[PATCH] don't swapon mounted devices

[Bjorn Helgaas]

The attached patch against 2.4.21-pre3 fixes a problem in sys_swapon.
Previous behavior:

	* boot kernel with "root=/dev/sda3 init=/bin/bash" (ext3 root)
	* mount -t proc proc /proc
	* /sbin/swapon /dev/sda3    <--- NOTE: same as root!
	* mount -n -o remount,rw /
	* oops in __make_request because superblock buffer_head is
	  no longer mapped (via sys_swapon -> set_blocksize ->
	  kill_bdev-> truncate_inode_pages -> do_flushpage ->
	  block_flushpage -> discard_bh_page -> discard_buffer)

I think 2.5 avoids this problem by attempting to get exclusive
use of the device with bd_claim (see
http://linux.bkbits.net:8080/linux-2.5/cset@1.369.32.10?nav=index.html|src/.|src/mm|related/mm/swapfile.c)

bd_claim is not in 2.4, so this seems next-best. 

Bjorn

--- 1.25/mm/swapfile.c	Sat Dec 28 16:18:13 2002
+++ edited/mm/swapfile.c	Wed Feb  5 10:32:27 2003
@@ -915,6 +915,11 @@
 		struct block_device_operations *bdops;
 		devfs_handle_t de;
 
+		if (is_mounted(dev)) {
+			error = -EBUSY;
+			goto bad_swap_2;
+		}
+
 		p->swap_device = dev;
 		set_blocksize(dev, PAGE_SIZE);