Re: Fwd: [Bug 213069] New: kernel BUG at arch/powerpc/include/asm/book3s/64/hash-4k.h:147! Oops: Exception in kernel mode, sig: 5 [#1]

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 5/17/21 11:17 AM, Christophe Leroy wrote:
> +aneesh
> +linuxppc-dev list
> 
> Le 17/05/2021 à 07:44, Anshuman Khandual a écrit :
>> Hello Christophe,
>>
>> DEBUG_VM_PGTABLE has now been re-enabled on powerpc recently ? was not
>> aware about this. From the error log, it failed explicitly on 4K page
>> size hash config.
>>
>> static inline pmd_t hash__pmd_mkhuge(pmd_t pmd)
>> {
>>          BUG();        ------> Failed
>>          return pmd;
>> }
>>
>> static inline pmd_t __pmd_mkhuge(pmd_t pmd)
>> {
>>          if (radix_enabled())
>>                  return radix__pmd_mkhuge(pmd);
>>          return hash__pmd_mkhuge(pmd);
>> }
>>
>> pmd_t pfn_pmd(unsigned long pfn, pgprot_t pgprot)
>> {
>>          unsigned long pmdv;
>>
>>          pmdv = (pfn << PAGE_SHIFT) & PTE_RPN_MASK;
>>
>>          return __pmd_mkhuge(pmd_set_protbits(__pmd(pmdv), pgprot));
>> }
>>
>> It seems like on powerpc, where pfn_pmd() makes a huge page but which
>> is not supported on 4K hash config thus triggering the BUG(). But all
>> pfn_pmd() call sites inside the debug_vm_pgtable() test are protected
>> with CONFIG_TRANSPARENT_HUGEPAGE. IIUC unlike powerpc, pfn_pmd() does
>> not directly make a huge page on other platforms.
>>
>> Looking at arch/powerpc/include/asm/book3s/64/hash-4k.h, all relevant
>> THP helpers has BUG() or 0 which indicates THP might not be supported
>> on 4K page size hash config ?
>>
>> But looking at arch/powerpc/platforms/Kconfig.cputype, it seems like
>> HAVE_ARCH_TRANSPARENT_HUGEPAGE is invariably selected on PPC_BOOK3S_64
>> platforms which I assume includes 4K page size hash config as well.
>>
>> Is THP some how getting enabled on this 4K page size hash config where
>> it should not be (thus triggering the BUG) ? OR am I missing something
>> here.
>>
>

We should put those  pfn_pmd()  and pfn_pud() after

	if (!has_transparent_hugepage())
		return;


On hash with 4K page size, we can't support leaf page table entry and 
PMD and PUD level. Hence we don't support THP for them.

-aneesh