run you out of all available
memory. (Note: If you don't specify rdsize= on the kernel command line, it will
not convert rootfs to tmpfs).

See attached.

  -Byron

On Wed, 3 Apr 2013, Rob Landley wrote:

> Attached is my quick and dirty hack to make rootfs be tmpfs when CONFIG_TMPFS 
> is
> enabled. It can't be this easy or somebody would have done it in the 
> _eight_years_
> since https://lkml.org/lkml/2006/7/31/145
>
> Yes, it's got an #ifdef and out of place prototypes. Yes, it manually calls a 
> module
> init function and compensates by making it reentrant. But it works, and when 
> I
> "cat /dev/zero > filename" the filesystem fills _up_ instead of panicing the 
> kernel.
>
> So now that I've posted the error, would someone please tell me how I 
> _should_ have done it?
>
> Rob
>
> P.S. If I actually change the filesystem type to a name other than "rootfs", 
> it panics on the way up because various bits of the kernel are looking for 
> that magic name. Sigh.
>
> P.P.S. removing MS_NOUSER is actually intentional, there's a local cray patch 
> that does the same thing because otherwise you can't --bind mount directories 
> out of this filesystem, which is a thing they wanted to do.--
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[-- Attachment #2: Type: TEXT/x-diff; name=root-tmpfs.patch, Size: 1592 bytes --]

--- linux/init/initramfs.c.orig	2012-12-05 21:39:09.000000000 -0500
+++ linux/init/initramfs.c	2012-12-10 12:15:49.000000000 -0500
@@ -569,14 +569,56 @@
 }
 #endif
 
+static char * __initdata ramdisk_size;
+
+static int __init rdsize_setup(char *str)
+{
+	ramdisk_size = str;
+	return 1;
+}
+__setup("rdsize=", rdsize_setup);
+
+static int __init change_root_to_tmpfs(void)
+{
+	char size[38], *s;
+
+	sprintf(size, "size=%.20s,nr_inodes=0", ramdisk_size);
+	if ((s = strchr(size, ',')))
+		*s = '\0';
+
+	if (!sys_mkdir("/root", 0700) &&
+	    !sys_mount("/dev/root", "/root", "tmpfs", 0, size) &&
+	    !sys_chdir("/root") &&
+	    !sys_mount(".", "/", NULL, MS_MOVE, NULL) &&
+	    !sys_chroot("."))
+	        return 0;
+
+	panic("Failed to mount tmpfs as root filesystem");
+}
+
+
 static int __init populate_rootfs(void)
 {
-	char *err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
+	char *err;
+
+	if (ramdisk_size)
+		change_root_to_tmpfs();
+
+	err = unpack_to_rootfs(__initramfs_start, __initramfs_size);
 	if (err)
 		panic(err);	/* Failed to decompress INTERNAL initramfs */
 	if (initrd_start) {
 #ifdef CONFIG_BLK_DEV_RAM
 		int fd;
+		if (ramdisk_size) {
+			printk(KERN_INFO "Unpacking initramfs...\n");
+			err = unpack_to_rootfs((char *)initrd_start,
+				initrd_end - initrd_start);
+			if (err)
+				panic(err);
+			free_initrd();
+			return 0;
+		}
 		printk(KERN_INFO "Trying to unpack rootfs image as initramfs...\n");
 		err = unpack_to_rootfs((char *)initrd_start,
 			initrd_end - initrd_start);