powerpc: Do not assert pte_locked for hugepage PTE entries

With CONFIG_DEBUG_VM, an assertion is made when changing the protection
flags of a PTE that the PTE is locked. Huge pages use a different pagetable
format and the assertion is bogus and will always trigger with a bug looking
something like

 Unable to handle kernel paging request for data at address 0xf1a00235800006f8
 Faulting instruction address: 0xc000000000034a80
 Oops: Kernel access of bad area, sig: 11 [#1]
 SMP NR_CPUS=32 NUMA Maple
 Modules linked in: dm_snapshot dm_mirror dm_region_hash
  dm_log dm_mod loop evdev ext3 jbd mbcache sg sd_mod ide_pci_generic
  pata_amd ata_generic ipr libata tg3 libphy scsi_mod windfarm_pid
  windfarm_smu_sat windfarm_max6690_sensor windfarm_lm75_sensor
  windfarm_cpufreq_clamp windfarm_core i2c_powermac
 NIP: c000000000034a80 LR: c000000000034b18 CTR: 0000000000000003
 REGS: c000000003037600 TRAP: 0300   Not tainted (2.6.30-rc3-autokern1)
 MSR: 9000000000009032 <EE,ME,IR,DR>  CR: 28002484  XER: 200fffff
 DAR: f1a00235800006f8, DSISR: 0000000040010000
 TASK = c0000002e54cc740[2960] 'map_high_trunca' THREAD: c000000003034000 CPU: 2
 GPR00: 4000000000000000 c000000003037880 c000000000895d30 c0000002e5a2e500
 GPR04: 00000000a0000000 c0000002edc40880 0000005700000393 0000000000000001
 GPR08: f000000011ac0000 01a00235800006e8 00000000000000f5 f1a00235800006e8
 GPR12: 0000000028000484 c0000000008dd780 0000000000001000 0000000000000000
 GPR16: fffffffffffff000 0000000000000000 00000000a0000000 c000000003037a20
 GPR20: c0000002e5f4ece8 0000000000001000 c0000002edc40880 0000000000000000
 GPR24: c0000002e5f4ece8 0000000000000000 00000000a0000000 c0000002e5f4ece8
 GPR28: 0000005700000393 c0000002e5a2e500 00000000a0000000 c000000003037880
 NIP [c000000000034a80] .assert_pte_locked+0xa4/0xd0
 LR [c000000000034b18] .ptep_set_access_flags+0x6c/0xb4
 Call Trace:
 [c000000003037880] [c000000003037990] 0xc000000003037990 (unreliable)
 [c000000003037910] [c000000000034b18] .ptep_set_access_flags+0x6c/0xb4
 [c0000000030379b0] [c00000000014bef8] .hugetlb_cow+0x124/0x674
 [c000000003037b00] [c00000000014c930] .hugetlb_fault+0x4e8/0x6f8
 [c000000003037c00] [c00000000013443c] .handle_mm_fault+0xac/0x828
 [c000000003037cf0] [c0000000000340a8] .do_page_fault+0x39c/0x584
 [c000000003037e30] [c0000000000057b0] handle_page_fault+0x20/0x5c
 Instruction dump:
 7d29582a 7d200074 7800d182 0b000000 3c004000 3960ffff 780007c6 796b00c4
 7d290214 7929a302 1d290068 7d6b4a14 <800b0010> 7c000074 7800d182 0b000000

This patch fixes the problem by not asseting the PTE is locked for VMAs
backed by huge pages.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/mm/pgtable.c

@@ -219,7 +219,8 @@ int ptep_set_access_flags(struct vm_area_struct *vma, unsigned long address,
 		entry = do_dcache_icache_coherency(entry);
 	changed = !pte_same(*(ptep), entry);
 	if (changed) {
-		assert_pte_locked(vma->vm_mm, address);
+		if (!(vma->vm_flags & VM_HUGETLB))
+			assert_pte_locked(vma->vm_mm, address);
 		__ptep_set_access_flags(ptep, entry);
 		flush_tlb_page_nohash(vma, address);
 	}