[PATCH] mm/page_alloc: fix a crash in free_pages_prepare()

[Qian Cai]

On architectures like s390, arch_free_page() could mark the page unused
(set_page_unused()) and any access later would trigger a kernel panic.
Fix it by moving arch_free_page() after all possible accessing calls.

 Hardware name: IBM 2964 N96 400 (z/VM 6.4.0)
 Krnl PSW : 0404e00180000000 0000000026c2b96e
(__free_pages_ok+0x34e/0x5d8)
            R:0 T:1 IO:0 EX:0 Key:0 M:1 W:0 P:0 AS:3 CC:2 PM:0 RI:0 EA:3
 Krnl GPRS: 0000000088d43af7 0000000000484000 000000000000007c
 000000000000000f
            000003d080012100 000003d080013fc0 0000000000000000
 0000000000100000
            00000000275cca48 0000000000000100 0000000000000008
 000003d080010000
            00000000000001d0 000003d000000000 0000000026c2b78a
 000000002717fdb0
 Krnl Code: 0000000026c2b95c: ec1100b30659 risbgn %r1,%r1,0,179,6
            0000000026c2b962: e32014000036 pfd 2,1024(%r1)
           #0000000026c2b968: d7ff10001000 xc 0(256,%r1),0(%r1)
           >0000000026c2b96e: 41101100  la %r1,256(%r1)
            0000000026c2b972: a737fff8  brctg %r3,26c2b962
            0000000026c2b976: d7ff10001000 xc 0(256,%r1),0(%r1)
            0000000026c2b97c: e31003400004 lg %r1,832
            0000000026c2b982: ebff1430016a asi 5168(%r1),-1
 Call Trace:
 __free_pages_ok+0x16a/0x5d8)
 memblock_free_all+0x206/0x290
 mem_init+0x58/0x120
 start_kernel+0x2b0/0x570
 startup_continue+0x6a/0xc0
 INFO: lockdep is turned off.
 Last Breaking-Event-Address:
 __free_pages_ok+0x372/0x5d8
 Kernel panic - not syncing: Fatal exception: panic_on_oops
00: HCPGIR450W CP entered; disabled wait PSW 00020001 80000000 00000000
26A2379C

Signed-off-by: Qian Cai <cai@xxxxxx>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 3334a769eb91..a54ff6a60649 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1175,11 +1175,11 @@ static __always_inline bool free_pages_prepare(struct page *page,
 		debug_check_no_obj_freed(page_address(page),
 					   PAGE_SIZE << order);
 	}
-	arch_free_page(page, order);
 	if (want_init_on_free())
 		kernel_init_free_pages(page, 1 << order);
 
 	kernel_poison_pages(page, 1 << order, 0);
+	arch_free_page(page, order);
 	if (debug_pagealloc_enabled())
 		kernel_map_pages(page, 1 << order, 0);
 
-- 
1.8.3.1