__test_page_isolated_in_pageblock() in mm/page_isolation.c has a comment
saying that the caller must hold zone->lock. But the only caller of that
function, test_pages_isolated(), does not hold zone->lock and the lock is
also not acquired anywhere before. This patch adds the missing zone->lock
to test_pages_isolated().

We reproducibly run into BUG_ON(!PageBuddy(page)) in __offline_isolated_pages()
during memory hotplug stress test, see trace below. This patch fixes that
problem, it would be good if we could have it in 2.6.27.

kernel BUG at /home/autobuild/BUILD/linux-2.6.26-20080909/mm/page_alloc.c:4561!
illegal operation: 0001 [#1] PREEMPT SMP
Modules linked in: dm_multipath sunrpc bonding qeth_l3 dm_mod qeth ccwgroup vmur
CPU: 1 Not tainted 2.6.26-29.x.20080909-s390default #1
Process memory_loop_all (pid: 10025, task: 2f444028, ksp: 2b10dd28)
Krnl PSW : 040c0000 801727ea (__offline_isolated_pages+0x18e/0x1c4)
 R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:0 CC:0 PM:0
Krnl GPRS: 00000000 7e27fc00 00000000 7e27fc00
 00000000 00000400 00014000 7e27fc01
 00606f00 7e27fc00 00013fe0 2b10dd28
 00000005 80172662 801727b2 2b10dd28
Krnl Code: 801727de: 5810900c l %r1,12(%r9)
 801727e2: a7f4ffb3 brc 15,80172748
 801727e6: a7f40001 brc 15,801727e8
 >801727ea: a7f4ffbc brc 15,80172762
 801727ee: a7f40001 brc 15,801727f0
 801727f2: a7f4ffaf brc 15,80172750
 801727f6: 0707 bcr 0,%r7
 801727f8: 0017 unknown
Call Trace:
([<0000000000172772>] __offline_isolated_pages+0x116/0x1c4)
 [<00000000001953a2>] offline_isolated_pages_cb+0x22/0x34
 [<000000000013164c>] walk_memory_resource+0xcc/0x11c
 [<000000000019520e>] offline_pages+0x36a/0x498
 [<00000000001004d6>] remove_memory+0x36/0x44
 [<000000000028fb06>] memory_block_change_state+0x112/0x150
 [<000000000028ffb8>] store_mem_state+0x90/0xe4
 [<0000000000289c00>] sysdev_store+0x34/0x40
 [<00000000001ee048>] sysfs_write_file+0xd0/0x178
 [<000000000019b1a8>] vfs_write+0x74/0x118
 [<000000000019b9ae>] sys_write+0x46/0x7c
 [<000000000011160e>] sysc_do_restart+0x12/0x16
 [<0000000077f3e8ca>] 0x77f3e8ca
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>