powerpc/mm: Remove DEBUG_VM_PGTABLE support on powerpc

The test is broken w.r.t page table update rules and results in kernel
crash as below. Disable the support until we get the tests updated.

[   21.083519] kernel BUG at arch/powerpc/mm/pgtable.c:304!
cpu 0x0: Vector: 700 (Program Check) at [c000000c6d1e76c0]
    pc: c00000000009a5ec: assert_pte_locked+0x14c/0x380
    lr: c0000000005eeeec: pte_update+0x11c/0x190
    sp: c000000c6d1e7950
   msr: 8000000002029033
  current = 0xc000000c6d172c80
  paca    = 0xc000000003ba0000   irqmask: 0x03   irq_happened: 0x01
    pid   = 1, comm = swapper/0
kernel BUG at arch/powerpc/mm/pgtable.c:304!
[link register   ] c0000000005eeeec pte_update+0x11c/0x190
[c000000c6d1e7950] 0000000000000001 (unreliable)
[c000000c6d1e79b0] c0000000005eee14 pte_update+0x44/0x190
[c000000c6d1e7a10] c000000001a2ca9c pte_advanced_tests+0x160/0x3d8
[c000000c6d1e7ab0] c000000001a2d4fc debug_vm_pgtable+0x7e8/0x1338
[c000000c6d1e7ba0] c0000000000116ec do_one_initcall+0xac/0x5f0
[c000000c6d1e7c80] c0000000019e4fac kernel_init_freeable+0x4dc/0x5a4
[c000000c6d1e7db0] c000000000012474 kernel_init+0x24/0x160
[c000000c6d1e7e20] c00000000000cbd0 ret_from_kernel_thread+0x5c/0x6c

With DEBUG_VM disabled

[   20.530152] BUG: Kernel NULL pointer dereference on read at 0x00000000
[   20.530183] Faulting instruction address: 0xc0000000000df330
cpu 0x33: Vector: 380 (Data SLB Access) at [c000000c6d19f700]
    pc: c0000000000df330: memset+0x68/0x104
    lr: c00000000009f6d8: hash__pmdp_huge_get_and_clear+0xe8/0x1b0
    sp: c000000c6d19f990
   msr: 8000000002009033
   dar: 0
  current = 0xc000000c6d177480
  paca    = 0xc00000001ec4f400   irqmask: 0x03   irq_happened: 0x01
    pid   = 1, comm = swapper/0
[link register   ] c00000000009f6d8 hash__pmdp_huge_get_and_clear+0xe8/0x1b0
[c000000c6d19f990] c00000000009f748 hash__pmdp_huge_get_and_clear+0x158/0x1b0 (unreliable)
[c000000c6d19fa10] c0000000019ebf30 pmd_advanced_tests+0x1f0/0x378
[c000000c6d19fab0] c0000000019ed088 debug_vm_pgtable+0x79c/0x1244
[c000000c6d19fba0] c0000000000116ec do_one_initcall+0xac/0x5f0
[c000000c6d19fc80] c0000000019a4fac kernel_init_freeable+0x4dc/0x5a4
[c000000c6d19fdb0] c000000000012474 kernel_init+0x24/0x160
[c000000c6d19fe20] c00000000000cbd0 ret_from_kernel_thread+0x5c/0x6c
33:mon>

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200902040122.136414-1-aneesh.kumar@linux.ibm.com

Documentation/features/debug/debug-vm-pgtable/arch-support.txt

@@ -22,7 +22,7 @@
     |       nios2: | TODO |
     |    openrisc: | TODO |
     |      parisc: | TODO |
-    |     powerpc: |  ok  |
+    |     powerpc: | TODO |
     |       riscv: |  ok  |
     |        s390: |  ok  |
     |          sh: | TODO |

arch/powerpc/Kconfig

@@ -116,7 +116,6 @@ config PPC
 	#
 	select ARCH_32BIT_OFF_T if PPC32
 	select ARCH_HAS_DEBUG_VIRTUAL
-	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_ELF_RANDOMIZE
 	select ARCH_HAS_FORTIFY_SOURCE